First Steps in Growth of a Polypeptide toward β-Sheet Structure

The full conformational energy surface is examined for a molecule in which a dipeptide is attached to the same spacer group as another peptide chain, so as to model the seminal steps of beta-sheet formation. This surface is compared with the geometrical preferences of the isolated dipeptide to extract the perturbations induced by interactions with the second peptide strand. These interpeptide interactions remove any tendency of the dipeptide to form a CS ring structure, one of its two normally stable geometries. A C7 structure, the preferred conformation of the isolated dipeptide, remains as the global minimum in the full molecule. However, the stability of this structure is highly dependent upon interpeptide H-bonds with the second chain. The latter forces include not only the usual NH ... O interaction, but also a pair of CH ... O H-bonds. The secondary minimum is also of C7 type and likewise depends in part upon CH ... O H-bonds for its stability. The latter interactions also play a part in the tertiary minimum. A two strand beta-sheet structure is not yet in evidence for this small model system, requiring additional peptide units to be added to each chain.