C. C. Moser

Initial charge separation kinetics of bacterial photosynthetic reaction centers in oriented Langmuir-Blodgett films in an applied electric field

Abstract The free energy of the initial charge separation in bacterial photosynthetic reaction centers has been modified by placing oriented Langmuir-Blodgett films of the purified protein between external electrodes in a planar capacitor and applying a field of nearly 106 V/cm. The near-infrared transient absorption changes associated with the decay of the excited state of the bacteriochlorophyll dimer and the initial charge separation was measured with 300 fs time resolution with and without applied field. The surprisingly small field induced rate changes of the oriented systems compared to unoriented systems suggest that modulation of the energy gap between excited bacteriochlorophyll dimer and the charge separated state with bacteriochlorophyll monomer reduced is the principal influence of electric field on rate. The field induced quantum yield failure observed at longer timescales appears to be associated with modulation of the bacteriopheophytin to bacteriochlorophyll dimer charge recombination.

Far-infrared perturbation of electron tunneling in reaction centers?

Abstract We use the intense pulsed far-infrared laser of the UCSB FEL as a probe for the role that low-frequency protein modes could have on the tunneling rates of photosynthetic reaction centers. We will present our results and try to convince the skeptical reader that this is not yet another stupid paper trying to find simple minded physics in complex biological systems.

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.

Femtosecond Infrared Studies of Photosynthetic Reaction Centers: New Charge Transfer Bands and Ultrafast Energy Redistribution

New low-energy, charge transfer-like excited states of the special pair in the photosynthetic reaction center are identified.

Optical Coherence and Anisotropy Studies of the First Events in Photosynthesis

Femtosecond optical coherence and anisotropy measurements are reported for the photosynthetic reaction center (Rb. Sph) revealing rapid electronic dephasing of both P* and B*, and novel polarization responses.

Picosecond Infrared Spectroscopy of the Photosynthetic Reaction Center

We report the first picosecond infrared spectroscopic study on the reaction center of bacterial photosynthesis. Infrared difference spectra in the range from 1560 to 1800 cm−1 were recorded and investigated from 50 ps to 4 ns.after photooxidation of the primary donor. Bands at 1755, 1707 and 1683 cm−1 can be assigned to vibrations of cofactor carbonyl groups. Protein contributions at 1644, 1661 and 1665 cm−1 (amide I) and at 1727 cm−1 (carboxylic residues) and their kinetics reflect the fast protein response to intermolecular charge transfer.

Pump Probe Anisotropy Studiesof the PhotosyntheticReaction Center

The photosynthetic reaction center of Rb. sphaeroides is investigated by unique pump-probe anisotropy methods using 13 fs optical pulses tunable throughout the near IR absorption band. The time and wavelength resolved pump probe anisotropy transients yield detailed experimental descriptions of the energy levels, Py

ELECTRONIC AND VIBRATIONAL COHERENCE IN PHOTOSYNTHETIC COFACTORS" COMPARISON OF SOLUTIONS AND PROTEINS

The interaction between photosynthetic cofactors and the surrounding bath or protein environment is addressed via experimental measurements of the optical coherence responses from bacteriochlorophylla (Bchla) chromophores within the photosynthetic reaction center (RC) of Rhodobacter sphaeroides and solutions of Bchla monomers in THF and pyridine. The results indicate that both the spectrum of fluctuations and chromophore bath coupling strengths vary between solutions and protein. In particular, the protein environment yields faster dephasing, faster spectral diffusion, and significantly more inhomogeneity than solutions.

Femtosecond Infrared Spectroscopy on Reaction Centers of Rb. Sphaeroides

In this paper we will introduce some of the theory of ultrafast visiblepump IR-probe spectroscopy which is employed to understand recent experimental studies on chromophore vibrations in the RC. New methods to generate femtosecond probe pulses tunable in the IR are presented. These pulses have been used to study energy transfer in the RC as well as low lying electronic states in the special pair.

Vibrational Dynamics in Condensed Phases and Proteins

Ultrafast processes in condensed phase are explored with transient vibrational spectroscopy. Examples include chemical reactions, protein dynamics and energy transfer. Direct interrogation of molecular vibrations by infrared or Raman scattering is complementary to optical spectroscopic studies of ultrafast processes. The vibrational spectroscopies have some advantage in as much as the structurally important modes can be observed directly even when electronic transitions may be inaccessible. Although there are as yet no table top ultrashort pulse lasers in the structurally useful region of the infrared spectrum, say from 500 to 4000 cm- I the recent advances in ultrafast technology in combination with advances in nonlinear materials, permit effective generation of IR pulses and shorter time resolution. Free electron lasers represent another approach with promise. At time of writing the shortest IR pulse used in an experiment was 160 fs (I). There appears to be no technological barrier in obtaining useable pulses of about one-half this width. It is therefore evident that ultrafast processes can be studied directly via the vibrations and the purpose of this talk is to summarize some of the recent results obtained in this laboratory using a variety of IR and finally Raman methods. First a very brief summary of the current capabilities of transient IR methods will be given. The methods themselves have been adequately described in recent review articles (2, 3). There then follows an account of various areas of research in which these methods have been employed by us. For this talk, the following topics will be considered in more detail: