CONFORMATIONAL AND DATABASE STUDY ON THE INTRAMOLECULAR CH...P AROMATIC BOND AND ITS POSSIBLE INFLUENCE ON THE STEREOCHEMISTRY OF POLYPEPTIDE CHAINS

The presence of intramolecular CH: : : ¼ aromatic bonds in organic molecules has been analyzed by means of CSD searches. In particular, molecules containing aromatic and methyl groups have been examined as a function of the length of the chain separating the interacting groups. The type of bonds in the chain and its geometry have also been taken into account. The maximum number of CH...pi interactions have been found for five-membered chains joining the aromatic and methyl moieties. In addition, this interaction seems favored when the chain contains two sp2 hybridized atoms. The interactions involving polypeptides have been carefully examined. Interest has focused on compounds containing both aromatic and methylic residues, i.e., a combination of phenylalanine (phe), tyrosine (tyr), and tryptophan (trp) with alanine (ala), valine (val), leucine (leu), and isoleucine (ile). The maximum number of CH ... pi interactions have been found when five atoms constituted the chain, i.e., in the sequences phe–ala and ala–phe