Whereas CH⋯O H-bonds are usually weaker than interpeptide NH⋯O H-bonds, this is not necessarily the case within proteins. The nominally weaker CH⋯O are surprisingly strong, comparable to, and in some cases stronger than, the NH⋯O H-bonds in the context of the forces that hold together the adjacent strands in protein β-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⋯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⋯N interaction. This approach is stabilized by the same sort of electron transfer from the N lone pair to the P–H σ* antibond that characterizes the PH⋯N H-bond.
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