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April 18, 2013
A Short Peptide And A Small Molecule
Just as a quick example of how odd molecular recognition can be, have a look at this paper from Chemical Communications. It's not particularly remarkable, but it's a good example of what's possible. The authors used a commercial phage display library (this one, I think) to run about a billion different 12-mer peptides past the simple aromatic hydrocarbon naphthalene (immobilized on a surface via 2-napthylamine). The usual phage-library techniques (several rounds of infection into E. coli followed by more selectivity testing against bound naphthalene and against control surfaces with no ligand) gave a specific 12-mer peptide. It's HFTFPQQQPPRP, for those who'd like to make some. Note: I typo-ed that sequence the first time around, giving it only one phenylalanine, unhelpfully.
Now, an oligopeptide isn't the first thing you'd imagine being a selective binder to a simple aromatic hydrocarbon, but this one not only binds naphthalene, but it has good selectivity versus benzene (34-fold), while anthracene and pyrene weren't bound at all. From the sequence above, those of you who are peptide geeks will have already figured out roughly how it does it: the phenylalanines are pi-stacking, while the proline(s) make a beta-turn structure. Guessing that up front would still not have helped you sort through the possibilities, it's safe to say, since that still leaves you with quite a few.
But the starting phage library itself doesn't cover all that much diversity. Consider 20 amino acids at twelve positions: 4.096 times ten to the fifteenth. The commercial library covers less than one millionth of the possible oligopeptide space, and we're completely ignoring disulfide bridges. To apply the well-known description from the Hitchhiker's Guide to the Galaxy, chemical space is big. "Really big. You just won't believe how vastly, hugely, mindbogglingly big it is. . ."
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