| Abstract
| - Although the existence of Cα−H···OC hydrogen bonds in protein structures recently has beenestablished, little is known about their strength and, therefore, the relative importance of these interactions.We have discovered that similar interactions occur in N,N-dimethylformamide dimers. High level ab initiocalculations (MP2/aug-cc-pTZV) yield electronic association energies (De) and association enthalpies (ΔH298)for four dimer geometries. These data provide a lower limit of De = −2.1 kcal mol-1 for the Cα−H···OChydrogen bond. A linear correlation between C−H···O bond energies and gas-phase proton affinities is reported.The gas-phase anion proton affinity of a peptide Cα−H hydrogen was calculated (355 kcal mol-1) and usedto estimate values of De = −4.0 ± 0.5 kcal mol-1 and ΔH298 = −3.0 ± 0.5 kcal mol-1 for the Cα−H···OChydrogen bond. The magnitude of this interaction, roughly one-half the strength of the N−H···OC hydrogenbond, suggests that Cα−H···OC hydrogen bonding interactions represent a hitherto unrecognized, significantcontribution in the determination of protein conformation.
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