Hole-burning spectroscopy of proteins in external fields: Human serum albumin complexed with the hypericinate ion

We investigated human serum albumin complexed with the hypericinate ion by using optical hole-burning techniques. The response of the burned-in holes to external pressure variations and electric fields was measured as a function of burn frequency within the inhomogeneous band. The results for the protein-dye complex were compared with the respective behavior of the dye in a host glass. In addition, the spectral properties of the serum albumin hypericinate complex in an electric field were compared with the respective properties of protoporphyrin substituted myoglobin. On the basis of our results we concluded that there must be two stereoisomers with a relative weight of about 50%. Further, the binding site in human serum albumin seems to be quite shallow. The dye is not shielded from the host glass, contrary to the respective behavior of protoporphyrin-substituted myoglobin.