The carbonyl group is ubiquitous in both chemistry and biology. Apart from being involved in numerous chemical transformations, it can also participate in various distinct weak noncovalent interactions including hydrogen bonding (C=O···H), carbonyl-chalcogen interactions (C=O···X; X = S, Se, Te) and nucleophile-carbonyl interactions. In recent years, carbonyl-carbonyl (CO···CO) interaction has emerged as an important noncovalent interaction that was observed in various small molecules, polyesters, peptoids, protein secondary structures and collagen-like peptides. It is believed that this interaction is n→π* in nature where the lone pair of oxygen of the donor carbonyl group is delocalized over the antibonding π orbital of C=O bond of another nearby acceptor carbonyl group. In this talk, I will discuss our current understanding of CO···CO interactions with a focus on the “reciprocal” variant of n→π* interaction that we recently discovered. I will also discuss about the various structural motifs of CO···CO interactions and put forward a hypothesis based on CO···CO interactions that could explain the stabilization of polyproline II and collagen triple helices.
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