Non-classical helix-stabilizing interactions: C-H⋯O H-bonding between Phe and Glu side chains in α-helical peptides

The classical picture of H-bonds has evolved considerably. In contrast to earlier expectations, C-H⋯O H-bonds are now known to be prevalent in both small organic and large biological systems. However, there are few reports on the energetic contribution of C-H⋯O H-bonds in protein or polypeptide systems and we do not know whether such interactions are stabilizing. Here we investigate C-H⋯O H-bonding interactions between Phe and Glu side chains by determining their effects on the helicity of model α-helical peptides using a combination of CD and NMR spectroscopy. The results suggest that Glu/Phe C-H⋯O H-bonding interactions stabilize helical structure, but only in the orientation Glu→Phe (N→C). Each Glu→Phe (N→C) interaction can contribute approximately -0.5 kcal mol^-1 to the stability of helical peptide. In the reverse orientation, Phe→Glu (N→C) appears to contribute negligibly. pH titrations provide further evidence for the existence of C-H⋯O H-bonds. The C-H⋯O H-bonding interactions in these peptides are insensitive to the screening effect of added neutral salt. Our results provide quantitative energetic information on C-H⋯O H-bonds that should be useful for empirical force-field calibration.