00:01
Right. Let’s look briefly at RedOx reactions
and enzymes. So, we’re going to touch upon
biochemical reactions in living organisms
which are essentially transfers of energy
from one thing to the other. Often, they occur
together linked in the chain in what is referred
to as a RedOx or oxidation-reduction reaction.
00:21
As we said before when we were looking at
other types of ionic reaction, one chemical
is oxidised and its electrons are then passed
to another chemical and is thus reduced. Such
coupled reactions are referred to as RedOx
reactions and are very important to every
facet of every element of even the most basic
biochemical process such as the ability for
us to convert sugar into adenosine triphosphates.
00:50
Important metabolic processes, such as glycolysis,
which involves the conversion of a sugar into
acetyl co-enzyme A, the Krebs cycle which
involves the conversion amongst other things
of acetyl co-enzyme A into NADH and finally,
electron-transport phosphorylation which uses
NADH in the conversion of ADP to ATP. This
all involves the transfer of electrons via
RedOx reactions and is very important.
01:23
These are often catalysed, as we’ll see,
and this goes back to the original discussion
about protein and, of course, DNA structure
from whence proteins are derived.
01:33
Enzymes are proteins that can accelerate a
chemical reaction and effectively are just
organic or biological catalysts. Let’s have
a quick look at oxidation and reduction again
in terms of our alcohols and in terms of our
carboxylic acids.
01:50
If we look at the lower set, you’ll see
we have, at one end, a terminal, primary alcohol.
01:57
Next along is an aldehyde and finally, is
a carboxylic acid.
02:02
At the top, we have examples of alkanes, alkenes
and alkynes. And in much the same way that
we consider an alkyne to be partially more
oxidised than an alkane or an alkene, we also
consider the carboxylic acid to be more oxidised
than an aldehyde or primary alcohol.
02:24
Something we also have is H+ ions along, so
reduction also becomes the gain of H and the
oxidisation the loss of H. So, let’s have
a quick look at a biological application of
this.
02:37
You recall me referring earlier to what happens
within the body. So, this is a very simple
biological process by which alcohol, which
is highly fat soluble amongst other things
and, of course, can disrupt ion channels creating,
again, a degree of toxicity, can be oxidised
via alcohol dehydrogenase shown here. This
converts the alcohol into an aldehyde, in
this case, acetaldehyde to use its old name
or ethanal to use the accepted IUPAC name.
03:09
This aldehyde, which is one of the things
that’s actually responsible for the hangover
associated with excess consumption of alcohol,
is then converted by aldehyde dehydrogenase
ALDH2 into the correlating carboxylic acid,
which in of itself, can either serve as a
fuel for the Krebs cycle or itself can be
excreted in the urine.
03:34
Another application of alcohol in chemistry
in this particular regard is the use or the
old use of oxidising agents within a breathalyser.
These days it’s a little bit more advanced,
but previously, you used to have a bag containing
potassium dichromate crystals here which are
yellowy-orange in colour. Of course, the reaction
with an alcohol and its conversion into a
carboxylic acid is spontaneous on contact
with these and the result is that the oxidised,
remember chromium and its +6 oxidation state,
can be reduced by the alcohol in the process
of its own oxidation to bring the oxidation
state of chromium down to 3+. This, for reasons
we will not go into during the course of this
course, convert the colour of the chromium
salt from yellowy-orange down to green. Chromium
(III) ions, that is to say chromium in their
3+ oxidation state, are green and chromium
in its 6+ oxidation state is a yellowy-orange
colour.
04:38
And, for the purposes of reference, it allows
you to, therefore, calculate the concentration
of alcohol assuming that you have an approximate
correlation between the amount of alcohol
in your breath and the amount of alcohol you
can infer within your bloodstream.
04:57
Newer breathalysers use spectroscopic analysis
of the breath trapped in the sample and this
is an analytical technique which we will not
discuss here.
05:06
Approximately, 2000mL of breath contains the
same amount of alcohol as approximately 1mL
of blood. However, it should be borne in mind
that we are not all the same and it should
be considered that, for example, somebody
who has a particularly large volume of blood,
more so than others, will perhaps give a result
which could be considered inaccurate.