Photophysics of hypericin and hypocrellin A in complex with subcellular components: Interactions with human serum albumin

Time-resolved fluorescence and absorption measurements are performed on hyp ericin complexed with human serum albumin, HSA (1:4, 1:1 and similar to 5:1 hypericin: HSA complexes), Detailed comparisons with hypocrellin A/HSA com plexes (1:4 and 1:1) are made. Our results are consistent with the conclusi ons of previous studies indicating that hypericin binds to HSA by means of a specific hydrogen-bonded interaction between its carbonyl oxygen and the N-1-H of the tryptophan residue in the IIA subdomain of HSA, (They also ind icate that some hypericin binds nonspecifically to the surface of the prote in.) A single-exponential rotational diffusion time of 31 ns is measured fo r hypericin bound to HSA, indicating that it is very rigidly held. Energy t ransfer from the tryptophan residue of HSA to hypericin is very efficient a nd is characterized by a critical distance of 94 Angstrom, from which we es timate a time constant for energy transfer of similar to 3 x 10 (15) s, Alt hough it is tightly bound to HSA, hypericin is still capable of executing e xcited-state intramolecular proton (or hydrogen atom) transfer in the simil ar to 5:1 complex, albeit to a lesser extent than when it is free in soluti on, It appears that the proton transfer process is completely impeded in th e 1:1 complex. The implications of these results for hypericin (and hypocre llin A) are discussed in terms of the mechanism of intramolecular excited-s tate proton transfer, the mode of binding to HSA and the light-induced anti viral and antitumor activity.