PAPQMD PARAMETRIZATION OF MOLECULAR-SYSTEMS WITH CYCLOPROPYL RINGS - CONFORMATIONAL STUDY OF HOMOPEPTIDES CONSTITUTED BY L-AMINOCYCLOPROPANE-L-CARBOXYLIC ACID

The suitability of ab initio, semiempirical and density functional met hods as sources of stretching and bending parameters has been explored using the PAPQMD (Program for Approximate Parametrization from Quantu m Mechanical Data) strategy. Results show that semiempirical methods p rovide parameters comparable to those compiled on empirical force fiel ds. In this respect the AMI method seems to be a good method to obtain parameters at a minimum computational cost. On the other hand, harmon ic force fields initially developed for proteins and DNA have been ext ended to include compounds containing highly strained three-membered r ings, Like 1-aminocyclopropane-1-carboxylic acid. For this purpose the cyclopropyl ring has been explicitly parametrized at the AMI level co nsidering different chemical environments. Finally, the new set of par ameters has been used to investigate the conformational preferences of homopeptides constituted by 1-aminocyclopropane-1-carboxylic acid. Re sults indicate that such compounds tend to adopt a helical conformatio n stabilized by intramolecular hydrogen bonds between residues i and i + 3. This conformation allows the arrangement of the cyclic side chai ns without steric clashes.