Ab initio calculations are used to analyze the CH‚‚‚O interaction between F n H 3-n CH as proton donor and H 2 O, CH 3 OH, and H 2 CO as acceptor. The interaction is quite weak with CH 4 as donor but is enhanced by 1 kcal/mol with each F added to the donor. The CH‚‚‚O interaction behaves very much like a conventional OH‚‚‚O H-bond in most respects, including shifts in electron density that accompany the formation of the bond and the magnitudes of the various components of the interaction energy. The two sorts of H-bonds also gravitate toward a similar equilibrium geometry and are comparably sensitive to deformations from that structure. In a quantitative sense, while both CH‚‚‚O and OH‚‚‚O prefer a linear configuration, the former is somewhat more easily bent and is less sensitive to stretches from its equilibrium H-bond length. Whereas the OH bond has been shown to stretch and undergo a red shift in its vibrational frequency upon formation of a H-bond, the CH bond of the molecules studied here follows the opposite trend, a contraction and a blue shift. Analysis demonstrates that this opposite pattern is not due to any fundamental distinction between the two interactions, since the same sets of forces are acting on both. It is concluded that the CH‚‚‚O interaction can, indeed, be categorized as a true H-bond.
Although the peptide C␣ H group has historically not been thought to form hydrogen bonds within proteins, ab initio quantum calculations show it to be a potent proton donor. Its binding energy to a water molecule lies in the range between 1.9 and 2.5 kcal/mol for nonpolar and polar amino acids; the hydrogen bond (H-bond) involving the charged lysine residue is even stronger than a conventional OH⅐⅐O interaction. The preferred H-bond lengths are quite uniform, about 3.32 Å. Formation of each interaction results in a downfield shift of the bridging hydrogen's chemical shift and a blue shift in the C ␣ H stretching frequency, potential diagnostics of the presence of such an H-bond within a protein.
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