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 gamma 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 phi, psi, chi 1, or chi2 torsion angles, unlike the results seen with other amino acids. There is, however, a linear relation between computed C gamma 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 pi-electron density and 13C shifts for classical 4n + 2 (n=0, 1, 2) pi-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 pi electron density results in an increase in C gamma atomic charge and increased shielding (of sigma 11 and sigma 22, perpendicular to the pi 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 gamma 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.