TIME-RESOLVED RESONANCE RAMAN SPECTRA OF THE PHOTOCYCLE INTERMEDIATES OF ACID AND DEIONIZED BACTERIORHODOPSIN

Probing Structure and Reaction Dynamics of Proteins Using Time-Resolved Resonance Raman Spectroscopy

The mechanistic understanding of protein functions requires insight into the structural and reaction dynamics. To elucidate these processes, a variety of experimental approaches are employed. Among them, time-resolved (TR) resonance Raman (RR) is a particularly versatile tool to probe processes of proteins harboring cofactors with electronic transitions in the visible range, such as retinal or heme proteins. TR RR spectroscopy offers the advantage of simultaneously providing molecular structure and kinetic information. The various TR RR spectroscopic methods can cover a wide dynamic range down to the femtosecond time regime and have been employed in monitoring photoinduced reaction cascades, ligand binding and dissociation, electron transfer, enzymatic reactions, and protein un- and refolding. In this account, we review the achievements of TR RR spectroscopy of nearly 50 years of research in this field, which also illustrates how the role of TR RR spectroscopy in molecular life science has changed from the beginning until now. We outline the various methodological approaches and developments and point out current limitations and potential perspectives.

Partial dehydration of the retinal binding pocket and proof for photochemical deprotonation of the retinal Schiff base in bicelle bacteriorhodopsin crystals

In bicelle bacteriorhodopsin (bcbR) crystals, the protein has a different structure from both native bacteriorhodopsin (bR) and in-cubo bR (cbR) crystals. Recently, we studied the ability of bcbR crystals to undergo the photocycle upon laser excitation, characterized by the appearance of the M intermediate by single crystal resonance Raman spectroscopy. Calculation of the M lifetime by flash photolysis experiments demonstrated that in our bcbR crystals, the M rise time is much faster than in the native or cbR crystals, with a decay time that is much slower than these other two forms. Although it is now known that the bcbR crystals are capable of photochemical deprotonation, it is not known whether photochemical deprotonation is the only way to create the deprotonated Schiff base in the bcbR crystals. We measured both the visible and Raman spectra of crystals dried under ambient lighting and dried in the dark in order to determine whether the retinal Schiff base is able to thermally deprotonate in the dark. In addition, changes in the visible spectrum of single bcbR crystals under varying degrees of hydration and light exposure were examined to better understand the retinal binding environment.

Nature of the chromophore binding site of bacteriorhodopsin: the potential role of Arg82 as a principal counterion

The nature of the chromophore binding site of light-adapted bacteriorhodopsin is analyzed by using modified neglect of differential overlap with partial single and double configuration interaction (MNDO-PSDCI) molecular orbital theory to interpret previously reported linear and nonlinear optical spectroscopic measurements. We conclude that in the absence of divalent metal cations in close interaction with Asp85 and Asp212, a positively charged amino acid must be present in the same vicinity. We find that models in which Arg82 is pointed upward into the chromophore binding site and directly stabilizes Asp85 and Asp212 are successful in rationalizing the observed one-photon and two-photon properties. We conclude further that a water molecule is strongly hydrogen bonded to the chromophore imine proton. The chromophore "1Bu*+" and "1Ag*-" states, despite extensive mixing, exhibit significantly different configurational character. The lowest-lying "1Bu*+" state is dominated by single excitations, whereas the second-excited "1Ag*-" state is dominated by double excitations. We can rule out the possibility of a negatively charged binding site, because such a site would produce a lowest-lying "1Ag*-" state, which is contrary to experimental observation. The possibility that Arg82 migrates toward the extracellular surface during the photocycle is examined.

Photochemistry in dried polymer films incorporating the deionized blue membrane form of bacteriorhodopsin

The preparation and photochemical properties of dried deionized blue membrane (dIbR600; lambdamax approximately 600 nm, epsilon approximately 54, 760 cm-1 M-1, f approximately 1.1) in polyvinyl alcohol films are studied. Reversible photoconversion from dIbR600 to the pink membrane (dIbR485; lambdamax approximately 485 nm) is shown to occur in these films under conditions of strong 647-nm laser irradiation. The pink membrane analog, dIbR485, has a molar extinction coefficient of approximately 39,000 cm-1 M-1 (f approximately 1.2). The ratio of pink --> blue and blue --> pink quantum efficiencies is 33 +/- 5. We observe an additional blue-shifted species (dIbR455, lambdamax approximately 455 nm) with a very low oscillator strength (f approximately 0.6, epsilon approximately 26,000 cm-1 M-1). This species is the product of fast thermal decay of dIbR485. Molecular modeling indicates that charge/charge and charge/dipole interactions introduced by the protonation of ASP85 are responsible for lowering the excited-state all-trans --> 9-cis barrier to approximately 6 kcal mol-1 while increasing the corresponding all-trans --> 13-cis barrier to approximately 4 kcal mol-1. Photochemical formation of both 9-cis and 13-cis photoproducts are now competitive, as is observed experimentally. We suggest that dIbR455 may be a 9-cis, 10-s-distorted species that partially divides the chromophore into two localized conjugated segments with a concomitant blue shift and decreased oscillator strength of the lambdamax absorption band.

Influence of cation binding on the electro-optically determined electric moments of purple membranes

Analysis of the electro-optically determined permanent dipole moment and electric polarizability of purple membrane fragments reveals the complex nature of the membrane electric moments. The problem to distinguish between the contribution of the membrane structural charges (charged groups of the polypeptide chain and polar lipid headgroups), bound cations and the electric double layer structure deserves particular attention not only because of its importance for electro-optics but also in respect to the relation of the membrane surface electric properties to the membrane transport function. The removal of divalent cations (Ca2+ and Mg2+) bound to purple membrane in the native state induces a cation-free species of purple membrane (deionized--blue membrane) with drastically changed spectroscopic properties and function. The present paper summarizes our study on the electric moments of blue membrane and their changes during the blue to purple transition. We intended to provide an insight into the possible regulation of this reversible transition (purple-to-blue and blue-to-purple) through changes of the asymmetric charge distribution and the importance of the asymmetric interfacial charge distribution for the proton transfer in purple membranes. The changes in the electric moments (permanent and induced dipole moments) of purple membrane fragments upon di- and trivalent cations binding to cation-depleted purple membranes were studied by electric light scattering (rotational electrokinetics) in d.c. and a.c. electric fields, and by electric pulses with reversing polarity. The results show a recovery of the membrane charge asymmetry (permanent dipole moment) though not of the induced dipole moment.

The binding site of the strongly bound Eu3+ in Eu(3+)-regenerated bacteriorhodopsin

A Scatchard plot for the strongly bound Eu3+ to deionized bacteriorhodopsin (bR) was made using a method based on measuring the concentration of unbound Eu3+ from its fluorescence intensity. The results suggest that the first mole of Eu3+ added to a mole of bR is strongly bound by displacing 2-3 protons. In order to reconcile this result with the previous time-resolved fluorescence studies on Eu(3+)-regenerated bR, which showed the presence of 3 sites of comparable binding constants, one is forced to conclude that the emission from the strongly bound Eu3+ is completely quenched, e.g. by energy transfer to the retinal. For this to take place, the Eu3+ must be within a few A from the retinal, i.e. within the retinal pocket (the active site). The possible importance of this conclusion to the deprotonation mechanism of the protonated Schiff base, the switch of the proton pump in bR, is discussed.