Weak H-bonds. Comparisons of CH center dot center dot center dot O to NH center dot center dot center dot O in proteins and PH center dot center dot center dot N to direct P center dot center dot center dot N interactions

Whereas CH center dot center dot O H-bonds are usually weaker than interpeptide NH center dot center dot center dot O H-bonds, this is not necessarily the case within proteins. The nominally weaker CH center dot center dot center dot O are surprisingly strong, comparable to, and in some cases stronger than, the NH center dot center dot center dot O H-bonds in the context of the forces that hold together the adjacent strands in protein beta-sheets. The peptide NH is greatly weakened as proton donor in certain conformations of the protein backbone, particularly extended structures, and forms correspondingly weaker H-bonds. The PH group is a weak proton donor, but will form PH center dot center dot center dot N H-bonds. However, there is a stronger interaction in which P can engage, in which the P atom, not the H, directly approaches the N electron donor to establish a direct P center dot center dot center dot N interaction. This approach is stabilized by the same sort of electron transfer from the N lone pair to the P-H sigma* antibond that characterizes the PH center dot center dot center dot N H-bond.