Hello, my name is Jennifer Hiscock and I'm a lecturer in the School of Physical
Sciences here at the University of Kent. I am a supramolecular chemist. What does
that mean? That means I study non-covalent interactions between
molecules. What are non-covalent interactions? Well,
the definition's in the word: anything that doesn't involve a covalent bond. So,
what can we consider under this non- covalent interactions title? Ion–ion
interactions, ion–dipole interactions, dipole–dipole interactions and pi–pi
interactions and van der Waals forces. In particular, I have a little bit of an
interest in a specific form of dipole–dipole interaction: the hydrogen
bond. So, non-covalent interactions, you can think of them as molecular velcro.
Molecules can come together, fall apart, come together ,fall apart, come together,
fall apart, with no damage at all to that molecule, just through the formation of
these reversible interactions. But why do we care about how molecules come
together, how they fall apart, how they can aggregate? Well, this
principle underpins soft material development, such as the hair gels that
you buy at your local supermarket, the medicinal surface gels you put on your
arm for pain relief, so development and understanding of these materials is very
important. We can also look at them for new drug development. We all know today
that the development of novel antimicrobials and antibiotics is really
important,as antimicrobial resistance to our drugs that we normally take to fight
infections increases. So, if we can use these associative interactions to look
at the formulation of new drugs, we're on to a winner. We can also use these
molecular self-associations to produce novel capsules to carry drugs to a
specific site within the human body, therefore the drugs don't come in
contact with the entire system, just to a specific site, and we can also
look at these interactions within surfactant systems. What are surfactants, why are they
important? Well, you use soap every day, washing up
your dishes and shampooing your hair, and also it's a big thing in the automotive
industry for removing and breaking down oils. We can also look at these
associative interactions to produce novel catalysts. Why do we want to
produce novel catalysts? Well, we all want to decrease the amount of energy it
takes for chemical reactions to take place. When you do these chemical
reactions on huge scales, it can take a lot of energy to drive them to
completion. Catalysts reduce this energy so using these
molecular interactions to bring specific parts of a molecular system together
to produce effective catalysts is a good way forwards. But how do we study
molecular interactions? Molecules are really small. Well, the first way perhaps
is looking in the solid state. So we can take a solution of your molecules such
as shown here and we can allow the solution to evaporate. When the solutions
evaporate, you can produce crystals. You can use a microscope to look at these
crystals and see if they're single in nature. If they're single in nature, then
we can use a single crystal x-ray diffractometer to mount these crystals,
fire x-rays at them, the x-rays then defract off the crystals and are
collected by the detector. The detector can then produce information that
through some complex mathematics allows us to see how these molecules interact
within the crystal structure. This is one of our compounds. We can see that it
forms a dimer: two molecules come together through the formation of four
hydrogen bonds. The NHs act as a hydrogen bond donor and the oxygens attached to the
sulphur act as the hydrogen bond acceptors. So now we know that this
particular type of molecule can come together and form dimers through the
formation of four hydrogen bonds. But if we're going to look at the production of
novel drug carriers, bits and pieces like that,
I don't know about you, but I'm mostly liquid, so we can't really rely on solid
state studies – we have to look into the solution state. What can we use to do
this? Well, we can use NMR, OK, nuclear magnetic resonance spectroscopy, and what
we can also see in the spectra shown behind me is a proton NMR. This proton
NMR, the different signals, correspond to the different proton environments within
our molecule so each different spike is a different proton on our molecular
structure. Now if we layer some of these proton experiments together, again do
some really funky maths with it, we can produce a DOSY experiment –
diffusion orderd spectroscopy. What's the importance of this? Well, it looks at how
quickly things diffuse through the solution. Big things diffuse slowly, small
things diffuse really fast. From these diffusion rates, we can calculate the
size of the molecular species. If we have two molecules stuck together, their size
is going to be quite small. If we have 10 molecules stuck together, it's going to
be bigger, 20, even bigger, a thousand, massive, and this is going to be
something we can see using this technique. So, from the spectra shown
behind me, basically indicates that our molecules exist either as monomers or
dimers in the solution, which is exactly what we expect from the solid
state experiments. But do we have hydrogen bonding? Are these monomers held
together by the formation of hydrogen bonds and how strong are they? Well, we
can use another really simple proton experiment. We can take a solution of our
compound and we can look at the signals that correspond to the hydrogen bond
donors, those NH groups, and as we dilute our sample we can look at how those
signals change. Plotting those signals and doing some more funky mathematics we
can calculate the strength of that molecular interaction. So now we know
that these molecules come together probably as dimers in the solution state
and how strong those interactions are. But what does this mean about global
properties, okay? How do these molecules affect the solutions that they're in?
Well, we can observe surface tension using a tensiometer, such as the one
shown behind me. This machine basically looks at a droplet that's produced. This
droplet contains some of our compound and from the shape of that droplet we
can calculate the surface tension. So, at a low concentration with not very much
of our compound within the solution, we see a high surface tension. We increase
the concentration of our compound and we start to decrease the surface tension.
But why is this happening? Well, this is because all of our little molecules are
aggregating at the surface of that droplet. This lowers the surface tension,
so we add more and more compound, we lower the surface tension, but it gets to
a point the surface tension doesn't lower any more. We keep adding compound, we
just get a flat line. Why is this? Well, it's because the surface of that droplet
is saturated, there is literally no more room to aggregate any molecules at the
surface, so they have to stay inside, but if they're staying inside, what are they
doing? They can't be aggregating at the surface so they've got to be doing
something. Cue the fact these molecules are intrinsically fluorescent? What does that
mean? Well, this means that we can use microscopy techniques to look at the big
structures – not the little ones, the big ones that exist in solution. Here, we have
an image of the structures formed in DMSO. You can see that they look fairly
solid, they're circular in size and, to be honest, to me don't look too
interesting. Here, on the other hand, is what happens when we have a solution
that's mostly water. This is 95% water and only 5% DMSO. What do we have? We have
a hollow bubble, with a fluorescent edge to it. What does this mean? This means all
of our fluorescent molecules are aggregated at the edge of that bubble so
our molecules are coming together and interacting to form this hollow
structure, and that's pretty cool. We can start to put things inside this
and we can start to use it to do more interesting things. But we want to be
able to control the formation of these structures in solution, so we start to
look for trends. So here's some information from five of our compounds
and we can see that as we increase the dimerization constant, so as the strength
of the interaction between those molecules when they come together
increases, we get a decrease in critical micelle concentration, so this critical
micelle concentration is that turning point that we saw on the graph earlier
where you no longer get a decrease in surface tension but you get that
flat line, so just where those two things meet is our critical micelle
concentration. This is pretty cool because if I want to get a high yield of
my circular hollow species in solution, what do I need to do? Well, I now have
some information that allows me to design a molecule that does that better.
I increase the strength of that dimerization. I get a lower critical
micelle concentration and therefore more of my molecules can be used to form the
spherical species in solution. So if you'd like to learn any more about this
and look at it in a bit more detail, if you just follow the DOI from this
chemical science paper shown behind me, it'll give you all the information you
need. Thank you very much for listening.