ON THE ELECTRONIC NATURE OF LOW-BARRIER HYDROGEN-BONDS IN ENZYMATIC-REACTIONS

The electronic nature of low-barrier hydrogen bonds (LBHBs) in enzymat ic reactions is discussed based on combined low temperature neutron an d x-ray diffraction experiments and on high level ab initio calculatio ns by using the model substrate benzoylacetone. This molecule has a LB HB, as the intramolecular hydrogen bond is described by a double-well potential with a small barrier for hydrogen transfer. From an ''atoms in molecules'' analysis of the electron density, it is found that the hydrogen atom is stabilized by covalent bonds to both oxygens, Large a tomic partial charges on the hydrogen-bonded atoms are found experimen tally and theoretically. Therefore, the hydrogen bond gains stabilizat ion from both covalency and from the normal electrostatic interactions found for long, weak hydrogen bonds, Based on comparisons with other systems having short-strong hydrogen bonds or LBHBs, it is proposed th at all short-strong and LBHB systems possess similar electronic featur es of the hydrogen-bonded region, namely polar covalent bonds between the hydrogen atom and both heteroatoms in question.