MODELING VIBRATIONAL-SPECTRA OF AMINO-ACID SIDE-CHAINS IN PROTEINS - THE CARBONYL STRETCH FREQUENCY OF BURIED CARBOXYLIC RESIDUES

To improve the use of vibrational spectra for modeling Asp and Glu environments buried in proteins, nu(C=0) frequencies of aliphatic carboxylic acids and N-acetylaspartic acid alpha-amide were compared in several different solvents. These data indicate that propionic acid and longer-chain aliphatic carboxylic acids are all quite similar, and serve as better models for Asp and Glu residues buried in proteins than does acetic acid. For propionic acid, nu(C=0) frequencies above 1745 cm(-1) are observed only in non-H-bonding solvents. Furthermore, in such non-H-bonding solvents, the nu(C=0) frequency exhibits a linear correlation with Onsager's parameter, 2(epsilon - 1)/(2 epsilon + 1), which is expected to be proportional to the strength of the solute-induced electrostatic (''reaction'') field of the solvent. We also measured a nu(C=0) frequency of 1742 cm(-1) for the protonated Asp-26 residue of thioredoxin which is known to be surrounded principally by nonpolar groups. These results are used to model the environment of the Asp-85 residue of bacteriorhodopsin, for which the nu(C=0) frequency has been measured previously in several photointermediate states. In the unphotolyzed (bR) state, the Asp-85 residue (nu(C=0) similar or equal to 1730 cm(-1)) is in a relatively polar hydrogen-bonding environment, but this environment is drastically changed upon photoconversion to the M state (nu(C=0) = 1762 cm(-1)). We conclude that in the latter state, the Asp-85 COOH group is in a highly nonpolar environment, characterized by the absence not only of hydrogen bonding but also of other forms of external dielectric stabilization.