Subpicosecond time-resolved absorption and steady-state resonance Raman (RR) studies are reported for Rhodobacter capsulatus reaction centers (RCs) that incorporate an Asp in place of Val M131 near the ring V keto group of the M-side bacteriopheophytin (BPhM). The L-side C2-symmetry analogue of residue M131 is Glu L104, which is known to form a hydrogen bond to the ring V keto group of BPhL in the wild-type RC. The effects of the V(M 131)D mutation were probed in the triple mutant V(M131)D/G(M201)D/L(M212)H, which also incorporates an Asp at M201 near the L-side bacteriochlorophyll (BChlL) as well as the "beta" mutation L(M212)H that results in replacement of the native BPhL with a BChl molecule (denoted β). The primary photochemistry in the triple mutant (denoted V(M131)D-DH) is similar to that reported previously for the G(M201)D/L(M212)H double mutant (denoted DH). Upon excitation, P* decays with a time constant of 15 ps via a combination of electron transfer to the L side (70%), decay to the ground state (15%), and electron transfer to the M side to form P+BPhM- (15%). The bleaching of the QX ground-state absorption band of BPhM observed upon formation of P+BPhM- is red shifted 1-2 nm from ∼527 nm in the DH RC to ∼529 nm in the V(M131)D-DH mutant, and the BPhM anion band is shifted 20 nm from 645 to 665 nm. RR experiments reveal that the ring V keto vibration of BPhM at 1705 cm-1 in the wild-type RC downshifts to 1697 cm-1 in the V(M131)D mutant. Collectively, these results indicate that the Asp introduced at M131 forms a hydrogen bond to the ring V keto group of BPhM in both ground (neutral) and anionic states of this cofactor, and further demonstrate that M-side electron transfer to form P+BPhM- occurs in both mutants. Comparison of the effects engendered by the addition of the hydrogen to BPhM with those found previously upon removal of the Glu L104 hydrogen bond to BPhL give insights into specific and global effects of the protein on the properties of these symmetry-related cofactors.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry