¹³C NMR reveals no evidence of n-π* interactions in proteins

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 n-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.