Photoinduced Structural Volume Changes in Aqueous Solutions of Blepharismin

Abstract

Abstract— Employing time‐resolved photoacoustics we measured the structural volume changes (ΔVri) occurring after photo‐excitation of blepharismin (BPR) aqueous solutions; an expansion occurring in the subnanosecond timescale is followed by a back contraction within some hundreds of nanoseconds. The magnitude of the ΔVri strongly depends on pH, allowing the determination of the pKa of BPR with this method. The values so measured are very close to those found by means of UV‐visible absorption spectroscopy. The presence of water‐soluble electron acceptors or donors (hexacyanoferrate [III] hexacyanofer‐rate [II]) as well as the concentration of oxygen do not affect the magnitude or the kinetics of the structural volume changes. On the contrary, we detect a strong deuterium effect; this suggests that the observed ΔVri are related to an altered hydrogen bond pattern of the excited state of the pigment with respect to the ground state. Comparative measurements with the parent compound hypericin are also reported, suggesting that the photo‐induced expansion‐contraction pattern is a general characteristic of polyhydroxylated quinones.

Primary Photoprocesses Involved in the Sensory Protein for the Photophobic Response ofBlepharisma japonicum

“…Application of the Förster cycle yields Δ pK a = p K a * − p K a = −2.3 for OxyBP and −2.2 dOxyBP, the 0−0 transition location being estimated by the average wavenumber position of the absorption and fluorescence maxima. Such values are similar to the Δp K a value of −2.7 reported for the perihydroxyl groups of Hyp by Eloy et al It may be argued that since the p K a of the OxyBP perihydroxyl groups is situated around 11 in solution (reported value for Hyp , and blepharismin), one would expect the corresponding excited-state p K a in the range of 8−9. In order to explain that some excited-state deprotonation of OBIP occurs at pH 7.4, one would therefore need to hypothesize that the effective perihydroxyl p K a of OxyBP is somewhat reduced to a lower value upon binding to the protein, as is for example the case for the phenol group of the chromophore covalently bound to the Photoactive Yellow Protein .…”