Heterodimeric versus homodimeric structure of the primary electron donor in Rhodobacter sphaeroides reaction centers genetically modified at positionM202

Using light-induced Fourier-transform infrared (FTIR) difference spectrosco py of the photo-oxidation of the primary donor (P) in chromatophores from R hodobacter sphaeroides, we examined a series of site-directed mutants with His M202 changed to Gly, Ser, Cys, Asn or Glu in order to assess the abilit y of these side chains to ligate the Mg atom of one of the two bacteriochlo rophylls (BChl) constituting P. In the P(+)Q(A)(-)/PQ(A) FTIR difference sp ectra of the mutants HG(M202), HS(M202), HC(M202) and HN(M202), the presenc e of a specific electronic transition at similar to 2650-2750 cm(-1) as wel l as of associated vibrational (phase-phonon) bands at similar to 1560, 148 0 and 1290 cm-L demonstrate that these mutants contain a BChI/ BChI homodim er like that in native reaction centers with the charge on pf shared betwee n the two coupled BChl, In contrast, the absence of all of these bands in H E(M202) shows that this mutant contains a BChl/bacteriopheophytin heterodim er with the charge localized on the single BChl, as previously determined f or the mutant HL(M202). Furthermore, the spectra of the heterodimers HE(M20 2) and HL(M202) are very similar in the 4000-1200 cm(-1) IR range. Perturba tions of the 10a-ester and 9-keto carbonyl modes for both the P and P+ stat es are observed in the homodimer mutants reflecting slight variations in th e conformation and/or in position of P. These perturbations are likely to b e due to a repositioning of the dimer in the new protein cavity generated b y the mutation.