(13)C NMR Reveals No Evidence of n−π* Interactions in Proteins

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