Hydrogen-bond cooperativity, vibrational coupling, and dependence of helix stability on changes in amino acid sequence in small 3 10-helical peptides. A density functional theory study.

Five pentapeptides, GGGGG, GAGGG, GVGGG, GLGGG, and GIGGG, have been completely optimized in the 3(10)-helical and open beta-strand conformations at the B3LYP/D95 level. The energies of the helices relative to the beta-strands vary from -2.1 to -3.6 kcal and depend on the amino acid residue sequence. The energies of substituting A, V, L, or I for G in the second position are also presented. Vibrational analyses were performed on the optimized structures. Vibrational coupling through the individual H-bond chains of the helices is confirmed to be stronger than that through space or through the covalent bonds. The cooperative interactions of the H-bonds are evident from both the structures and the coupling of the amide I, amide II, and N-H vibrations.