Conformational studies on β‐bend containing a cis peptide unit

Conformational studies have been carried out on the X‐cis‐Pro tripeptide system (a system of three linked peptide units, in the trans‐cis‐trans configuration) using energy minimization techniques. For X, residues Gly, L‐Ala, D‐Ala and L‐Pro have been used. The energy minima have been classified into different groups based upon the conformational similarity. There are 15, 20, 18 and 6 minima that are possible for the four cases respectively arid these fall into 11 different groups. A study of these minima shows that, (i) some minima contain hydrogen bonds ‐ either 4→1 or 1→2 type, (ii) the low energy minima qualify themselves as bend conformations, (iii) cis′ and trans′ conformations are possible for the prolyl residue as also the C^γ‐endo and C^γ‐exo puckerings, and (iv) for Pro‐cis‐Pro, cis′ at the first prolyl residue is ruled out, due to the high energy. The available crystal structure data on proteins and peptides, containing cis‐Pro segment have been examined with a view to find the minima that occur in solid state. The data from protein show that they fall under two groups. The conformation at X in X‐cis‐Pro is near extended when it is a non‐glycyl residue. In both peptides and proteins there exists a preference for trans′ conformation at prolyl residue over cis′ when X is a non‐glycyl residue. The minima obtained can be useful in modelling studies.