Calculation of the CONH dipole contribution to lattice energies of amides, polyamides, and polypeptides

Abstract

The dipole energy of a lattice of point dipoles in the configurations of the polyamides 6.6 and 6, of tetradecanamide, and of α and β forms of poly-L-alanine are calculated. The dipole forces contribute 4·8 kcal/mole to the lattice energy of nylon 6.6 of which 93 % arises from the collinear arrays of CONH dipoles along the ‘a ’ axis. For nylon 6 the total energy is 4·6 kcal/ mole. In tetradecanamide the total energy is 3·9 kcal/mole of which only 0·7 kcal/mole arise from the dimer pairs of CONH₂ groups. The calculated energy for β poly-L-alanine is 5·7 kcal/ mole but only 1·8 kcal/mole for the α helix. The dipole forces only stablize the α helix for molecules containing more than 14 CONH groups. Below this length the dipole energy is repulsive. CO...HN hydrogen bonding contributes to the stability of the helix. Changes in configuration of protein molecules from α to β forms are probably facilitated by a balance between dipole forces and hydrogen bonding which vary reciprocally with changing configurations of the CONH group contacts.