Experimental evidence for the correlation of bond distances in peptide groups detected in ultrahigh-resolution protein structures.

The structural analysis of a deamidated derivative of ribonuclease A, determined at 0.87 A resolution, reveals a highly significant negative correlation between CN and CO bond distances in peptide groups. This trend, i.e. the CO bond lengthens when the CN bond shortens, is also found in seven out of eight protein structures, determined at ultrahigh resolution (<0.95 A). In five of them the linear correlation is statistically significant at the 95% confidence level. The present findings are consistent with the traditional view of amide resonance and, although already found in small peptide structures, they represent a new and important result. In fact, in a protein structure the fine details of the peptide geometry are only marginally affected by the crystal field and they are mostly produced by intramolecular and solvent interactions. The analysis of very high-resolution protein structures can reveal subtle information about local electronic features of proteins which may be critical to folding, function or ligand binding.