Abstract
Hydrogen bonding in the α‐helix and β‐sheet has been studied by ab initio molecular orbital calculations carried out on complexes of formamide. Hydrogen‐bond geometries were taken from x‐ray crystallography of polypeptides. Positive cooperativity is found in all cases. The limiting value for infinite chains is obtained by use of a double‐reciprocal plot and indicates an increase in the effective bond strength of 25% over that of a single isolated bond. Parallel calculations based on a classical electrostatic model yield qualitatively similar trends but underestimate the cooperativity by half. Charge redistribution accompanying cooperativity is characterized by a new type of charge‐density difference plot, the cooperativity map. The magnitude and distance over which cooperativity acts suggest several significant biological consequences. Thus the average of α‐helices and the number of β‐sheet strands found in protein may be influenced by cooperativity. Cooperativity in the interpeptide hydrogen bond may also be partly responsible for the rapid formation of secondary structure in renaturing proteins and help stabilize secondary structure relative to the random‐coil conformation.