No abstract

Halobacterium halobium is attracted by green and red light and repelled by blue-green and shorter wavelength light. A photochromic, rhodopsin-like protein in the cell membrane, sensory rhodopsin sR587, has been identified as the receptor for the long-wavelength and near-UV stimuli. Discrepancies between the action spectrum for the repellent effect The archaeobacterium H. halobium thus uses two different mechanisms for color discrimination; it uses two rhodopsin-like receptors with different spectral sensitivities and also the photochromicity of at least one of these receptors to distinguish between three regions covering the visible and near-UV spectrum.Halobacterium halobium uses light energy to drive ion fluxes across its membrane. This biological light-energy transduction is mediated by two small retinylidene proteins, bacteriorhodopsin (bR) and halorhodopsin (hR), which have broad absorption bands near 570 nm (for recent reviews, see refs. 1 and 2). In addition, the cells show sensory light reactions. They are attracted by long-wavelength light, green and red, and repelled by short-wavelength light and, thus, migrate to regions of illumination that are optimal for their light-energy transduction systems (3, 4). These phototactic responses are also mediated by retinal pigments (5). Spudich and Bogomolni have recently identified a third retinylidene protein, sensory rhodopsin (sR). It is similar in its physical and chemical properties to bR and hR and mediates both the attractant effect of green and red and the repellent effect of near-UV light (6,7). sR has a strong absorption band centered at 587 nm and undergoes a cyclic photoreaction with a long-lived intermediate, which absorbs maximally at 373 nm. The intermediate, S373, is rapidly photoconverted back to sR587, and this has a repellent effect, whereas absence of near-UV light causes an attractant effect.Discrepancies have been noted between action spectra predicted from the absorption spectrum of sR587 and its photocycle intermediate S373 and those actually observed in several strains of H. halobium. Several small maxima or shoulders between 540 and 640 nm in the broad actionspectrum peak for the attractant response are not present in the absorption spectrum of sR (8, 9, §); more importantly, a repellent effect of blue light may be observed under conditions where no significant amount of S373 is expected to be present or to absorb light, and the crossover from the attractant to the repellent effect varies but usually occurs near 500 nm (4, 5), whereas the isosbestic point in the sR587/S373 difference spectrum occurs at 420 nm (6). These observations prompted the search for a repellent receptor, which absorbs at longer wavelength than does S373 and does not require long-wavelength background illumination.

The static and dynamic response of optical waveguides coated with a thin protein film of bacteriorhodopsin was investigated. The size and kinetics of the light-induced refractive index changes of the adlayer were determined under different conditions of illumination. The results demonstrate the applicability of this protein as an active, programmable nonlinear optical material in all-optical integrated circuits. (C) 2002 American Institute of Physics

Several inorganic and organic materials have been suggested for utilization as nonlinear optical material performing light-controlled active functions in integrated optical circuits, however, none of them is considered to be the optimal solution. Here we present the first demonstration of a subpicosecond photonic switch by an alternative approach, where the active role is performed by a material of biological origin: the chromoprotein bacteriorhodopsin, via its ultrafast BR->K and BR->I transitions. The results may serve as a basis for the future realization of protein-based integrated optical devices that can eventually lead to a conceptual revolution in the development of telecommunications technologies.

According to our earlier pioneering study, a dry film containing native bacteriorhodopsin (bR) shows unique nonlinear optical properties (refractive index change, controllable by light of different colors, greater than 2 x 10(-3)) that are in many respects superior to those of the materials presently applied in integrated optics. Here, we report on the first integrated optical application based on a miniature Mach-Zehnder interferometer (see Figs. 1 and 2) demonstrating a real switching effect by bR (efficiency higher than 90%) due to the M-state. Our results also imply that the refractive index change of the K-state (9 x 10(-4)) is high enough for fast switching.