Primary photoprocesses in oxyblepharismin interacting with its native protein partner

The primary photoprocesses in the photoreceptor for the step-up photophobic response of the light-adapted cells of Blepharisma japonicum (OBIP, OxyBlepharismin bInding Protein) have been studied by ultrafast UV–vis transient spectroscopy. The results are rationalized in terms of heterogeneity of the OBIP sample. Two independent classes of chromoprotein are proposed: a “reactive” species, which presents a specific 680-nm band decaying in 4 and 56 ps and a “non-reactive” one, which behaves like the free chromophore (OxyBP) in solution. A bimolecular photooxidation of OxyBP in the presence of 1,4-benzoquinone was performed to record the absorption spectrum of the OxyBP radical cation. Comparison with reactive OBIP suggests that an electron transfer could be involved in the primary photoprocesses of OBIP and possibly trigger the sensory transduction chain of B. japonicum. In addition, the specificity of the chromophore–protein interaction was investigated by studying the artificial complex that OxyBP forms with human serum albumin (HSA). OxyBP–HSA happens to be spectroscopically much closer to free OxyBP than to OBIP. This highlights the specific nature of the interaction between OxyBP and its native protein partner and further supports the proposal that OBIP is the actual photoreceptor for the photophobic response of B. japonicum.