Tryptophan Chemical Shift in Peptides and Proteins: A Solid State Carbon-13 Nuclear Magnetic Resonance Spectroscopic and Quantum Chemical Investigation

We have obtained the carbon-13 nuclear magnetic resonance spectra of a series of tryptophan-containing peptides and model systems, together with their X-ray crystallographic structures, and used quantum chemical methods to predict the ¹³C NMR shifts (or shieldings) of all nonprotonated aromatic carbons (C^γ, Cδ², and C^ε2). Overall, there is generally good accord between theory and experiment. The chemical shifts of Trp C^γ in several proteins, hen egg white lysozyme, horse myoglobin, horse heart cytochrome c, and four carbonmonoxyhemoglobins, are also well predicted. The overall Trp C ^γ shift range seen in the peptides and proteins is 11.4 ppm, and individual shifts (or shieldings) are predicted with an rms error of ∼1.4 ppm (R value = 0.86). Unlike C^α and N^H chemical shifts, which are primarily a function of the backbone φ,ψ torsion angles, the Trp C^γ shifts are shown to be correlated with the side-chain torsion angles χ₁ and χ₂ and appear to arise, at least in part, from γ-gauche interactions with the backbone C′ and N^H atoms. This work helps solve the problem of the chemical shift nonequivalences of nonprotonated aromatic carbons in proteins first identified over 30 years ago and opens up the possibility of using aromatic carbon chemical shift information in structure determination.