Protein Sequence- and pH-Dependent Hydration Probed by Terahertz Spectroscopy

Solvation free energy changes induced by protein folding and function are comparable to the corresponding overall free energy changes. Yet the structure, dynamics, and energetics of the protein itself have received more attention because they are easier to probe. Here we use terahertz (far-infrared) spectroscopy to directly probe the effect of mutations and solvent pH on the solvent shell−protein interaction. We study absorption spectra of the 80 residue viral protein, a five helix bundle, in the 2.1−2.8 THz region. We find that the wild type at pH 7 has a much more pronounced effect on long-distance solvation water than mutants replacing a single polar glutamine side chain with aromatic residues (tyrosine, histidine). This is true both in the context of enhanced and decreased helix stability (via alanine and glycine substitutions). Bringing the wild type and mutants closer to the unfolding transition by lowering the pH likewise reduces the long distance solvation effect. Thus terahertz spectroscopy can be used to probe both local and global solvation dynamics around proteins.