SynopsisHydrogen bonding in the a-helix and P-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 a-helices and the number of P-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.

Ab initio molecular orbital calculations at the MP 2/6-31 + G** and MP 4/6-31 + G** levels were performed to determine the preferential site of hydrogen bond formation in small molecules where more than one such site exists. For HOF, HNO, H2NF, and H2NOH the better hydrogen bond, as measured by bond strength, occurred when the proton acceptance site was the less electronegative atom. Structures and energies for all configuations were determined.

We report here predictions for the geometries of fluorine azide and fluorine nitrate and an analysis of their electronic structure based on ab initio calculations. Although the compounds were synthesized many years ago and some data exist on their reactivity,1-3 both are explosive and there has been much uncertainity

Nitrosyl fluoride, of some interest in atmospheric chemistry, has three atoms which could potentially serve as proton acceptors in the formation of hydrogen bonds. The optimized structure of FNO and H(2)O was determined at the MP4/6-31G** level of calculation. In the resulting structure, the hydrogen bonds at the fluorine with an energy of 5.15 kcal/mol, contrary to the prediction that less electronegative atoms make better proton acceptors/electron donors but consistent with results of proton affinity studies and with the contribution of a resonance structure with no bond between the N and F and a negative charge on the fluorine atom. The resulting structure also shows a significant increase in the length of the NF bond from 1.51 to 1.61 A.

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