The coupling between electron transfer and protein dynamics in the bacterial photosynthetic reaction center: trapping of conformational substates in room temperature amorphous matrices

The photosynthetic reaction center (RC) from purple bacteria is becoming a prototype in exploring the coupling between internal protein motions and long-range electron transfer (ET). This interplay has been extensively investigated by hampering RC substate interconversion and relaxations at low temperatures (McMahon et al., 1998, Biophys. J. 74, 2567). As a complementary approach for limiting RC dynamics at room temperature we embedded the protein within a dehydrated trehalose matrix. Spectroscopic studies and molecular dynamics simulations performed on myoglobin/trehalose/water systems have shown that the nonharmonic contributions to the protein motions (attributed to thermal fluctuations among conformational substates) are greatly reduced in a dry trehalose matrix (Librizzi et al., 2002, J. Chem. Phys. 116, 1193) when the protein is confined within a network of hydrogen bonds connecting protein groups, residual water and trehalose molecules (Cottone et al., 2002, J. Chem. Phys. 117, 9862). These properties can be put in relation with the unique efficacy of trehalose in the preservation of biostructures. We analysed the effect of progressive dehydration of RC-trehalose-water matrices on two lightinduced reactions: (a) ET from the primary (QA-) to the secondary (QB) quinone acceptor; (b) recombination of QA- with the photoxidized primary donor P+. Reaction (a) is slowed by more than four orders of magnitude in relatively wet glasses (Francia et al., 2003, Biophys. J. 85, 2760), consistently with a conformational gate mechanism. When the amount of residual water in the matrix is further reduced (to approximately 0.5 water per trehalose molecule), the kinetics of reaction (b) at room temperature becomes broadly distributed and accelerated as observed in glycerol-water mixtures only at cryogenic temperatures. These effects have been taken to reflect a drastic inhibition of the RC relaxation from the dark-adapted to the lightadapted conformations as well as of the interconversion between conformational substates (Palazzo et al., 2002, Biophys. J. 82, 558). Comparison of the effects of matrix dehydration on the two ET reactions examined points to a highly selective correlation (slaving) between the specific internal motions governing different ET processes and the structure and dynamics of the external medium at the protein surface.