PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

Quantitative molecular volume changes of photoreactions in photosynthesis and visual system revealed by pulsed photoacoustic spectroscopy. Harvey Jian-Min Hou^*,1, Thomas Sakmar², David Mauzerall², ¹ Department of Chemistry, Gonzaga University, Spokane, WA, USA² The Rockefeller University, New York, NY, USA

ABSTRACT- Pulsed photoacoustics (PA) is used to determine quantitatively the molecular volume changes (due to electrostriction and/or conformational change) associated with photoreactions in PSI and rhodopsin (Rho). In PSI, several lines of evidences showed that phylloquinone is replaced by a plastoquinone (AP) in the menA- or menB- mutants. They also demonstrated that the electron transfer rate from AP to FX slows about 1000-fold. Our PA data on the nanosecond and microsecond time scales reveal that the volume change of charge separation in both mutants is -17+/-2 A3. In contrast, the volume change in WT PSI is -26+/-1 A3. We suggest that the thermodynamics of early electron transfer steps in PSI is affected in these mutants. To test the hypothesis that Schiff base counterion switching occurs in the early photointermediates during rhodopsin activation, we studied recombinant expressed rhodopsin and mutant E181Q by PA. Our PA data show that the volume changes in E181Q mutant and Rho are +72+/-7 and +37+/-4 A3, respectively, on the microsecond time scale. The apparent significant difference in conformational change between E181Q and Rho supports a functional role for E181 in early steps of Rho photoactivation.

KEY WORDS: PS1, rhodopsin, photoacoustics, volume change