Unveiling the n→π☆ interactions in dipeptides

Numerous studies have suggested that the n ->pi(star) interactions between carbonyls could contribute significantly to the stability of proteins. Nevertheless, their evaluation is challenging because of the solvent environment or crystal packing forces in solids. Here we study the rotational spectrum of HGlyProOH dipeptide, a very common sequence found in collagen, the most abundant protein in vertebrates, in isolated conditions. Three different structures are unequivocally characterized in the gas phase. Interestingly, the most abundant structure is stabilized by an n ->pi(star) interaction and adopts the same conformation as is found in crystalline collagen. This observation serves to support the importance of the n ->pi(star) interactions between carbonyl groups.