Effect of twisting a polypeptide on its geometry and electron distribution

The geometry of N-formyltriglycine amide, which possess four amidic bonds, is completely optimized at the Hartree-Fock level using the 6-31+G* basis, starting with a set of geometrical parameters characteristic of the 3.613 α-helix. Optimization results in the partial unfolding of this geometry to one characteristic of the 310 helix. The effect of allowing for nonplanarity of the amide bonds is separately determined in arriving at the final geometry. A hydrogen-bonded α-helix structure is obtained by optimizing the bond lengths and angles of the starting structure with fixed torsion angles. The existence of the hydrogen bond in this molecule and in C5 and C7 structures of blocked N-formylglycine amide is confirmed by the presence of a corresponding bond path in the electron density ρ, and their properties are characterized in terms of p at the associated bond and ring critical points. The changes in the charge distribution caused by the twisting of the peptide chain are summarized in terms of the atomic properties and properties of the bond critical points. It is shown that charge neutrality of a peptide group, a necessary requirement for its transferability, is preserved in the helical structures. The study indicates that it should be possible to assign characteristic properties to the atoms of a peptide chain by taking into account their dependence on the torsion angles and nonplanarity of the amide bond. © 1994 American Chemical Society.