13C NMR Reveals No Evidence of n-π* Interactions in Proteins

An n-pi* interaction between neighboring carbonyl groups has been postulated to stabilize protein structures. Such an interaction would affect the C-13 chemical shielding of the carbonyl groups, whose paramagnetic component is dominated by n-pi* and pi-pi* 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 C-13 chemical shift assignments verifies this correlation and provides no evidence for an inter-carbonyl n-pi* interaction.