Date of this Version
Worley B, Richard G, Harbison GS, Powers R (2012) 13C NMR Reveals No Evidence of n2p* Interactions in Proteins. PLoS ONE 7(8): e42075. doi:10.1371/ journal.pone.0042075
An n ‒ Π* interaction between neighboring carbonyl groups has been postulated to stabilize protein structures. Such an interaction would affect the 13C chemical shielding of the carbonyl groups, whose paramagnetic component is dominated by n ‒ Π* and Π ‒ Π* excitations. Model compound calculations indicate that both the interaction energetics and the chemical shielding of the carbonyl group are instead dominated by a classical dipole-dipole interaction. A set of high resolution protein structures with associated carbonyl 13C chemical shift assignments verifies this correlation and provides no evidence for an inter-carbonyl n ‒ Π* interaction.