Electronic and vibronic coupling of the special pair of bacteriochlorophylls in photosynthetic reaction centers from wild-type and mutant strains of Rhodobacter sphaeroides
Sprache des Titels:
The photosynthetic reaction center (RC) is an integral membrane protein that carries out the initial charge-separation reactions of photosynthesis. Upon light excitation, a pair (P) of bacteriochlorophylls (Bchls) donates an electron to a bacteriopheophytin (H(L)), generating an ion-pair state (P(+)H(L)(-)). Previous ENDOR studies of RCs from the purple bacterium Rhodobacter sphaeroides have shown that the unpaired electron of P(+) is distributed unequally between the two Bchls of P, with about 2/3 of the unpaired spin and positive charge residing on the Bchl bound to subunit L (P(L)) and 1/3 on the Bchl bound to M (P(M)). To investigate the protein's role in establishing the energies of the cations P(L)(+) and P(M)(+) through long-range electrostatic interactions, we mutated Arg L135 and Arg M164 individually to Leu or Glu and measured the effects on the Special TRIPLE and FTIR spectra of P(+). These highly conserved residues occupy homologous positions on either side of P but have no hydrogen bonds or steric interactions with the pigments. Previous work has shown that replacing Arg by Leu at either site lowers the midpoint potential (E(m)) of P/P(+) by about 15 mV; replacing it by Glu lowers the E(m) by about 35 mV. We found that the mutations also alter the spin distribution in P(+). The mutation R(L135)E stabilize ...