Qy-excitation resonance Raman scattering from the special pair in Rhodobacter sphaeroides reaction centers. Implications for primary charge separation

Excited State Frequencies of Chlorophyll f and Chlorophyll a and Evaluation of Displacement through Franck-Condon Progression Calculations

We present ground and excited state frequency calculations of the recently discovered extremely red-shifted chlorophyll f. We discuss the experimentally available vibrational mode assignments of chlorophyll f and chlorophyll a which are characterised by particularly large downshifts of 13¹-keto mode in the excited state. The accuracy of excited state frequencies and their displacements are evaluated by the construction of Franck–Condon (FC) and Herzberg–Teller (HT) progressions at the CAM-B3LYP/6-31G(d) level. Results show that while CAM-B3LYP results are improved relative to B3LYP calculations, the displacements and downshifts of high-frequency modes are underestimated still, and that the progressions calculated for low temperature are dominated by low-frequency modes rather than fingerprint modes that are Resonant Raman active.

Intermolecular Vibrational Coherence in Bacteriochlorophyllawith Clustered Polar Solvent Molecules

We show that resonant impulsive excitation of the Qy absorption band of bacteriochlorophyll a (BChl) launches a rapidly damped (gamma < 200 fs) ground-state coherent wave-packet motion that arises from intermolecular modes with clustered solvent molecules. Femtosecond pump-probe, dynamic-absorption signals were obtained at room temperature with BChl solutions in pyridine, acetone, and 1-propanol. The vibrational coherence observed in the 0-800-fs regime is modeled in the time domain by two (or three, in the case of 1-propanol) modulation components with asymmetric, inhomogeneously broadened line shapes and frequencies in the 100-200-cm(-1) range. The mean frequency of the vibrational coherence exhibits at least a quadratic dependence on the dipole moment of the solvent molecules and a y-intercept in the 100-cm(-1) regime. This trend is modeled by an expression for the natural frequency of a "6-12" potential composed of attractive terms from van der Waals forces and a repulsive term from the exchange (Pauli exclusion) force. The model suggests that comparable contributions to the potential are provided by the dipole-dipole and London dispersion interactions. These results support the hypothesis that the low-frequency vibrational modes in the 100-cm(-1) regime that are coupled to the light-driven charge-separation reactions in the reaction center from purple bacteria are derived from intermolecular vibrational modes between the chromophores and the surrounding protein medium.

Low-frequency resonance Raman studies of the H(M202)G cavity mutant of bacterial photosynthetic reaction centers

Low-frequency (90-435 cm(-1)) NIR-excitation (875-900 nm) resonance Raman (RR) studies are reported for the H(M202)G cavity mutant of bacterial photosynthetic reaction centers (RCs) from Rb. sphaeroides that was first described by Goldsmith et al. [(1996) Biochemistry 35: 2421-2428]. In this mutant, the His residue that axially ligates the Mg ion of the M-side bacteriochlorophyll (BChl) of the special pair primary donor (P) is replaced by a non-ligating Gly residue. Regardless, the Mg ion of P(M) in the H(M202)G RCs remains pentacoordinates and is presumably ligated by a water molecule, although this axial ligand has not been definitively identified. The low-frequency RR studies of the H(M202)G RCs are accompanied by studies of RCs exchanged with D(2)O and incubated with imidazole (Im). The RR studies of the cavity mutant RCs reveal the following: (1) The structure of P(M) in the H(M202)G RCs is different from that of the wild-type, consistent with an altered BChl core. (2) A water ligand for P(M) in the H(M202)G RCs is generally consistent with the low-frequency RR spectra. The Mg-OH(2) stretching vibration is tentatively assigned to a band at 318 cm(-1), a frequency higher than that of the Mg-His stretch of the native pigment ( approximately approximately 235 cm(-1)). (3) The BChl core structure of P(M) in the cavity mutant is rendered similar (but not identical) to that of the wild-type when the adventitious water axial ligand is replaced by Im. (4) Exchange with D(2)O results in more global structural changes, likely involving the protein, which in turn affect the structure of the BChls in P. (5) Assignment of the low-frequency vibrational spectrum of P is generally more complex than originally suggested.

Vibrational coherence from the dipyridine complex of bacteriochlorophyll a: intramolecular modes in the 10-220-cm(-1) regime, intermolecular solvent modes, and relevance to photosynthesis

We present the first observations of vibrational coherence in the 10-220-cm-1 region from bacteriochlorophyll a (BChl) in solution. A distinction can be made for the first time between BChl's intramolecular normal modes and intermolecular modes between BChl and solvent. The results show that the low-frequency vibrations that accompany the initial electron-transfer reaction from the paired BChl primary electron donor, P, in photosynthetic reaction centers arise predominantly from intramolecular modes of histidine-ligated BChl macrocycles. The results also suggest that polar-solvent interactions can significantly perturb the electronic properties of BChl in a manner that might have important functional consequences.

Coupling of nuclear wavepacket motion and charge separation in bacterial reaction centers

The mechanism of the charge separation and stabilization of separated charges was studied using the femtosecond absorption spectroscopy. It was found that nuclear wavepacket motions on potential energy surface of the excited state of the primary electron donor P* leads to a coherent formation of the charge separated states P(+)B(A)(-), P(+)H(A)(-) and P(+)H(B)(-) (where B(A), H(B) and H(A) are the primary and secondary electron acceptors, respectively) in native, pheophytin-modified and mutant reaction centers (RCs) of Rhodobacter sphaeroides R-26 and in Chloroflexus aurantiacus RCs. The processes were studied by measurements of coherent oscillations in kinetics at 890 and 935 nm (the stimulated emission bands of P*), at 800 nm (the absorption band of B(A)) and at 1020 nm (the absorption band of B(A)(-)) as well as at 760 nm (the absorption band of H(A)) and at 750 nm (the absorption band of H(B)). It was found that wavepacket motion on the 130-150 cm(-1) potential surface of P* is accompanied by approaches to the intercrossing region between P* and P(+)B(A)(-) surfaces at 120 and 380 fs delays emitting light at 935 nm (P*) and absorbing light at 1020 nm (P(+)B(A)(-)). In the presence of Tyr M210 (Rb. sphaeroides) or M195 (C. aurantiacus) the stabilization of P(+)B(A)(-) is observed within a few picosseconds in contrast to YM210W. At even earlier delay (approximately 40 fs) the emission at 895 nm and bleaching at 748 nm are observed in C. aurantiacus RCs showing the wavepacket approach to the intercrossing between the P* and P(+)H(B)(-) surfaces at that time. The 32 cm(-1) rotation mode of HOH was found to modulate the electron transfer rate probably due to including of this molecule in polar chain connecting P(B) and B(A) and participating in the charge separation. The mechanism of the charge separation and stabilization of separated charges is discussed in terms of the role of nuclear motions, of polar groups connecting P and acceptors and of proton of OH group of TyrM210.

Selective enhancement of resonance Raman spectra of separate bacteriopheophytins in Rb. sphaeroides reaction centers

Tunable dye laser excitation of carefully prepared samples of Rb. sphaeroides reaction centers provides richly detailed resonance Raman (RR) spectra of the bacteriopheophytins, H, and the accessory bacteriochlorophylls, B. These spectra demonstrate selective enhancement of the separate bacteriopheophytins on the active (H(L)) and inactive (H(M)) sides of the reaction centers. The spectra are assigned with the aid of normal coordinate analyses using force fields previously developed for porphyrins and reduced porphyrins. Comparison of the H(L) and H(M) vibrational mode frequencies reveals evidence for greater polarization of the acetyl substituent in H(L) than H(M). This polarization is expected to make H(L) easier to reduce, thereby contributing to the directionality of electron transfer from the special pair, P. In addition, the acetyl polarization of H(L) is increased at low temperature (100 K), helping to account for the increase in electron transfer rate. The polarizing field is suggested to arise from the Mg(2+) of the neighboring accessory bacteriochlorophyll, which is 4.9 A from the acetyl O atom. The 100 K spectra show sharpening and intensification of a number of RR bands, suggesting a narrowing of the conformational distribution of chromophores, which is consistent with the reported narrowing of the distribution in electron transfer rates. Excitation at 800 nm produces high-quality RR spectra of the accessory bacteriochlorophylls, and the spectral pattern is unaltered on tuning the excitation to 810 nm in resonance with the upper exciton transition of P. Either the resonance enhancement of P is weak, or the bacteriochlorophyll RR spectra are indistinguishable for P and B.

Nuclear wavepacket motion producing a reversible charge separation in bacterial reaction centers

The excitation of bacterial reaction centers (RCs) at 870 nm by 30 fs pulses induces the nuclear wavepacket motions on the potential energy surface of the primary electron donor excited state P*, which lead to the fs oscillations in stimulated emission from P* [M.H. Vos, M.R. Jones, C.N. Hunter, J. Breton, J.-C. Lambry and J.-L. Martin (1994) Biochemistry 33, 6750-6757] and in Qy absorption band of the primary electron acceptor, bacteriochlorophyll monomer B(A) [A.M. Streltsov, S.I.E. Vulto, A.Y. Shkuropatov, A.J. Hoff, T.J. Aartsma and V.A. Shuvalov (1998) J. Phys. Chem. B 102, 7293-7298] with a set of fundamental frequencies in the range of 10-300 cm(-1). We have found that in pheophytin-modified RCs, the fs oscillations with frequency around 130 cm(-1) observed in the P*-stimulated emission as well as in the B(A) absorption band at 800 nm are accompanied by remarkable and reversible formation of the 1020 nm absorption band which is characteristic of the radical anion band of bacteriochlorophyll monomer B(A)-. These results are discussed in terms of a reversible electron transfer between P* and B(A) induced by a motion of the wavepacket near the intersection of potential energy surfaces of P* and P+B(A)-, when a maximal value of the Franck-Condon factor is created.

Qy-excitation resonance Raman spectra of chlorophyll a and bacteriochlorophyll c/d aggregates. Effects of peripheral substituents on the low-frequency vibrational characteristics

Low-frequency (80-700 cm-1) Qy-excitation resonance Raman (RR) spectra are reported for thin-solid-film aggregates of several chlorophyll (Chl) a and bacteriochlorophyll (BChl) c/d pigments. The pigments include Chl a, pyrochlorophyll a (PChl a), methylpyrochloropyllide a (MPChl a), methylbacteriochloropyllide d (MBChl d), [E,M] BChl cS, [E,E] BChl cF, and [P,E] BChl cF. The BChl c/d's are the principal constituents of the chlorosomal light-harvesting apparatus of green photosynthetic bacteria. Together, the various Chl a's and BChl c/d's represent a series in which the peripheral substituent groups on the chlorin macrocycle are varied in systematic fashion. All of the Chl a and BChl c/d aggregates exhibit rich low-frequency vibrational patterns. In the case of the BChl c/d's, certain modes in the very low-frequency region (100-200 cm-1) experience exceptionally strong Raman intensity enhancements. The frequencies of these modes are qualitatively similar to those of oscillations observed in femtosecond optical experiments on chlorosomes. The RR data indicate that the low-frequency vibrations are best characterized as intramolecular out-of-plane deformations of the chlorin macrocycle rather than intermolecular modes. The coupling of the out-of-plane modes in turn implies that the Qy electronic transition(s) of the aggregate have out-of-plane character. The RR spectra of the BChl c/d's also reveal that the nature of the alkyl substituents at the 8 and 12 positions of the macrocycle plays an important role in determining the detailed features of the low-frequency vibrational patterns. The frequencies of the modes are particularly sensitive to larger substituent groups whose conformations may be more easily perturbed in the tightly packed aggregates. These factors also make aggregates of pigments containing larger substituents more susceptible to structural, electronic, and vibrational inhomgeneities. Collectively, the RR studies of the various pigments delineate the factors which influence the low-frequency vibrational characteristics of chlorosomal aggregates.

Near-infrared resonance Raman spectra of Chloroflexus aurantiacus photosynthetic reaction centers

Resonance Raman spectra of the photosynthetic reaction center isolated from the green bacterium Chloroflexus aurantiacus have been obtained with excitation in the near-infrared absorption bands of the special pair (P) and the accessory bacteriochlorophyll (B) using shifted-excitation Raman difference spectroscopy (SERDS). These spectra are compared with the previously reported Raman spectra of P and B in reaction centers from the purple bacterium Rhodobacter sphaeroides. The spectra of P and B from the two species are nearly identical. Common and distinctive attributes of these spectra include enhanced low-frequency (30-200 cm-1) modes in P and the absence of strong Raman activity in modes higher than 1200 cm-1 in both P and B. Also, the absolute scattering cross sections with excitation in the P band are unusually weak in both reaction centers, indicating that their excited states are rapidly vibronically dephased. The striking similarities between the P and B spectra in reaction centers from two very different bacterial species suggest that the common nuclear and electronic dynamics identified here are characteristic of photosynthetic reaction centers.

Coherent dynamics during the primary electron-transfer reaction in membrane-bound reaction centers of Rhodobacter sphaeroides

The temporal evolution of the near-infrared stimulated emission band of the special pair excited state (P*) in the reaction center of Rhodobacter sphaeroides has been studied in intracytoplasmic membranes of the antenna-deficient RCO1 mutant at 10 K with a resolution of 30 fs. On the 100-fs time scale the emission band gradually shifts to longer wavelengths. After 150 fs the band shifts back to shorter wavelengths and continues to develop on the picosecond time scale in a damped oscillatory manner (most prominent fundamental frequencies around 15 cm-1 and at 92, 122, and 153 cm-1). These phenomena are shown to be due to low-frequency vibrational motions in the P* excited state that conserve their phase on the time scale of electron transfer. These results imply that the vibrational manifold of P* is not thermalized during the electron-transfer reaction in functional reaction centers. The initial Stokes shift dynamics are largely determined by the modes in the 90-160-cm-1 frequency range, which probably involve motions of several chromophores, including the bacteriopheophytin electron acceptor HL.

Changes in primary donor hydrogen-bonding interactions in mutant reaction centers from Rhodobacter sphaeroides: identification of the vibrational frequencies of all the conjugated carbonyl groups

Specific changes in the hydrogen-bonding states of the primary donor, P, in reaction centers from Rhodobacter sphaeroides bearing mutations near P were determined using near-infrared excited Fourier transform (FT) Raman spectroscopy. This technique, using 1064-nm excitation, provides the preresonantly enhanced vibrational spectrum of P in its reduced state selectively over the contributions of the other reaction center chromophores and protein and yields structural information concerning P and its hydrogen-bonding interactions. The mutations studied were as follows: Leu M160-->His, Leu L131-->His, the D9 double mutant (Leu M160-->His + Leu L131-->His), Phe M197-->His, and His L168-->Phe. These mutations were designed to introduce new, or to break existing, hydrogen bonds to the C9 and C2 carbonyl groups of P. On the basis of previous assignments [Mattioli, T. A., Hoffmann, A., Robert, B., Schrader, B., & Lutz, M. (1991) Biochemistry 30, 4648-4654], the FT Raman spectra of these mutants show the predicted changes in hydrogen bond interactions of P carbonyl groups with the protein. The results of this study have permitted us to unambiguously identify the C2 and C9 carbonyl vibrators of P in Rb. sphaeroides. The genetically introduced hydrogen bond interactions are discussed in terms of other physicochemical properties of P including the redox potential and electronic asymmetry in the P+ state. It is discussed that changes in protein hydrogen bonding to the conjugated carbonyl groups of P alone are not the sole factor that contributes to the sizeable modifications of the P/P+ redox midpoint potentials, and that the chemical nature of the hydrogen bond donor plays a significant role in this modification.