Topological analysis of hydrogen bonds and weak interactions in protein helices via transferred experimental charge density parameters.

Helices represent the most abundant secondary structure motif in proteins and are often involved in various functional roles. They are stabilized by a network of hydrogen bonds between main chain carbonyl and amide groups. Several surveys scrutinized these hydrogen bonds, mostly based on the geometry of the interaction. Alternatively, the topological analysis of the electron density provides a powerful technique to investigate hydrogen bonds. For the first time, transferred experimental charge density parameters (ELMAM database) were used to carry out a topological analysis of the electron density in protein helices. New parameters for the description of the hydrogen bond geometry are proposed. Bonding contacts between the amide N and carbonyl O atoms (N···O) of helices, poorly addressed in the literature so far, were characterized from topological, geometrical, and local energetic analyses. Particularly, a geometrical criterion allowing for the discrimination between hydrogen bonds and N···O contacts is proposed.