Primary electron transfer in Rhodobacter sphaeroides R-26 reaction centers under dehydration conditions

The photoinduced charge separation in QB-depleted reaction centers (RCs) from Rhodobacter sphaeroides R-26 in solid air-dried and vacuum-dried (~10−2 Torr) films, obtained in the presence of detergent n-dodecyl-β-D-maltoside (DM), is characterized using ultrafast transient absorption spectroscopy. It is shown that drying of RC-DM complexes is accompanied by reversible blue shifts of the ground-state absorption bands of the pigment ensemble, which suggest that no dehydration-induced structural destruction of RCs occurs in both types of films. In air-dried films, electron transfer from the excited primary electron donor P⁎ to the photoactive bacteriopheophytin HA proceeds in 4.7 ps to form the P+HA− state with essentially 100% yield. P+HA− decays in 260 ps both by electron transfer to the primary quinone QA to give the state P+QA− (87% yield) and by charge recombination to the ground state (13% yield). In vacuum-dried films, P⁎ decay is characterized by two kinetic components with time constants of 4.1 and 46 ps in a proportion of ~55%/45%, and P+HA− decays about 2-fold slower (462 ps) than in air-dried films. Deactivation of both P⁎ and P+HA− to the ground state effectively competes with the corresponding forward electron-transfer reactions in vacuum-dried RCs, reducing the yield of P+QA− to 68%. The results are compared with the data obtained for fully hydrated RCs in solution and are discussed in terms of the presence in the RC complexes of different water molecules, the removal/displacement of which affects spectral properties of pigment cofactors and rates and yields of the electron-transfer reactions. [Display omitted] •Dehydration of RCs in air/vacuum leads to blue shifts of the main absorption bands.•Dehydration does not cause structural destruction of RCs in air- and vacuum-dried films.•Dehydration of RCs in air/vacuum affects the primary photochemistry in the RC.•Bound water molecules play a role in the electron-transfer functionality of the RC.