Sequence-Specific Recognition and Cooperative Dimerization of N-Terminal Aromatic Peptides in Aqueous Solution by a Synthetic Host

This article describes the selective recognition and noncovalent dimerization of N-terminal aromatic
peptides in aqueous solution by the synthetic host compound, cucurbit[8]uril (Q8). Q8 is known to bind two
aromatic guests simultaneously and, in the presence of methyl viologen, to recognize N-terminal tryptophan
over internal and C-terminal sequence isomers. Here, the binding of Q8 to aromatic peptides in the absence
of methyl viologen was studied by isothermal titration calorimetry (ITC), 1H NMR spectroscopy, and X-ray
crystallography. The peptides studied were of sequence X-Gly-Gly, Gly-X-Gly, and Gly-Gly-X (X = Trp,
Phe, Tyr, and His). Q8 selectively binds and dimerizes Trp-Gly-Gly (1) and Phe-Gly-Gly (4) with high affinity
(ternary K = 109−1011 M-2); binding constants for the other 10 peptides were too small to be measured by
ITC. Both peptides bound in a stepwise manner, and peptide 4 bound with positive cooperativity. Crystal
structures of Q8·1 and Q8·42 reveal the basis for selective recognition as simultaneous inclusion of the
hydrophobic aromatic side chain into the cavity of Q8 and chelation of the proximal N-terminal ammonium
group by carbonyl groups of Q8. The peptide sequence selectivity and positively cooperative dimerization
reported here are, to the best of our knowledge, unprecedented for synthetic hosts in aqueous solution.
Specific peptide recognition and dimerization by synthetic hosts such as Q8 should be important in the
study of dimer-mediated biochemical processes and for the separation of peptides and proteins.