A solid state ¹³C NMR, crystallographic, and quantum chemical investigation of phenylalanine and tyrosine residues in dipeptides and proteins

We report the results of a solid-state NMR and quantum chemical investigation of the ¹³C^γ NMR chemical shifts in phenylalanine and tyrosine in dipeptides and proteins. Accurate computation of the experimental shifts is shown to require a good description of local electrostatic field effects, and we find the best results (R² = 0.94, rmsd = 1.6 ppm, range = 17.1 ppm, N = 14) by using a self-consistent reaction field continuum model. There are no obvious correlations with φ, ψ, χ₁, or χ₂ torsion angles, unlike the results seen with other amino acids. There is, however, a linear relation between computed C^γ atomic charges and shifts for the 14 peptide as well as 18 protein residues investigated. This result is similar to the correlation reported in the 1960s between π-electron density and ¹³C shifts for classical 4n + 2 (n = 0, 1, 2) π-electron aromatic species, such as cyclopentadienide and the tropylium cation, and in fact, we found that the shielding/atomic charge correlation seen in the peptides and proteins is virtually identical to that seen with a broad range of aromatic carbocations/carbanions. These results suggest the dominance of an electrostatic field polarization model in which increasing π electron density results in an increase in C^γ atomic charge and increased shielding (of σ₁₁ and σ₂₂, perpendicular to the π orbital) in Phe and Tyr, as well as in the other aromatic species. These results are of general interest since they demonstrate the importance of electrostatic field effects on Phe and Tyr C^γ chemical shifts in peptides and proteins and imply that inclusion of these effects will be necessary in order to interpret the shifts of other aromatic species, such as drug molecules, bound to proteins.