On the conformational preferences of nylons-n: Analysis of the intramolecular interactions in even nylons-n

The relative stability between the fully-extended and quasi-extended conformations characteristic of the alpha and gamma forms of nylons, respectively, has been investigated for both even and odd nylons-n using ab initio quantum mechanical calculations. For nylon-2 the fully-extended is the most favored conformation and its stability clearly increases with the number of residues. On the other hand, for nylons-4 and -6 the quasi-extended conformation is slightly favored with respect to the fully-extended one, although the relative energy between the two conformations decreases with the number of residues. Thus, the fully-extended conformation is predicted as the most stable conformation in compounds with a larger number of residues for nylons-4 and -6. On the other hand, the quasi-extended conformation is the most favored for odd nylons-n and even nylons-n with n > 6. The results have been explained in terms of the intramolecular electrostatic interactions that appear in nylons-2, -4 and -6 between the N-H and C=O groups of a residue.