Tuesday, May 16, 2006

Are chemists designers?

Not according to a provocative article by Martin Jansen and Christian Schön in Angewandte Chemie. They argue that 'design' in the strict sense doesn't come into the process of making molecules, because the freedom of chemists is so severely constrained by the laws of physics and chemistry. Whereas a true designer shapes and combines materials plastically to make forms and structures that would never have otherwise existed, chemists are simply exploring predefined minima in the energy landscape that determines the stable configurations of atoms. Admittedly, they say, this is a big space (the notion of 'chemical space' has recently become a hot topic in drug discovery) – but nonetheless all possible molecules are in principle predetermined, and their structures cannot be varied arbitrarily. This discreteness and topological fixity of chemical space means (they say) that "the possibility for 'design' is available only if the desired function can be realized by a structure with essentially macroscopic dimensions." You can design a teapot, but not a molecule.

Chemists won't like this, because they (rightly) pride themselves in their creativity and often liken their crafting of molecules to a kind of art form. Having spoken in two books and many articles about molecular and materials design, I might be expected to share that response. And in fact I do, though I think that Jansen and Schön's article is extremely and usefully stimulating and makes some very pertinent points. I suppose that the most immediate and, I think, telling objection to their thesis is that the permutations of chemical space are so vast that it really doesn't matter much that they are preordained and discrete. One estimate gives the number of small organic molecules alone as 10^60, which is more than we could hope to explore (at today's rate of discovery/synthesis) in a billion years.

Given this immense choice, chemists must necessarily use their knowledge, intuition and personal preferences to guide them towards molecules worth making – whether that is just for the fun of it or because the products will fulfil a specific function. Designers do the same – they generally look for function and try to achieve it with a certain degree of elegance. The art of making a functional molecule is generally not a matter of looking for a complete molecular structure that does the job; it usually employs a kind of modular reasoning, considering how each different part of the structure must be shaped to play its respective role. We need a binding group here, a spacer group there, a hydrophilic substituent for solubility, and so on. That seems a lot like design to me.

Moreover, while it's true that one can't in general alter the length or angle of a bond arbitrarily, one can certainly establish principles that enable a more or less systematic variation of such quantities. For example, Roald Hoffmann and his colleagues have recently considered how one might compress carbon-carbon bonds in cage structures, and have demonstrated (in theory) an ability to do this over a wide range of lengths (see the article here). The intellectual process here surely resembles that of 'design' rather than merely 'searching' for stable states.

Jansen and Schön imply that true design must include an aesthetic element. That is certainly a dimension open to chemists, who regularly make molecules simply because they consider them beautiful. Now, this is a slippery concept – Joachim Schummer has pointed out that chemists have an archaic notion of beauty, defined along Platonic lines and thus based on issues of symmetry and regularity. (In fact, Platonists did not regard symmetry as aesthetically beautiful – rather, they felt that order and symmetry defined what beauty meant.) I have sometimes been frustrated myself that chemists' view of what 'art' entails so often falls back on this equating of 'artistic' with 'beautiful' and 'symmetric', thus isolating themselves from any real engagement with contemporary ideas about art. Nonetheless, chemists clearly do possess a kind of aesthetic in making molecules – and they make real choices accordingly, which can hardly be stripped of any sense of design just because they are discrete.

Jansen and Schön suggest that it would be unwise to regard this as merely a semantic matter, allowing chemists their own definition of 'design' even if technically it is not the same as what designers do. I'd agree with that in principle – it does matter what words mean, and all too often scientists co-opt and then distort them for their own purposes (and are obviously not alone in that). But I don't see that the meaning of 'design' actually has such rigid boundaries that it will be deformed beyond recognition if we apply it to the business of making molecules. Keep designing, chemists.


Philip Ball said...

We welcome very much the thoughtful comments by Philip Ball on our essay, and appreciate in particular the correct and concise summary of our ideas as given in his introductory part. Also, most of the issues he subsequently raises were already addressed in our essay. Thus, we are wondering why Philip Ball is reluctant in drawing the rather obvious conclusions from applying the energy landscape concept to compositions and structures of chemical compounds. Perhaps this might be due to some misinterpretations of our statements, which we would like to dispel.

(1) We have made the point, and agree, that chemists are right to be proud of their intellectual input and of their great synthetic achievements. (Don’t forget the main focus of one of the authors, M. J., is synthetic solid state chemistry!).

(2) We have addressed the “combinatorial explosion” as an archetypical feature of chemistry. In spite of the “enumerably infinite” number of possible compounds, the landscape of compounds is discrete in nature. Irrespective of the scientists attitude in exploring it, e. g. following intuition, personal preferences, even speculation or employing more rational tools like computational chemistry, the action taken by a chemist always contributes a discovery and not a creation, and is lacking essential attributes of “design”, even in its most general sence. We do not question the high degree of control achieved in some subdisciplines of chemistry by purposefully functionalizing molecules, but we would like to emphasize that also such educts or intermediary molecules correspond to local minima and are thus both predefined and discrete.

(3) It is well documented (by experience) that the bond lengths among a given pair of atoms may vary substantially, depending on its chemical surrounding. However, this length is fixed for a specific position in a specific compound. This also holds true for extremely long or short bonds which cannot be varied either, and thus making such compounds by purpose again is not synthesis by “design”.

(4) We have mentioned that in its main meaning design also includes an aesthetic element. However, we have also pointed out that shaping and producing a certain object by design does not necessarily include such an element. We are quite aware that in selecting a molecule as a target for synthesis chemists are sometimes lead by aesthetic considerations. We agree, too, that in this respect chemists make real choices. But selecting or choosing out of a multitude of items is normally not associated with the term “design”.

Languages are living entities, and the meaning of a word may be subject to change and extension. From the way some chemists are using the term “design” one can easily see directly that their intention is not to change its meaning but to highlight and adorn their (often excellent) achievements. Admittedly, such kind of euphemistic use of words is very common in politics and in some media, however, chemists should be scientist first and foremost, and politicians or journalists at most as an afterthought or by cruel necessity.

We regard using the correct terminology as indispensable for avoiding a wrong understanding. Employing precise terms is easy, and costs nothing.

Keep discovering new marvel of nature, chemists!

M. Jansen
C. Schön

(Posted by Phil)

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