Warren F. Beck
Biophysical Chemistry: Ultrafast Spectroscopy of Protein Reaction Dynamics; Single-Molecule Spectroscopy and Optical Trapping
My group's current research activities are focused on the reaction dynamics of proteins and their intrinsic active centers. The topics we have considered so far include the mechanism of excitation transport and radiationless decay in photosynthetic light-harvesting proteins, the photophysics of pairs of chlorophylls in the subunits of purple-bacterial light-harvesting proteins and reaction centers, charge-transfer dynamics in blue copper proteins, and the nature of protein-matrix solvation dynamics around electronic excited states in small globular proteins. The latter topic relates to the nature of structural fluctuations in proteins and in protein-folding intermediate structures. We are now beginning work on the reaction dynamics of single protein molecules using far-field microscopic techniques in combination with optical trapping (tweezer) methods.
The approach in these studies is to exploit a combination of state-of-the-art spectroscopic methods with biochemical control of the target systems. As an example, in order to detect the exquisitely fast radiationless decay processes that occur between the exciton states in the cyanobacterial light-harvesting proteins allophycocyanin and C-phycocyanin, we relied on a comparison of the ultrafast spectroscopic response of monomeric proteins containing just a single chromophore in the isolated limit with that of intact protein arrays carrying pairs of chromophores. In this work, we combined the information from conventional femtosecond transient-hole-burning (pump-continuum probe) spectroscopy with the results of a new type of stimulated-photon-echo spectroscopy.
We employ state-of-the-art femtosecond spectroscopic instrumentation, both home-built and commercially prepared. At Michigan State University, two rooms in the LASER laboratory are currently being renovated to permit installation of three spectrometers that will be used by my group. One spectrometer is based around a 10-fs titanium-sapphire oscillator, constructed using the Murnane/Kapteyn design, that is pulse-picked using a acoustooptical device (Bragg cell). The 10-fs oscillator is pumped by a continuous-wave solid-state green pump laser (frequency doubled Nd3+-YVO4; Coherent Verdi). Another spectrometer is based around a complete 250-kHz repetition rate amplified titanium-sapphire laser setup from Coherent Laser Group (Sabre 25-W argon-ion continuous pump laser, Mira-F oscillator, RegA regenerative amplifier, and OPA9400 optical parametric amplifier). The amplified instrument will also be equipped with a home-built noncollinearly pumped optical parametric amplifier that will be capable of emitting 10-fs pulses tunable throughout the visible and near-IR regime. The third spectrometer will combine a 50-fs Murnane/Kapteyn titanium-sapphire oscillator with a stage-scanned confocal microscope; the intention is to perform single-molecule spectroscopy with real time CCD detection of spectral fluctuations in the spontaneous fluorescence emitted following absorption of femtosecond pulse sequences.
The main area of current research involves coherent wave packet dynamics on charge-transfer surfaces, in proteins and in bulk molecular solvents. We are interested in determining how low-frequency vibrational modes promote surface-crossing events that result in transfer of an electron from one molecule to another or from one part of a molecule to another. As an example, we have recently observed excited-state vibrational coherence and a fast anisotropy decay at room temperature in the bacteriochlorophyll a dimer in the B820 subunit of the LH1 light-harvesting subunit of Rhodospirillum rubrum G9; the results suggest that intradimer charge transfer results in the excitation of medium-frequency vibrational modes that do not have Franck-Condon activity. This type of chemistry may be involved in the earliest events that lead to long-distance charge-transfer in the photosynthetic reaction center.
We are also continuing work on the solvation dynamical properties of the protein matrix that surrounds the active centers. The main methods are those of transient-grating spectroscopy and stimulated photon-echo peak-shift spectroscopy (3PEPS), but we also exploit dynamic pump-probe absorption spectroscopy with 10-fs pulses to observe directly the dynamic Stokes shift of the stimulated emission. Using these methods, we have demonstrated that the interior of a protein exhibits a fast phase of solvation response that is analogous to the inertial phase of solvation that occurs in bulk polar solvents. The inertial phase of the protein-matrix response occurs on the < 100-fs time scale. We are examining how this phase of protein dynamics influences electron transfer and energy transfer in photosynthetic proteins. Newly obtained results suggest that the diffusive, conformational part of the protein-matrix solvation response is faster in the partially folded, molten globule state of a protein than it is in the native, fully folded state; this is a result that may stimulate future work on the protein-folding problem.
Finally, the new initiative in single-molecule spectroscopy involves trapping of single proteins in solution so that we can watch them react and change structure. One goal is to observe the dynamics of single protein molecules as they fold from the random coil to native structures; another goal is to compare the spectral fluctuations of native proteins with those of molten-globule structures, which lack the protein-protein contacts that define the tertiary structure. This work will complement the work being done in parallel using stimulated-photon-echo spectroscopy on protein dynamics in native and molten-globule structures.