An electrochemical investigation of the redox properties of bacteriochlorophyll and bacteriopheophytin in aprotic solvents

Abstract

Knowledge of solvent effects on the redox properties of bacteriochlorophyll (BChl) and bacteriopheophytin (BPhco) is important for understanding their possible role(s) as intermediate electron acceptors in the primary photochemistry of photosynthetic bacteria. In the present study, an investigation of the electrochemical behavior of these compounds by cyclic voltammetry (CV) and cyclic differential pulse voltammetry (CDPV) in several aprotic solvents has shown that BChl aggregation and ligation interactions have a significant effect on its redox potentials. In methylene chloride, the one-electron reduction potential of BChl was found to shift positively by 200 mV to a value nearly identical with that of BPheo in the same solvent. The shift is most readily explained by the presence of BChl aggregates in this solvent. The one-electron oxidation potential is relatively unaffected by aggregation. In contrast, the formation of six-coordinate BChl in tetrahydrofuran (two molecules of solvent coordinated to the Mg atom of BChl) affects both the one-electron reduction and one-electron oxidation potential, with the greatest effect on the latter. Solvent effects on the redox properties of BPheo were found to be much smaller, a finding consistent with its inability to undergo aggregation and coordination interactions similar to those of BChl.During the past decade, considerable progress has been made toward understanding the primary photochemical events in photosynthetic bacteria.2 The reaction center (RC) complex has been purified3 and found to contain four bacteriochlorophyll (BChl) and two bacteriopheophytin (BPheo) molecules in addition to one quinone and three polypeptides. (See recent reviews in ref 4-8.) Two of the BChl molecules, the special pair, which absorb a t 870 nm (P870) are now generally accepted as the primary electron donor in photoinduced charge separ a t i~n .~-'~ The quinone molecule has been shown to act as the primary electron a c~e p t o r '~, '~ as defined on a millisecond time scale.I5 However, with the advent of picosecond (ps) absorption spectroscopy techniques, an intermediate electron acceptor with a lifetime of 250 ps has been d e t e~t e d . '~. ' 'The intermediate was identified by Fajer and co-workers18 as one of the BPheo molecules present in the R C by a comparison of the optical properties of the intermediate with those of the BPheo anion radical prepared electrochemically in methylene chloride. Subsequently, BPheo has been identified as the intermediate electron acceptor in a variety of photosynthetic bacterialThe picosecond data have recently been r e v i e~e d ,~~.~~ and possible artifacts of the method have been d i s c u~s e d .~~~~~Although the role of the remaining two BChl molecules and the BPheo molecule in the charge separation process has not been unambiguously demonstrated, there are some experimental observations that suggest BChl (PSOO) may be involved in some manner in the transfer of an electron from P870 to PBheo. These observations include the picosecond absorption stud...

Infrared spectroelectrochemistry of bacteriochlorophylls and bacteriopheophytins: Implications for the binding of the pigments in the reaction center from photosynthetic bacteria

“…Simultaneous coulometric measurements show that at this potential one electron per BChl-a is removed. The spectrum of the electrolysis product is in close agreement with published spectra of the cation radical of BChl-a generated by electrochemical (23) or chemical (13) oxidation. Upon reversal of the potential, about 80-90% of the neutral BChl-a are regenerated, and little degradation product is formed, as is seen by the almost complete absence of a band at 680 nm (data not shown).…”

“…1 Inset. At this potential, the ir-monocation radical is generated according to cyclic voltammograms (20,23). The time course of oxidation as followed by the absorbance at 772 nm (data not shown) indicates that a quantitative conversion is achieved within a S e0 a few minutes.…”

“…The chemical or electrochemical oxidation of the pigments involved in photosynthesis has been widely used to obtain optical (13,14,23,25), EPR (14,25), electron nuclear double resonance (27,28), or RR (29, 30) spectra of the anionic or cationic form of the pigments in addition to spectra of their neutral state. We have extended these investigations to the IR absorption range by using a spectroelectrochemical cell for the in situ generation of the cation and anion radicals.…”

The bacteriochlorophylla cation radical revisited. An ENDOR and TRIPLE resonance study

“…The addition of methanol is necessary to stabilize the BChl a as monomeric species in solution; i.e., the alcohol acts as ligand to the central Mg atom and leads to hexacoordinated BChl a [34]. This prevents self-aggregation of BChl a [34,35]. In pure CH2C12 the BChl _q+" BChl a +" could also be generated -but with a lower yield -and the ENDOR spectra were different, in particular the four largest couplings.…”

Temperature Dependence of Electron Transfer to the M-Side Bacteriopheophytin in Rhodobacter capsulatus Reaction Centers

“…[55][56][57] Additionally, calculations generally place the P + B -state higher in free energy than the P + H -state on both L and M branches by 0.15-0.25 eV, consistent with the difference in redox potentials of bacteriochlorophyll and bacteriopheophytin in vitro. [58][59][60][61] Calculations and experiments have provided estimates or bracketed ranges for the free energies of the charge-separated states in the wild-type RC: P + B L -0.05-0.1 eV below P*; 29,50,51,[62][63][64][65][66][67][68][69][70] P + H L -∼0.25 eV below P* when relaxed; [71][72][73][74][75] P + B M -0.1-0.2 eV above P* and P + H M -below P* by no more than ∼0.15 eV and probably within 0.1 eV. [50][51][52][76][77][78][79] Systematic efforts to manipulate the free energy differences of the L-and M-branch charge-separated states by site-directed mutagenesis of key amino acids, including those near B M and B L , have led to mutant RCs in which electron transfer to the M branch competes effectively with charge separation to the L branch, yielding P + H M -(reviewed in ref 80).…”