Towards Adaptive Chemistry
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Lindau Nobel Laureate Meetings278 / 340
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Transcript: English(auto-generated)
00:14
It's nice to be back in Lindau. Now, this is another setting. That's the one I had been in last time.
00:20
And it's quite a nice one. I must say I prefer that perhaps to the more modern type of setup. Anyway, so let me tell you about some perspectives in chemistry. And now we are very interested in developing an area which we may call adaptive chemistry. We'll try to explain what it is about. Now, as you know, we have chemists
00:43
are supposed to be able to make molecules, to design them, by using linking atoms to covalent bonds. Beyond that, there is an area which we have now developed for many years, which is supramolecular chemistry, which deals with the way in which molecules interact. And three main functions were studied over the years.
01:02
How do molecules recognize each other? We heard a lot about that, because in biology, any function starts first with a recognition process. Then how do they react with each other? How can they carry each other through a membrane? And recognition is the basis of this. But we think a bit more about what
01:21
happens beyond supramolecular chemistry. What can we learn from it? It is quite obvious, but it was taken for granted for many years, that supramolecular chemistry is a dynamic chemistry inasmuch as the pieces linked together by these non-covalent, rather labile interactions.
01:42
These pieces can exchange, and therefore, a supramolecular object can undergo variation in constitution by exchanging its components. Now, this then led to the idea that, in fact, this is a dynamic chemistry which relies on non-covalent, labile interactions
02:02
to exchange the pieces. The next step was to see whether or not one can introduce these dynamic features residing in the constitution of the object to molecular structures. And indeed, this can be done if a molecule is constituted by atoms linked through non-stable bonds, same bonds
02:26
which can break and then exchange also pieces. That was something a bit disturbing, because when you make molecules, you want them to be stable. And if they're not stable, you usually cool them down in a fridge or whatever just to keep them from breaking up.
02:42
But you can ask the question, let's turn the question around and see what would happen if you now make molecules which do the reverse, which exchange their pieces. And this then led to the idea that there must be perhaps an interest in studying a dynamic covalent chemistry. You can then combine these two under the same title,
03:02
the same roof, and calling this a dynamic chemistry residing in the constitution of the chemical object. That chemical object can be molecular, or it can be super molecular. So it is a dynamic chemistry based on the constitution of the chemical object.
03:20
And this opens then the possibility that your entities may be able to adapt, to open the possibility of an adaptive system where the system might respond in its constitution by exchanging its components to stimuli of physical nature or to chemical effectors.
03:41
And that opens possibly the road to an adaptive chemistry. Now, this constitution dynamic chemistry, what is it good for? First of all, it makes a mess. It makes a mixture. But you may call now a library. Of course, a much nicer name. In older times, you call that a mess and you throw it away.
04:00
But now we think that it might be very interesting to study the diversity generated. And indeed, this generates diversity on both the super molecular and the molecular level. And that makes possible something which is also a term which perhaps for biologists is more common, but which is interesting. The fact that you can select pieces,
04:20
you can select different constitutions of interest for biologically active substances, discovery of biological substances which are active, and implementing dynamics in nanodevices, and also perhaps making dynamic materials
04:40
like dynamic polymers. I will only discuss two parts of it, the biologically active substances and the dynamic materials. First, you have to choose a reversible chemical reaction. There are many. We have mostly studied an emblematic one, which is a condensation of a carbonyl with an NH2 giving an imine. That's a nicely reversible reaction.
05:02
It's not so simple as it looks on the screen, in fact, but it is a nice one which one can study and although there are good reasons for that, there are both organic molecules bearing it and of course, biomolecules. Other reactions can be studied. I think the group with Jeremy Sanders in Cambridge has studied mostly disulfide formation exchanged by making disulfide bonds
05:23
and exchanging the pieces. So, imine formation. This is just the framework, just the basic reaction, but I would like to illustrate the concepts behind. That's just the workhorse, so to say. So, how can you come to searching
05:40
for biologically active substances by using this kind of principle? If you go back to the image of Emil Fisher of the key and the lock, how can you make a key for biologically lock? This would be the key, would be the biologically active substance for binding to the biological target, the lock. So, the design chemistry would be to make one key,
06:04
the right one for the lock. That may be difficult because you have to know the lock. We heard a lot about each crystallography and also about dynamics. You have to know the lock, you have to build the key, you have to test it. Another approach is to try a sort of random,
06:21
random means of generating a lead substance by making very many keys, just trying them out and hoping that you will find one which may have some interesting activity and then you can improve on it. But this one, of course, is not teaching you much about how molecules recognize each other.
06:41
So, how can you combine the information which is present in the recognition process, one key for the right one, or the possibility to study many, many molecules, sort of a combinatorial approach. This would be nice if you can do a dynamic combinatorial which relies on the generation of fragments,
07:02
I come back to that, and this would then combine combinatorics, that means the way in which you can recombine the pieces so as to generate a very high diversity and informatics, the way in which these things interact, the information stored in the molecules and which leads to these recognition processes.
07:22
Just one slide to illustrate that because I would like to go to other things. If these are the components for different structures, different types of structures, with here a point, this black dot, which is a function group which can undergo reversible connection. When you mix those together, you generate an interconverting set of constituents.
07:44
You have access through this reversible reaction to all the possible combinations of the pieces. Now this gives a complex mixture of entities all the time interconverting. So what can you do then? You use a basic law of thermodynamics,
08:00
which is that you add the receptor and the receptor should best bind the combination which is the most suitable for this receptor site. And that means that the lock and key will be a sort of a dynamic key which the lock will then select and lead to the most stable binder.
08:23
In other words, the lock that's arriving, the lock assembles its own key. In fact, this can be used and for trying drug discovery processes. It has been used and applied for discovering enzyme inhibitors and also for looking at substrates
08:42
for biological artificial or biological receptors. I don't want to go more into that, but the principle has been demonstrated in a number of cases. Now I would like to switch to what you can do in the field of materials.
09:01
Dynamic materials look like something also one of sort of a thing which you might not like to do because you don't want to make really materials which fall apart. But they maybe have to have properties perhaps which are different from static materials because they might be self-healing, responsive, adaptive materials. Let's look at a class of materials,
09:23
dynamic polymers, which we like to call dynamers. These dynamic polymers can be of two types. One type is on the molecular level. If this is a monomer with two functional groups at each end of a given spacer, and these are two other,
09:40
this is another monomer with two functions at the end of a spacer, and these two functional groups are complementary, complementary meaning that they can undergo a reversible reaction. Then you get a chain which is dynamic in its constitution because these connections can break reversibly,
10:01
and that would then be a molecular dynamic polymer. On the other hand, if it is not a polycondensation, but the polyassociation through interactions, then these entities, the yellow and the red one, are complementary interactional groups undergoing molecular recognition.
10:21
And then it's a super molecular, non-covement dynamic polymer. Let's have a quick look at the super molecular polymer chemistry. This is a field which developed quite a bit over the last 30 years or so, and it was a bit difficult to have it to convince polymer chemists at the beginning, but now it's well accepted.
10:41
And let me just illustrate a point. What can one do on the basis of the super molecular polymer chemistry? How does it build up? I mention here the date when we published the principle, the first time we published that, where the principle was introduced in 1990, because I need this date 1990 in a moment, as you will see.
11:00
So the principle is simple. I just briefly mention it. You have a spacer, you have two end groups. These end groups here are hydrogen bonding groups undergoing donor, acceptor donor of hydrogen bond formation. And the complementary to DAD is then ADA, the acceptor donor acceptor,
11:21
so that if you mix this component with this component, the result is the formation of a super molecular polymer main chain, where the polymeric entity is linked, where the units are linked together by forming three hydrogen bonds between the components. Just to illustrate that there is a big difference
11:43
in material properties, these two are solids, and that's a liquid crystal. I don't want to go more into details, but this was just an illustration of the principle of making super molecular main chain polymers, which then was developed a lot, and especially by Bert Meyer in Eindhoven, where a lot of very nice work was done
12:01
to develop the ideas and to make a range of compounds and materials also by a number of other groups. This is an electron microscopy picture showing that indeed the material has fibers, which can connect together. I don't want to go more into the details.
12:21
Why do I mention that? First of all, because it is a dynamic polymer. It is a polymer based on non-covalent interactions, but also to illustrate how far, how long it can take to go from basic concept to an application. In 2003 of all, one can look at the possibility
12:43
to have self-healing materials. Let's have a look here. That's a transparent film which contains entities which are linked by non-covalent interactions. You can cut this film in two, superimpose the two ends, and then you press with your finger just for some time.
13:03
Not very quantitative, but you press with your finger and you can stretch and it sticks again. I have a movie for that, but I didn't want to show it. It takes a bit too much time just to get from one thing to the other one. But you can see this, you can stretch, and it has the mechanical properties almost the same as the starting film.
13:21
The reason that it are not the same is probably because it was not pressed long enough and this was not a very quantitative way of doing things. But this just demonstrates that supermolecular polymer films are self-healing materials. Now, the other point of importance is in addition
13:40
to having this practical material type of application, there's another possibility. And in 2013, in October of that year, I got an email from a small company saying we have developed supermolecular polymer materials which are biocompatible. And what they had done is using supermolecular materials,
14:01
this company Xeltis has used this type of material to make cardiovascular implants. These cardiovascular implants were then used for the reconstruction of the heart of children who had a congenital cardiac malformation.
14:20
It was indeed done, and here you see the first young girl who was implanted by Leo Boqueria. He's a cardiovascular surgeon at the Bakulev Scientific Center for Cardiovascular Research in Moscow. This was in 2013, it's now almost four years later
14:40
and she's fine. Many more children have now been implanted, at least 10, but I haven't followed all of it, so more than that. So that is, of course, for when you do basic research, it's a great satisfaction to see that some of the principles introduced many years earlier
15:01
finally came to a material which is now present and used in surgery for children. This has now been developed also by the same company to make a heart valve, which these are shown here, which was implanted in July 2016,
15:21
and which for the moment is also going well. They tell me that it's really a big change in surgical practice. Now, this is just about super molecular polymers, and as I said, this field has now developed very much and there's a lot of work going on in the area. What about now making dynamic polymers
15:42
which are covalent, where the connections between the pieces are covalently reversible? Then you would simply think of doing something which any chemist, it's sort of very simple kind of thing to do, it's a dicarbonyl, a diamine, you mix them together and you get a polyimine, no big deal, and you can make them for polyimines
16:02
or you can use hydrazides to make polyiside hydrazones. This is just something which is for a chemist, it's quite a normal thing to do. So, now what can one do with this type of polymers? What can one use them for, are they interesting or are they just some kind of idea
16:21
which doesn't need to match? If you make a film of dynamic polymers which can undergo dynamic component recombination, you can superimpose these two films. And let's now look at A and B as two components of the film.
16:41
A might be a diamine, B might be a bicarbonyl. So A, B, AB, they're lumped together forming that dynamic polymeric chain. You can then make another film which has another diamine, A prime,
17:01
and another bicarbonyl compound, B prime, and you make then the chain A prime, B prime, A prime, B prime. Now you superimpose the two films. What would happen, what can happen? This connection is in principle dynamic. And then, of course, it can disconnect
17:22
and recombine with the film below. In other words, A can now combine with B prime and A prime can combine with B. That means that you can generate inside your material the two new combinations which did not exist before.
17:41
And that can be quite interesting because they may have other properties. They may change the property of the material itself in a way which is a dynamic way. Let's assume these are the imains but they can be other functions of that type. And so how can one make use of it? Several ways have been tried and have been implemented.
18:03
One is to change the mechanical properties of the objects. And another one is to change the hydrophilicity or hydrophobicity. But the best illustration is by looking at optical properties because you see it. And these optical properties can be changed
18:23
off for such a material by just a recombination of the pieces inside the film. You can hope for generating changes in color and in fluorescence of these dynamic polymers. How? Again, you have a film AB.
18:41
I go back to the same nomenclature. You have a film A prime, B prime. You superimpose just a part of it, for instance, the corner. At that place where it's superimposed, what I just showed on the previous slide can happen, recombination of the components so as to generate AB prime and A prime B, A combining with B prime and A prime combining with B.
19:03
If one of these two combinations has a color, it will be generated, you will see it. Or if it has a fluorescence, you will see it also. In other words, you may optically see whether the recombination, which I'm talking about,
19:21
has occurred. Now, this was a collaboration with Mitsui Chemicals and they were sending collaborators from Japanese collaborators from their plant in Chiba to my lab. So we were developing the basic results in that and they were interested in potential applications.
19:41
So my Japanese coworkers then, they like these nice little pictures. This is a cat head where the head and the ears is AB. Inside of the ears, the eyes and the mustache is A prime B prime. You superimpose and then you just let the reaction go. You help it by heating.
20:01
You get color and you get fluorescence. Only when it's superimposed. That shows, of course, that the exchange works. That there is indeed a recombination at the interface between the two films. It also indicates that potentially it is possible to use, for instance, a heating laser to write at the interface between two films or if you make a stack of the films,
20:21
you might be able to write in three dimensions. We have not tried that, but I'm sure there might be bright engineers around the world making very good films and making very nice ways of handling them, superimposing them and trying them out for this type of purpose. So sort of making them, therefore,
20:40
materials which are responsive to external stimuli. This can be applied to the basic classes of biopolymers. We have done it in all three cases, but it's just scratching the surface for the moment. One can make dynamic nucleic acids where the components bearing a nuclear base
21:02
which can be connecting together to make a strand. One can apply it to dynamic peptoids. It's not really a peptide because you need to introduce the reversible function where, for instance here, the hydrazine of an amino acid can be combined either with a function contained on the other side of the amino acid
21:21
or with a complementary type of amino acid. And then you get a peptoid where the reversible connections and which then may exchange amino acid-like pieces. That is also quite interesting because if one of these combinations which is formed by exchange has a conformation which is particularly stable,
21:43
that's the one you should generate. Finally, you can also do it with sugars, carbohydrates, and make dynamic glycodynamics, like for instance this one which is an oligoarabinofuranose where you have here a reversible combination
22:00
and making a dynamic glycodynamic chain. These three areas are still, I think, of much interest. Of course, you can think of combining the amino acid subunit to a sugar subunit to make mixtures between glycosidic and peptoid type of entities.
22:21
And you can also think it, of course, with dynamic nucleic acids. So I think there's a lot of work to be done there and maybe also for interesting biological and maybe even drug applications generating combinations which might have interesting bindings to membrane surfaces, just to trapping molecules and so on.
22:40
I can imagine many things, but a lot of work has been involved. And for instance, for dynamic peptoids, Anna Hirsch is now continuing that work. Now, what about this adaptive chemistry then? Adaptive chemistry is to make entities which can undergo variation in their constitution, not shape, constitution, depending on some agent,
23:05
like the medium, the phase change, physical effector, can be heat, can be pressure, can be electric field, chemical effector, and even a change in conformation. This type of changes are included in a network.
23:22
I come back to what I need to a network. I just mentioned it here. And may give rise to adaptive chemical systems. I would like to illustrate that by following way. Suppose you have these two monomers which have a strong, a big hydrophobic core and the side chain which makes them water soluble.
23:41
This is a dicarbonyl, that's a dihydrozid. So if you mix those two, what do you get? You get the polycondensation. And we had independently studied that you get a polymer which is quite an interesting one, which is characteristic in as much as hydrophobic effects in water will try to minimize contact
24:01
between the hydrophobic core and the aqueous solution, the aqueous phase. Therefore, putting the core inside a rigid rod and having the chains which make it soluble in water sticking out. Okay, this in itself is an interesting polymer. But the interesting question you can then ask, if you now ask the system to solve a question,
24:22
to give an answer to a question, the question is, let's mix these two where you have a small hydrophobic core and a large hydrophobic core. And now ask this other component to choose between the two, which will be included
24:40
in the dynamic polymer. If you work in water, you would expect, and that goes, you have perhaps already guessed that, you will still get the same. And this will just stay out because the system, somewhat dynamically, will push towards excluding contacts between water and the hydrophobic core.
25:00
And indeed, that is what happens. You have a selective polymerization of this with that, despite the presence of the other one. But then you can do a counter experiment just to show this, if this is correct. If you go progressively from pure water to organo-water mixtures, you should lose the selectivity. And indeed, when you add acetonitrile,
25:21
you at 80% acetonitrile added, you have random incorporation. So you have lost these hydrophobic driving effects, which make the system select for one component and excludes the other one. So you have a component selection, which is based on exclusion of contacts
25:41
between hydrophobic pieces of the molecule and water. Another case of interest is work done in collaboration with one of my colleagues, Paolo Samui. It was much more had been done there, but let me just summarize one aspect related to what I'm telling you.
26:01
Suppose you have this long chain aldehyde, which deposits very well on a surface. And you mix it with these three diamines. You deposit that on a graphite surface and you look at the STM pictures of what happens. If you look first at the solution, the phenyl-octane solution,
26:21
what you find in that solution by NMR is simply monoemines, not diamines, because it's quite diluted, and only 1% or 5% do bicimines, and a mixture of all three, which is what you expect. Just a statistical mixture, but only a simple single imine.
26:41
Now you deposit that on a surface. What you now have is only one compound and only bicimine. Because that's the one which absorbs best. That means adaptation to the surface pushes the system to generate here to drive a complete reaction,
27:01
driving the reaction to completion, and selecting for the best, most stable bicimine. That means that adaptation occurs to the adsorption to the surface. It's adsorption forces, which leads to the selection of the right components, and it's a self-organization on the surface,
27:20
which leads to it. It's interesting that this is perhaps also interesting at the origin of life scenarios, because if things deposit, if selection can occur simply by deposition and interactions, there could be an interesting contribution there to select D by adsorption forces. Now let me come to this notion of networks.
27:43
This notion of networks is that you would like to try to see if these entities, they really talk to each other. You have a mixture where they exchange components, so each of them talks to the other one. Let's look at the simplest case. If you have, again, I come back to this nomenclature,
28:01
A, A prime, and B, B prime, which can react with each other to generate AB, AB prime, A prime, B, and A prime, B prime. These four exchange their pieces, and therefore they form a network. And this simple network is, in this case, the simplest you can imagine, a square. You can then put the four constituents
28:20
at the corners of the square and look at their relationships. On the edges, the constituents have a component in common. AB has in common A with AB prime, and B with A prime B. So if AB increases for some reason, it does it at the expense of these two,
28:43
which means these are antagonists. On the other hand, on the diagonals, the relationship is agonistic. If AB increases, it liberates A prime and B prime, which can make more A prime B prime. Therefore, you have two types of relationships,
29:02
antagonistic on the sides of the square and agonistic on the diagonals. Now what can I do with this? Of course, we have now more complicated networks with up to nine components, and this could be interesting, more complicated relationships there.
29:21
But let me look just at the simple one, because we can act on one of those dots, one of those constituents, and see how the other ones respond. And this would then be an adaptive chemical network, the chemical network which will respond to agents. So let's have a look at, first of all,
29:41
a very simple illustration, and then a somewhat more complicated one to conclude. Suppose you have a statistical mixture of these four constituents, where AB have what I said, A are the aldehydes, B are the hydrosides, and you have these four constituents. Now, you treat with a metal.
30:01
Among those four, there's one which will bind the metal very well, which is the one which has NNO3 coordination sides. If you add some base, you will ionize the NH and have a more stable complex. The result is, as you add the metal to this mixture of four, you get only two at the end, because you strongly amplify AB by binding of the metal.
30:25
But as the consequence, A prime, B prime will also be amplified. I come back to that, because I find that is one of the most important conclusions. So you have amplification of one constituent by driving the system with the metal.
30:43
So you have these four constituents, and you get this, and then you amplify, and you get this amplification of AB, and also of A prime, B prime. What does that mean? That means that you have an adaptation to the addition of the metal, and that you have what I call an agonist amplification.
31:02
In other words, the fittest for binding the metal amplifies also the unfittest. Unfittest for that property, but maybe good for something else, which might indeed be interesting, because the unfittest may have some other properties. Let me just conclude rapidly,
31:20
in view of time, I will be short, but just to illustrate an important driving force, which is self-organization, organization of system, which leads to a higher organization than a starting material. It is known, well known, that guanines form a guanine G quartet,
31:43
and they form then a gel. Now a gel is more organized than a solution. So what can you do with these forces of gelification of this organization? You can take a set of components, one of them be having a guanazine, a residue,
32:01
and the other ones having the connection possibilities, aldehydes, which can connect with, sorry, with this NH2 here, pyridoxal, or a sulfonated benzaldehyde, and another piece which is derived from alanine. So you mix that, what do you get? So this is from serine.
32:20
What you get here is that you get now a gel in the solution, which is included in this gel, in the pieces which are still a solution. You have three components. In the gel, you have only one. The one, of course, which forms a gel. But the interesting thing is not only that, that is quite obvious, that the one which forms a gel is the one which contains the guanine residues.
32:42
The one which forms the salt are the others. But you can see that the proportions are such that AB is strongly amplified by gel formation. As a consequence, AB is also strongly amplified. Again, therefore, you have an amplification
33:02
of the constituent which leads to the organized state, whereas the agonist, which is the counterpart which is in the solution, is also amplified. So you have a selection of components, either self-organization pressure. To summarize, in the beginning,
33:24
you have statistical distribution. You let gel formation occur. You have amplification of the one which forms a gel and the one which does certainly not form a gel, the least able to form a gel. This is a self-organization driving force.
33:42
And therefore, you can say that self-organization drives component selection in a way to generate the best for making an organized state and, of course, amplifies also its agonist. This adaptation results in adaptation under the pressure of self-organization.
34:03
Again, stabilizing the fittest here, not by a midline, but by self-organization forces, also drives amplification of the unfittest for that, the most soluble one. And this is perhaps also interesting, again, for prebiotic origin of matter,
34:20
because if a system is such that it may select for a more organized state, it will select the pieces it needs to form the more organized state. And this, before any selection can occur on biological nature, this is a purely chemical way of proceeding. So let me just summarize.
34:41
As chemists, we have to start with molecules. On basis of molecular chemistry, we can then develop a supramolecular chemistry, which leads to organized entities, which then are dynamic or can be made dynamic. For instance, when covalent, you have to make them dynamic, then they can become adaptive.
35:01
And that is now the thing which are mostly interesting, making systems which are adaptive, which respond to different type of forces like crystallization, like vaporization, like influence of metal ions, or like, and for the most intriguing one, is under the pressure of self-organization to lead to more complex states of matter.
35:22
I think in the future, having this evolution of chemistry, in fact you should say it's sort of evolutionary chemistry even, having forces which lead to more and more complex states by themselves, selecting the more complex states, the more organized states, by selecting the right pieces. I hope that is one of the perspectives of chemistry.
35:42
Thank you very much.
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