The CH--O Hydrogen Bond - Hiding in Plain Sight
The fundamental nature and strength of CH··O hydrogen bonds (HBs) are compared with their classical NH··O and OH··N analogues. In nearly all respects, the two are quite similar, with the single exception of the change in stretching frequency of the X-H covalent bond upon HB formation, a distinction which is probed. The hybridization of the C-atom proton donor affects both the strength of the HB and the direction of ?(CH) shift. The possibility of CaH groups of amino acid residues to engage in such HBs is examined. as are aromatic sidechain CH. The contributions of CH··O HBs to ß-sheet formation and to interpeptide unit interactions are evaluated. Several cases are described wherein a CH··O HB is directly responsible for the structure adopted by particular molecules. Spectroscopic means of identifying these HBs are proposed. Finally, the means by which introduction of charge into the proton donor magnifies the HB strength is described.
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