Lec 30 | 3.091 Introduction to Solid State Chemistry Video Lecture

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ocw.mit.edu one announcement even though
you will have no academic exercises
thursday and friday i know you'd feel a
sense of shock and disruption and dare I
say dislocation in your life if I didn't
stick to schedule so there'll be a skill
testing question on Tuesday based on the
homework 12 and that'll be the last one
because there the subsequent week is the
last week of semester and since there's
a final exam in this subject there can
be no tests during the last week of
class so we will not have any weekly
tests today I want to talk about
biochemistry we're going to spend the
next three lectures on biochemistry this
is the chemistry of living organisms and
I want to make several points by way of
introduction the first one is that
living organisms are chemical systems
and they're governed by the same laws
that apply to inanimate matter we don't
have a special chemistry in fact I came
across this comment that was made by the
Nobel laureate Richard Feynman who said
that if in some Cataclysm all of
scientific knowledge were to be
destroyed and only one sentence passed
on to the next generations of creatures
what statement would contain the most
information in the fewest words I
believe that it is the atomic hypothesis
or the atomic fact or whatever you wish
to call it that all things are made of
atoms little particles that move around
and perpetual motion attracting each
other when there are little distance
apart but repelling upon being squeezed
into one another the atomic hypothesis
all right very important second comment
I want to
make is the you none of us is unaware of
the boom and biotechnology today what is
the boom and biotechnology why do we
have the boom in biotechnology it's
really a boom in bio chemical technology
what's the modern biology called it's
called molecular biology and what is the
science what is molecular science this
is chemistry and more importantly it is
really not just chemical composition but
spatial arrangement so chemistry
material science solid-state chemistry
underlie the boom in biotech so that's
why we're going to spend some time we're
spending ten percent of our lectures in
3091 on this topic and the last comment
is why only now why didn't this happen
50 years ago what's enabling this boom
today and I think about the the ages of
like well the answer is complexity let's
get the answer out there the answer is
complexity of the chemistry and if you
think about how materials have evolved
over time you know we date our ages the
ages of man are dated in terms of
materials we have the Stone Age Bronze
Age Iron Age we might call the first
half of the 20th century the plastics
age might call the last half of the 20th
century the silicon age and we see
increasing complexity to work with stone
stone is found in nature it requires
only a change of shape but everything
else the metals the bronze the iron not
found in nature native so that requires
a change in shape after a change in
chemical composition chemistry is magic
chemistry is magic and chemists were
revered as sorcerers magicians they were
highly respected not today
they're compelling reasons to be alive
today but not for the respect given to
chemists but it's magic to take
something and convert its chemical
composition and that took time to get
the fires hot enough to discover the
chemistry that would allow us to take a
pile of sand and turn it into a
semiconducting device so that you could
talk on a phone or type a letter on a
solid-state device known as a computer
that took time so polymers come at the
dawn of the 20th 20th century
macromolecules it turns out that many of
the molecules in living organisms are
macromolecules so necessarily our
understanding of this chemistry had to
wait for the understanding of polymer
chemistry and finally now we have
information technology we have our
understanding of macromolecules and now
we can go forward with bio technology so
let's get down to business and start
talking about the chemistry of specific
biomolecules and they fall broadly into
four categories so let's document the
four categories of bio molecules so we
have first of all proteins carbohydrates
nucleic acids
and lipids and in the three lectures
that we have I'm going to focus
primarily on proteins nucleic acids and
lipids and I think this will get you far
enough along to enjoy subsequent
chemistry so let's get down to business
and start with proteins proteins comes
from the Greek word Proteus which means
of first importance and these are
macromolecules these are macromolecules
polymers of a sort and they're formed by
polymerization formed by polymerization
and the the mer groups that go into this
macro molecule are called amino acids
formed by polymerization of amino acids
so we better get a grip on what amino
acids are as the name implies they must
be compounds that have an acid group in
other words capability of donating
protons and amino means there must be
some kind of an NH formation so let's
draw the skeletal structure of a
prototypical amino acid you start at the
center with a carbon that's sp3
hybridized and one of the terminals
contains an amino group which consists
of this formation bearing nitrogen the
acid is given above this is a carboxylic
acid carbon double bond to oxygen and
then the oh h and this h is capable of
detaching so there is the amino acid the
third bond goes to oxygen so three of
the four bonds are uniquely specified in
every amino acid three of the four and
now the fourth one the cetera hedron's
so I'm trying to indicate that this bond
is receding into the back
is variable and I'm going to use the
carrier variable R and this is the term
for the substituent the substituent and
this is nature's choice this is nature's
choice so in principle anything that
will covalently bond with a sigma bond
can be put here so this means that there
are thousands and thousands of compounds
that qualify as amino acids but in
proteins only 20 only 20 found in
proteins so that's a short list it's a
rather compact library and let's take a
look at the types of substituents and
for that you can look at the table here
in your reading this is from the table
so this is actually spills over two
pages and what we see here I know that's
very hard to read but it's more iconic
just to sort of symbolize that maybe
somebody has done the reading and they
say I recognize that I've recognized
that table so there's 20 different r
groups 20 different substituents and
they themselves fall into four
categories so the r groups can be broken
down into four categories first one is
nonpolar the our group is nonpolar and
clearly under that circumstance we'd
have something that is hydrophobic
hydrophobic non polar groups would be
less inclined to bond to water we have
the option of polar groups polar with
hydrogen bonding capability with
hydrogen bonding capability and such
groups are going to be hydrophilic
there's a third group that has a net
charge so the our group is actually net
positive not positive and so that means
these are going to be strongly strongly
hydrophilic have a very high affinity
for water and it turns out they will
function as basic in aqueous solution
and then the last group is a set of
substituents that have a net negative
charge and these two are strongly
hydrophilic and will act acidic when
dissolved in water now our bodies can
synthesize to only 10 of the 20 amino
acids so you'll read in advertising that
certain foods contain some of the
essential amino acids meaning that
contain some of the 10 amino acids that
we're incapable of of processing and
failure to consume this in the diet over
a long period of time can have a
deleterious effects prompting in the
extreme you are exiting the carbon cycle
so you do want to watch your diet now
there's one other structural feature
that is important in the is important in
the amino acids and that i want to show
with reference to a specific amino acid
so let's choose this one's alanine in
alanine the substituent group is methyl
ch3 so i can draw alanine as follows
using the skeletal structure to the left
i can here's the amino group which i'm
just going to truncate as h2n and maybe
later i'm even going to get so bold as
to put NH 2 recognizing that it's not
the H that's bridging to the nitrogen
but let's do it this way the carboxylic
acid will go up here will go here and
then we'll just put the methyl group now
I could just as easily have written it
the following way I could have done
this I could have put the amino group to
the right the hydrogen to the left now
if you look at these two at first glance
you might say well I just got to turn it
and he's basically written the same
thing but upon closer inspection you'll
discover that it's impossible to convert
the one on the right to the one on the
left in fact they're not superimposable
what we're looking at here is
essentially a pair of gloves this is a
consider the left hand and this is the
right hand and you can close the gloves
you can make them fit one against the
other they will mirror one another
across the mirror plane but they're not
superimposable you know as opposed to
something like say propane if i take
propane which is also carbon sp3
hybridized propane and contrast has
methyl above and below and that clearly
is perfectly symmetric and doesn't have
such a glove-like feature such molecules
that have we say a handedness are called
chiral chiral from the greek word for
hand so chirality is the property of
having handedness non super imposable we
call these another term for these is
stereo isomers we can consider chiral
molecules stereo isomers stereo isomers
and the different hands if you will the
different hands are called enantiomer
enantiomer we don't refer to one
stereoisomer the other stereo meissen
where we refer to the enantiomer coming
from the greek word for mirror and anti
osas the greek word for mirror so these
are different mirror images the other
thing about these is that they are
optically active and what they will do
is they will take polarized light and
rotate it through
a degree of polarization here just to
give you another example of the
difference between something that's
chiral or not you see the boxes at the
top with no distinguishing features they
can be twisted and turned and made to
superimpose over one another by putting
these little features in the bottom
corner we render these boxes chiral and
no matter how you twist and turn this
box you won't be able to superimpose it
on top of the other box now here's a
more complicated molecule that can come
chemically identical but has a
right-handed version and a left-handed
version so let's talk about optical
activity the only reason I refer to this
is that it gives the labels that are
used in referring to one versus the
other so if you have a light source that
just gives light that's randomly
polarized that is to say it's not
polarized and then you put it through a
polarizing prison so that the light is
polarized in a certain direction other
words the electric vector is only in one
direction now now the polarized light
here it's indicating that the it's
polarized vertically this is the
optically active substance which could
include an aqueous solution of alanine
and if the aqueous solution of alanine
consists of only one enantiomer only one
enantiomer then when the polarized light
passes through that solution the plane
of polarization will be rotated a
certain amount by interaction of the
light with the electrons in the orbitals
in this particular molecule and what you
see here is a angle of rotation alpha
where alpha is proportional to the
concentration of the chiral species and
also the path length through the through
the solution so if we put in both
enantiomer as we expect no change if we
put in one enantiomer we will have one
direction and so it turns out that that
the different enantiomer is will cause
different directions of rotation and
this is how they are labeled so the one
that I've shown here on the right will
cause a clockwise rotation of the light
and so it is termed d as in
dextrorotatory from the Latin word for
right dextro rotatory this is the
dextrorotatory form and then the one
here on the Left will cause the light
and i'm going to write H nu here it's
the it's the it's the plane of
polarization of the light that's
rotating all right so the plane of
polarization if it went through a
solution that consisted of only the
enantiomer depicted on the left it would
move in the left direction and even
though the Latin word for left is
sinister they didn't call it Sinestro
rotatory they call it level rotatory I
guess they had to go to the Slavic
languages to get a root word so level
rotatory is this one dextrorotatory some
will cause call this the positive
enantiomer and the left one the negative
and auntie em er but in most of the
reading that you'll encounter it'll be
the the D and the L it turns out that
only the L form are only the elf form of
amino acids are found in proteins only L
enantiomer of amino acids found in
proteins but synthesis could give rise
to both synthesis could give rise to
both and certainly chemical synthesis
will give rise to both I'm not going to
spend much time on carbohydrates but
just for reference the carbohydrates are
also chiral molecules and it turns out
that in sugars in sugars only the deform
only deform is metabolized by our bodies
so you may have heard of invert sugar a
baker's sugar this is simply the elf
form of the various sugars and this
passes unrecognized through the body
honey contains the elf form of the carb
because it's processed by bees and the
bees have a different biological
apparatus so this gives rise to there's
through their science fiction where
somebody is after some grand event finds
themselves in a strange place on it it's
always got to be a desert island and
they end up eating fruit that happens to
be chock full of energy but it's the
wrong and an tiem ER and the people
although they're filled to satiety they
die because they can't process the food
so that's the basis that plot line has
been used already I'm sorry to tell you
so you can you can't use it now okay and
the a mix of both when both enantiomer
zar both enantiomers present this is
called racemic racemic and this is very
important I said it's a matter of life
and death it's very important and
pharmaceuticals there's a couple of I'll
tell you tell you one example back in
the 1960s there was a drug that was
produced by a German pharmaceutical
company called thalidomide if the lid
amide was developed as an anticonvulsant
and it and it was also good as a as a
sedative for people that were severely
depressed it had one of the properties
of thalidomide was that you could not
overdose on it the person took a huge
quantity of thalidomide it would make
the person comatose you go into a deep
sleep you'd be an extra on the set of
Rip Van Winkle but you could not kill
yourself an unintended benefit of
thalidomide was discovered by pregnant
women in Europe who learned that
consumption of thalidomide had a
tremendous pally defect
morning sickness particularly early in
the pregnancy so people started using it
for palliative against morning sickness
this was only in Europe the Food and
Drug Administration the United States
said it hasn't been subjected to enough
testing and refused to allow its
importation into the United States and
there was a lot of clamoring say oh the
FDA is is all old fashioned there in the
pockets of the drug companies they're
holding up the advance of of drugs and
so on you hear these charges with
respect to cancer drugs and drugs in the
treatment of AIDS where people are
impatient for solutions the FDA held its
ground but much of the drug did find its
way into the United States and then
shortly after its widespread consumption
it was found that this is a teratogen
and it produces hideous birth defects
and people said what went wrong why did
they not discover this was it not tested
was not tested on animals the answer is
it was tested on animal models it turns
out that all the animal models that it
was tested on those animals lack the
particular enzyme that processes
thalidomide the way it's processed in
humans so this is something that caused
quite a stir at the time what's the
punch line the punch line is it's a
chiral molecule only one enantiomer
causes the birth defects but nobody knew
at the time so if they'd known and
instead of selling a racemic form and
sold a form that has only the one
enantiomer they could have had the
beneficial effects without the
undesirable effects and now thalidomide
is back in the news some people have
discovered that it may have some benefit
as a component of a cocktail that's
being used in the treatment of AIDS but
the memory lingers on and there's a
tremendous thing by the way ritalin
ritalin which is used in the treatment
of ADHD it's also racemic and there's
some it's it's chiral forgive me it's
chiral and it's been produced without
regard to its chirality and it's now of
emerging that the
d that the D enantiomer the deform is
effective in the treatment of ADHD and
the elf form is ineffective and some of
the undesirable side effects of
long-term use of riddle and may in fact
be linked to the L enantiomer so what
you're seeing in more and more in drug
design is attention paid to the the
structure it's the structure in fact why
don't I cut to the chase this is what
drug design is all about this is in like
two minutes all right let's say this is
some receptor molecule some receptor
molecule and it's got little little
dipoles here this is a delta minus and
this is a delta plus and maybe this is a
delta minus and this is a delta plus
just arbitrarily and this is some
candidate drug that that is supposed to
come up to this receptor and the drug is
designed to react what's going on is
really a lock-and-key mechanism all
right and you know the Debye frequency
the Debye frequency is about 10 trillion
10 trillion hurts 10 trillion times a
second nature puts the system to the
test 10 trillion times a second so on
one of those tests suppose this this
drug that's supposed to bind to this
receptor where this is Delta minus this
is Delta plus where this is Delta plus
this is Delta minus and so on then the
two will stay together long enough to do
the math and figure out that there's
some more energy to be gained through a
further chemical reaction if on the
other hand this is somehow miss form so
that opposite this Delta minus is also a
delta minus during that collision there
won't be that staying power to keep
things together long enough and so the
activation will not be achieved and so
the reaction won't go forth so really
the message here is that atomic
arrangement atomic arrangement
is the key with a pun here it's the key
now this is a lock-and-key mechanism an
atomic arrangement is the key paying
attention to chirality of course the
other thing is let's understand the
chemistry to a such a high degree that
we're not relying on field testing to
sort of determine whether something's
acceptable an art we need to get rid of
determinism excuse me get rid of
empiricism and ramp up the determinism
all right let's talk about properties of
amino acids what are the properties of
these compounds well first of all
they're solids they're solids solids at
room temperature and they can
crystallize they can be crystal remember
these are the mer groups they're not the
polymers these are the mer groups that
have yet to be polymerized they can
crystallize I think it's fair to say
they're going to be colorless entirely
covalent high highly bonded and they're
moderately soluble in water moderately
soluble in water which is a good thing
for something that's going to function
as a biomolecule I think solubility and
waters is desirable and in fact I want
to look a little more closely at the
behavior and water and and to mention
that first thing that happens this thing
transforms and in the chart in the chart
that's in the book that that bio table 1
you don't see the amino acids drawn as
I've drawn them with the amino and
neutral and the carboxylic and neutral
they draw them as follows they show what
happens when the amino acid has already
been dissolved in water so let's
represent thats all you solvation
reaction so i'm going to compact the
notation so here's the central carbon
there's the hydrogen there's the amino
substituent on the bottom and on the top
i'm going to put the carboxylic acid and
let's ask what happens when this
dissolves in water at neutral pH and
this doesn't necessarily mean ph equals
7 it means neutral with respect to this
reaction that we're going to show what
happens when this species dissolves in
water is the proton leaves the
carboxylic acid and comes over and
attaches to the amino end I mean look at
the amino end this is really a bronsted
base isn't it this is a bronsted base
this is an arena assess it's also
bronsted acid so now we have from what
is a neutral molecule something that
looks like this protons attached here
rendering this and positive the R group
remains unchanged and the carboxylic
acid is now minus the proton so this
molecule it's still net neutral because
I've got charge neutrality to maintain
all I've done is relocate something
within the molecule but it's bipolar
isn't it it's bipolar that that is to
say it's locally it's locally positive
and locally negative but globally it is
net neutral so it's kind of a strange
ion isn't it is it an eye on or is it
not an eye on well it's its net neutral
so we wouldn't call it an eye on it's
some kind of instead of a 9 it's called
a vinter I on as Vitter ion which means
a double eye on it's as bitter iron or a
double eye on some kind of a hybrid so
that's really important chemistry
because that gives this molecule the
ability to respond to its environment
see somehow through the chemistry we
have to explain how matter becomes
animate how matter becomes animan what's
the chemistry here and how how do I do
this how do I move my hand how do I do
this this is all macromolecular so what
am i doing when I move my hand I'm
changing the confirmation of those
polymers
those polymers are changing shape
remember the c-17 age 36 i showed you
too long chains one was relatively
straight the one was coiled and i can go
from one confirmation to the other how
do i do that the electrical engineers
are going to say it's all electrical I
say no it's not electrical you can't do
this electrically where's the wires is
this chemical it's all chemical I'm
going to show you how and you say but
you can't do it that fast fast is fast
only in the mind of the beholder this
simply has to happen at a rate that is
keeping pace with the conversation on a
geological timescale this is a vanishing
short period of time right but otherwise
this is as long as our data rate of
understanding can keep pace with the
ability for us to change the
confirmation we live so let's look at
how this responds suppose this is uh I
don't know in the wall of the stomach
and you get a hankering for some Cola
beverage and you drink the cola the cola
is low pH right it's about ph 3
phosphoric acid benzoic acid so what's
going to happen is this Vitter ion will
respond in a defensive manner there's
there's a principle that's in chemistry
this is analogous to a Newton's third
law you know action reaction it's called
the lish atelier principle the lucia
atelier principle after the french
scientist who enunciated it lucia tele a
principle and it talks about the
restorative force the restorative force
exerted by a chemical system restorative
force exerted by a chemical system and
what is this restorative force designed
to do it's designed to minimize the
impact of any perturbation designed to
minimize impact or the effect of any
perturbation
by perturbation I mean not a physical
perturbation I mean a chemical shift any
kind of chemical perturbation so let's
say we have the zwitterion sitting
happily in a neutral solution and
suddenly the pH drops suddenly the pH
drops so how can the Vitter I and
respond what do i do to route to bring
the ph back to where it was before the
disturbance what is the manifestation of
pH its proton so somehow the proton
concentration suddenly went way up
driving the pH way down a nun's Vitter I
on sitting here there's veteran right
here what could Vitter ioan do to
minimize the impact of this flood of
proton well there's got two
possibilities how do you neutralize
something you either flood this with
hydroxyl all right start flooding this
with hydroxyl in order to neutralize I
look on zwitterion I see no hydroxyl so
that's no good but what could Vitter
ayan do there's at the carboxylic acid
end an attachment site for proton so
what's Vitter ayan can do is to gobble
up the excess proton in order to bring
the ph back to where it was before so
let's write that reaction it triggers
this response which is there's the amino
end
want to put the put the carboxylic acid
over to the right here so now this is
from the solution this is from the
solution it's trying to bring the ph
back to where it was before consuming
the proton and attaching the proton to
this carboxylic acid in so what's
happened now tis vitter on this end the
negative end has been capped so's vitter
i am now is net positive its net
positive now over here its net neutral
and it's gobbling up protons from the
solution and so this is really an
acid-base reaction for which i can write
a que an acid dissociation constant and
you know I'm getting tired writing all
these characters so I'm going to re
reformat this so I'm going to say this
is the veteran is I'm going to represent
as proton plus the rest and this is
proton and what's this thing here this
is now Sveta I on to which proton has
attached I've attached this proton to
the net neutral species okay so i can
rewrite this reaction in a more compact
notation so a captures all of this other
stuff minus the proton so i can write
the k for that reaction and k1 will
equal the concentration of proton times
the concentration of neutral zwitterion
over the concentration of this
protonated zwitterion thus Vitter ion
onto which we've glommed this proton
from the solution and we're going to
take a page out of Sorensen's book and
we'll take the log of both sides and
then we can say that p k 1 is equal to
ph
plus log and I'm writing ten here but
you know in engineering if you CLO G
it's typically log base 10 I know in
some of the math classes they they use
the log2 represent natural log but it's
uncommon in engineering alright so
here's the here's the Sorensen version
of the equation and what this tells us
is that at fifty percent fifty percent
association in other words when fifty
percent of the litter ion has gobbled up
the proton to generate this protonated
species this ratio is equal fifty
percent zitar ion unpro ten ated fifty
percent protonated so the log of 1 is 0
and that's the pH that gives us the
value of the of the acid dissociation
constant at fifty percent of consumption
of the Vitter ion we have the
concentration of bitter ion equally in a
concentration of the the protonated
zwitterion and therefore the ph is
exactly at the value of PK one now what
happens on the other extreme what
happens if all of a sudden there's a
rise in pH if there's a sudden rise in
pH how does Vitter ion respond sudden
rise in pH that means the concentration
of proton suddenly fell so we use the
leash Atelier principal again we use
Lucia telia principal will go back over
here and we look at svit orion and we
say okay what we've got right now is a
sudden dearth of a shortage of proton
howkins vitter iron respond to that
sudden shortage of proton well the
neutral zwitterion has protons sitting
right here so what vitter iron can do in
response to that sudden drop in proton
concentration is donate become a proton
donor and start adding protons to the
saloon
ocean sacrificing its neutrality in
order to minimize the impact of that
sudden rise in pH so let's let's
document that reaction that's this one
here that's where the Vitter ion does
the following we'll start with the
neutral again so that's over here h3 and
so here's the neutral zwitterion
experiences a sudden rise in PH and
responds by dumping protons into the
solution so now that the left end is
deprotonated leaving it neutral and this
proton is now donated to the solution
and if this solution is in fact high in
pH then this proton can go and attack
the excess hydroxyl and neutralize it so
this is how zitar iron responds and what
do we see here this is neutral this
isn't that neutral but now the positive
end has become neutralized by the loss
of the proton so this is now net
negative this is now net negative and so
we can call this reaction too and it has
a k2 and using the notation that I've
developed up there the neutral
zwitterion I'm denoting a che so this is
the Vitter I on- this proton so this is
just the a- mean it's making sense this
is net negative and if i take h away
from a che i get minus and then this is
the h plus so that's the reaction we're
looking at and we can write the K for
that reaction as concentration of proton
times concentration of deprotonated
zwitterion divided by the concentration
of the neutral zwitterion
and take the log base 10 of both sides
pk2 is ph plus log base 10 of I'm going
to flip this over so I don't have to put
a minus sign here so this will be H a
divided by a minus and again when we
have fifty percent neutralization then
that means that there'll be fifty
percent of this species consumed fifty
percent of this left the log of 1 is 0
and at that pH we have the the value of
the acid dissociation constant and I
think this this slide shows what's going
on here let all it does is it plots
these two equations if you take the
first equation here the logarithmic the
sorenson form of the first equation
that's the lower part of this graph the
lower part of this graph we're plotting
pH versus the ion dissociation so if you
turn this around ph versus the amount of
this that has been consumed and what you
see is that when you have fifty percent
this is fifty percent ions dissociated
per molecule at fifty percent we have
the PK one or both are equal the other
thing to note is that as this reaction
is taking place look at the gentle
change in pH the concentration here
changes from all roughly about ten
percent up to ninety percent so we have
an eighty percent variation in
concentration by that I mean an eighty
percent interval over which we are
changing the amount of attachment here
here there's no attachment here there's
total attachment and in the middle we
have from about ten percent to ninety
percent attachment the pH only moves
about two units so spit or ion is
fighting really really hard to get this
ph up to a certain level and hold it
there so we say that this vitter
and under these circumstances is acting
as a buffer so it's vitto I on Vitter I
on acts as a buffer so what it will do
it's got the capacity to either donate
protons or remove protons and do so in a
way that reduces the impact of the pH
and then here's the other part of the
graph which is up the upper part the
case where we have a sudden rise in pH
so the pH goes up and zwitterion is now
holding the ph is best it can around a
value of PK too and so then there's a
special point on this graph and it's
shown right here where we have maximum
undissociated zwitterion in other words
what is the pH the pH at which we have
one hundred percent undissociated
zwitterion okay just as Vitter I and
that's given by the balance between the
k1 and the k2 and that's called the
isoelectric point isoelectric point
because then everything is net neutral
isoelectric point and that's also given
the notation p I lowercase P capital I
for isoelectric and in this case is
simply the average of PK one and PK to
average of PK one and PK to and this can
be found for various for various amino
acids I don't expect you to know the
amino acid compositions by heart I would
always give you the chemical formula of
the amino acid and if you had to do
anything with such calculations I tell
you what the the k1 or the k2 is as the
as the case may be there's one other
twist on this there are a few amino
acids where the R groups where the R
groups are also capable of attaching or
donating protons we say that the R
groups are titratable
our groups are titratable that is to say
they can also be proton active and can
participate in this lucy atelier
principle in action and if you look on
the homework question 11 on the homework
goes through an analysis of how you deal
with a situation where there's a change
in pH and for example suppose you've got
a side group aspartic acid is a good
example in aspartic acid aspartic acid
the our group is is methylene and then
there's a carboxylic acid sticking off
this is the R group well clearly this is
a site of attachment so if i get a
sudden drop in pH there's a flood of
hydrogen ions the hydrogen ions can be
attaching not only the carboxylic acid
and here but they can also be attaching
on the R group so that brings a new
level of complexity and and we work that
out in the in the particular homework
example okay well let's let's take a
break here now and I'll leave you with a
five minutes on extreme kinetics earlier
in the semester i went up to halifax to
give a seminar and i was reminded of
this which actually our audio-visual
technician Dave Broderick had brought to
my attention a number of years ago we
talked about the extremes in kinetics
one of them being explosion the reason I
tell you about this it follows kinetics
and also it's apropos of the season
after thanksgiving day when you go
downtown you will see a feature that i'm
going to tell you about at the end of
this story on december six 1917 this was
in the middle of World War one and
Canada was part of the British Empire at
the time it was drawn into the war
Halifax was a port of exchange of a lot
of munitions
so there was the Belgian relief ship the
emo and the French supply ship Timon
Blanc and this is the cargo of the Mon
Blanc and you can see it was a potent
cargo to be sure at 8 45 a.m. there was
a collision the emo hit the hmong Blanc
and missed the TNT there's 20,000 tons
of it but it struck the picric acid
which was stored directly beneath the
drums of benzyl and there were some
sparks and so that's the way the story
begins and for this one in this light I
think I'll put on some glasses but I can
read it to you in modern English the
crew of the Mont Blanc aware of their
cargo immediately took to the lifeboats
screaming warnings that no one heated
they rode for Dartmouth which is across
the harbor the Mon belong drifted by
Halifax pier brushing it and setting it
ablaze members of the Halifax Fire
Department responded quickly and we're
positioning their engine up to the
nearest hydrant when the Mont Blanc
disintegrated in a blinding white flash
creating the biggest man-made explosion
before the nuclear age it was 90 5am
over 1,900 people were killed
immediately within a year the figure had
climbed to well over two thousand nine
thousand more injured many permanently
325 acres almost all of North End
Halifax destroyed and much of what was
not immediately leveled burned to the
ground but aided by winter stockpiles of
coal in cellars as for the Mont Blanc
all 3,000 tons of her were shattered
into little pieces that were blasted far
and wide the barrel of one of her
cannons Latin did 300 three and a half
miles away part of her anchor shank
weighing over half a tonne flew two
miles in the opposite direction windows
shattered 50 miles away and the
shockwave was felt in even in Sydney
Cape Breton 270 miles northeast there
was about 20 minutes between the
collision and the explosion was enough
time for spectators including many
children to run to the waterfront to
watch the ship burning thus coming into
close range was enough time for others
to gather at windows and thus an
exceptionally large number of people
were injured by flying glass many
sustained permanent eye damage not
surprisingly hospitals were unable to
cope with someone he wounded
this was also the misery was compounded
by a blizzard that struck the city the
following day dumping 16 inches of snow
over the ruins of their city oily
covering with astounding speed relief
efforts were set into motion money
poured in from as far away as China and
New Zealand the Canadian government gave
blah blah blah so much money so much
money but most halligan ians that's what
you call a resident of halifax most hal
ago nians remember the generosity of the
state of massachusetts which donated 750
thousand dollars in money and goods and
gave unstintingly in volunteer
assistance to the massachusetts halifax
relief committee people from the
hospital's messiah near went up because
so many people had sustained i damaged
and they got on trains and they went up
there to minister to these people to
this day halifax sends an annual
christmas tree to the city of Boston in
gratitude so when you go downtown I
don't know where it is now it used to be
in front of the pru but i heard recently
they relocated it to the Boston Common
if you see a 45-foot Christmas tree that
looks like it came from Canada
that's the reason okay have a happy
thanksgiving i'll see you next week