Hole burning spectroscopy of ribonuclease A.

We present pressure tuning hole burning experiments with the enzyme ribonuclease A using the UV-absorbing amino acid tyrosine as a probe. We show that, at 2 K, the protein is intact, and that at least four different regions which we associate with different tyrosine sites can be distinguished through their specific response to pressure. For one site we could determine the compressibility to 0.15 GPa(-1). Upon denaturing the protein with guanidine hydrochloride, one of the tyrosine sites is preserved to a large extent. Reducing the sulfur bonds has a more drastic effect: the tyrosine sites lose most of their individual features and their compressibilities come close to that of tyrosine in solution.