title: Temperature dependence of electron transfer to the M-side bacteriopheophytin in rhodobacter capsulatus reaction centers.
abstract: Subpicosecond time-resolved absorption measurements at 77 K on two reaction center (RC) mutants of Rhodobacter capsulatus are reported. In the D(LL) mutant the D helix of the M subunit has been substituted with the D helix from the L subunit, and in the D(LL)-FY(L)F(M) mutant, three additional mutations are incorporated that facilitate electron transfer to the M side of the RC. In both cases the helix swap has been shown to yield isolated RCs that are devoid of the native bacteriopheophytin electron carrier HL (Chuang, J. I.; Boxer, S. G.; Holten, D.; Kirmaier, C. Biochemistry 2006, 45, 3845-3851). For D(LL), depending whether the detergent Deriphat 160-C or N-lauryl-N,N-dimethylamine-N-oxide (LDAO) is used to suspend the RCs, the excited state of the primary electron donor (P*) decays to the ground state with an average lifetime at 77 K of 330 or 170 ps, respectively; however, in both cases the time constant obtained from single-exponential fits varies markedly as a function of the probe wavelength. These findings on the D(LL) RC are most easily explained in terms of a heterogeneous population of RCs. Similarly, the complex results for D(LL)-FY(L)F(M) in Deriphat-glycerol glass at 77 K are most simply explained using a model that involves (minimally) two distinct populations of RCs with very different photochemistry. Within this framework, in 50% of the D(LL)-FY(L)F(M) RCs in Deriphat-glycerol glass at 77 K, P* deactivates to the ground state with a time constant of approximately 400 ps, similar to the deactivation of P* in the D(LL) mutant at 77 K. In the other 50% of D(LL)-FY(L)F(M) RCs, P* has a 35 ps lifetime and decays via electron transfer to the M branch, giving P+HM- in high yield (> or =80%). This result indicates that P* --> P(+)H(M)(-) is roughly a factor of 2 faster at 77 K than at 295 K. In alternative homogeneous models the rate of this M-side electron-transfer process is the same or up to 2-fold slower at low temperature. A 2-fold increase in rate with a reduction in temperature is the same behavior found for the overall L-side process P* --> P(+)H(L)(-) in wild-type RCs. Our results suggest that, as for electron transfer on the L side, the M-side electron-transfer reaction P* --> P(+)H(M)(-) is an activationless process.
authors.affiliation: Department of Chemistry, Stanford University, Stanford, California 94305-5080, USA.
authors.forename: Jessica I
authors.initials: JI
authors.lastname: Chuang
authors.forename: Steven G
authors.initials: SG
authors.lastname: Boxer
authors.forename: Dewey
authors.initials: D
authors.lastname: Holten
authors.forename: Christine
authors.initials: C
authors.lastname: Kirmaier
mesh_terms: D004579:Electron Transport; D005990:Glycerol / Q000737:chemistry; D007096:Imidoesters / Q000737:chemistry; D008954:Models, Biological; D008958:Models, Molecular; D009154:Mutation; D044367:Phase Transition; D010674:Pheophytins / Q000737:chemistry* / Q000378:metabolism; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D016354:Rhodobacter capsulatus / Q000235:genetics / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D013055:Spectrophotometry, Infrared; D013696:Temperature; D013816:Thermodynamics
chemical_list: D007096:Imidoesters; D010674:Pheophytins; D045322:Photosynthetic Reaction Center Complex Proteins; C034266:N-lauryl iminodipropionate; C006678:bacteriopheophytin; D005990:Glycerol