ISOMER COMPOSITION and SPECTRA OF THE DARK and LIGHT ADAPTED FORMS OF ARTIFICIAL BACTERIORHODOPSINS

Retinal β-ionone ring-salinixanthin interactions in xanthorhodopsin: a study using artificial pigments

Xanthorhodopsin (xR) is a retinal protein that contains, in addition to the retinal chromophore, a carotenoid (salinixanthin) that functions as a light-harvesting antenna [Balashov, S. P., et al. (2005) Science 309, 2061-2064]. The center-center distance between the two polyene chains is 12-13 Å, but the distance between the two rings of retinal and salinixanthin is surprisingly small (~5 Å) with an angle of ~45° [Luecke, H., et al. (2008) Proc. Natl. Acad. Sci. U.S.A. 105, 16561-16565]. We aimed to clarify the role of the β-ionone ring in the binding of retinal to apo-xR, as well as a possible role that the β-ionone ring plays in fixation of the salinixanthin 4-keto ring. The binding of native retinal and series of synthetic retinal analogues modified in the β-ionone ring to apo-xR was monitored by absorption and circular dichroism (CD) spectroscopies. The results indicate that the β-ionone ring modification significantly affected formation of the retinal-protein covalent bond as well as the pigment absorption and CD spectra. It was observed that several retinal analogues, modified in the retinal β-ionone ring, did not bind to apo-xR and did not form the pigment. Also, none of these analogues induced the fixation of the salinixanthin 4-keto ring. In addition, we show that the native retinal within its binding site adopts exclusively the 6-s-trans ring-chain conformation.

Block Copolymer-Based Biomembranes Functionalized with Energy Transduction Proteins

Block copolymer-based membranes can be functionalized with energy transducing proteins to reveal a versatile family of nanoscale materials. Our work has demonstrated the fabrication of protein-functionalized ABA triblock copolymer nanovesicles that possess a broad applicability towards areas like biosensing and energy production. ABA triblock copolymers possess certain advantages over lipid systems. For example, they can mimic biomembrane environments necessary for membrane protein refolding in a single chain (hydrophilic(A)- hydrophobic(B)-hydrophilic(A)), enabling large-area membrane fabrication using methods like Langmuir-Blodgett (LB) deposition. Furthermore, the robustness of the polymer molecules/structure result in spontaneous and rapid protein-functionalized nano-vesicle formation that retains structure as well as protein functionality for up to several months, compared to one to two weeks for the lipid systems (e.g. POPC). The membrane protein, Bacteriorhodopsin (BR), found in Halobacterium Halobium, is a light-actuated proton pump that develops gradients towards the demonstration of coupled functionality with other membrane proteins, such as the production of electricity through Bacteriorhodopsin activity-dependent reversal of Cytochrome C Oxidase (COX), found in Rhodobacter Sphaeroides. Protein-functionalized materials have the exciting potential of serving as the core technology behind a series of fieldable devices that are driven completely by biomolecules.

Light-induced charge redistribution in the retinal chromophore is required for initiating the bacteriorhodopsin photocycle

Bacteriorhodopsin's photocycle is initiated by the retinal chromophore light absorption. It has usually been assumed that light primarily isomerizes a retinal double bond which in turn induces protein conformational alterations and biological activity. We have studied several artificial pigments derived from retinal analogues tailored to substantially reduce the light-induced chromophore polarization. The lack of chromophore polarization was reflected in an undetectable second harmonic generation (SHG) signal. It was revealed that these artificial pigments did not exhibit any detectable light-induced photocycle nor light acceleration of the hydroxylamine-bleaching reaction. We suggest that light-induced retinal polarization triggers protein polarization which controls the course of the isomerization reaction by determining the relative efficiency of forward versus back-branching processes.

Three electronic state model of the primary phototransformation of bacteriorhodopsin

The primary all-trans --> 13-cis photoisomerization of retinal in bacteriorhodopsin has been investigated by means of quantum chemical and combined classical/quantum mechanical simulations employing the density matrix evolution method. Ab initio calculations on an analog of a protonated Schiff base of retinal in vacuo reveal two excited states S1 and S2, the potential surfaces of which intersect along the reaction coordinate through an avoided crossing, and then exhibit a second, weakly avoided, crossing or a conical intersection with the ground state surface. The dynamics governed by the three potential surfaces, scaled to match the in situ level spacings and represented through analytical functions, are described by a combined classical/quantum mechanical simulation. For a choice of nonadiabatic coupling constants close to the quantum chemistry calculation results, the simulations reproduce the observed photoisomerization quantum yield and predict the time needed to pass the avoided crossing region between S1 and S2 states at tau1 = 330 fs and the S1 --> ground state crossing at tau2 = 460 fs after light absorption. The first crossing follows after a 30 degrees torsion on a flat S1 surface, and the second crossing follows after a rapid torsion by a further 60 degrees. tau1 matches the observed fluorescence lifetime of S1. Adjusting the three energy levels to the spectral shift of D85N and D212N mutants of bacteriorhodospin changes the crossing region of S1 and S2 and leads to an increase in tau1 by factors 17 and 10, respectively, in qualitative agreement with the observed increase in fluorescent lifetimes.