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Diatoms generate their cell walls by silica biomineralization. The cell walls are composed of silica and organic macromolecules and show a complex microscopic structure. Analysis of this structure by different atomic force microscopy (AFM) techniques revealed an unexpected nanostructured granular surface. Silaffins, proteins that are posttranslationally modified with long-chain polyamines, and oligo-N-methyl-propyleneamines were identified as the main organic constituents of diatom biosilica.¹ Silaffins as well as free propyleneamines of different chain lengths induce rapid precipitation of nanosized particles from silicic acid solutions in vitro. In a biomimetic approach, we reacted aqueous silicic acid solution with tripropylenetetramine in CHCl3 in a biphasic system. As a result, thin nanostructured silica layers that show a granular nanostructure very similar to that of the diatom cell walls were obtained. This finding may serve as a good model to study the mechanisms that lead to the nanostructure of the diatom cell walls.

It was found that bacterial transglutaminase (TGase) facilitates selective cross-linking of bacteriorhodopsin (BR) in purple membrane (PM) form under mild conditions. Fluorescent probes were used to detect that the membrane protein BR may act as a glutamine donor as well as a lysine donor for TGase. The binding sites were determined to be Gln-3 as the reactive glutamine, and Lys-129 is the corresponding lysine residue. Upon incubation of PM with TGase, cross-linking of PM patches can be achieved without an additional spacer molecule. To our knowledge, this is the first time that an intermembrane cross-linking of membrane-bound proteins is reported. Furthermore, this finding may provide the ability to achieve covalent linkage of complete purple membrane patches to synthetic polymers.

The M-lifetime of bacteriorhodopsin films for optical recording is a key parameter in obtaining a high light sensitivity, a high contrast ratio, and a high contrast decay time. An increase of the M-lifetime causes a proportional reduction of the light intensity required for optical applications. In bacteriorhodopsin variants such as BR-D96N, the M-lifetime can be tuned over several orders of magnitude by simply changing the pH value with respect to the proton availability in the matrix of the films. At low humidities, the proton transport steps linked to the photocycle limit the overall kinetics. A proton-diffusion-limited, two-state model (PDL2 model) for bacteriorhodopsin is introduced which allows us to model mathematically the optical excitation and thermal relaxation processes for both high and low humidities in bacteriorhodopsin films. Films containing wildtype bacteriorhodopsin and the variant D96N are compared in dependence on the pH value and the relative humidity at 20 °C. Of the investigated materials, only BR films containing BR-D96N can be used for recording at low light levels of 100 μW/cm². In a holographic interferometry experiment&sbd;a typical application where a high light sensitivity is a key issue&sbd;it is demonstrated to what high extent the water content in the films affects their suitability for recording at low light levels. Kinetically optimized bacteriorhodopsin films yield a more-than-30-fold improvement of sensitivity in holographic interferometry compared to dry bacteriorhodopsin films.

With Bacteriorhodopsin (BR) it will be demonstrated that biological systems - optimized by nature under evolutionary conditions - can be applied with great success to solve material requirements in technical systems. The possibility of genetic engineering further increases the technical applicability of these systems. The application of BR and its variants as a molecular processing unit in optical data processing will be shown in several examples.

The photochromic retinal protein bacteriorhodopsin which was obtained from Halobacterium salinarium has been studied as the photoactive component of recording materials for different dynamic holographic applications like degenerate - four - wave - mixing (DFWM) and interferometry. With the mutated BR-variant D96N resonantly induced reflectivities of up to 20% are observed at 647 nm in DFWM. The excellent reversibility of the BR-materials and the extended thermal decay time of this particular BR-variant support its application in real - time interferometry.

The cDNA clone for strictosidine synthase, the enzyme which catalyzes the stereospecific condensation of tryptamine with secologanin to form the key intermediate in indole alkaloid biosynthesis, strictosidine, has been identified with a synthetic oligodeoxynucleotide hybridization probe in a λgt11 cDNA library of cultured cells of Rauvolfia serpentina. The DNA has been sequenced, revealing an open reading frame of 1032 base pairs encoding 344 amino acids. The sequence of 60 nucleotides in the 5′-flanking region has been determined by primer extension analysis. The encoded protein has been expressed in E. coliDH5 as detected by immunoblotting of protein extracts with antibodies raised against the native enzyme.

The cDNA clone for strictosidine synthase, the enzyme which catalyzes the stereospecific condensation of tryptamine with secologanin to form the key intermediate in indole alkaloid biosynthesis, strictosidine, has been identified with a synthetic oligodeoxynucleotide hybridization probe in a λgt11 cDNA library of cultured cells of Rauvolfia serpentina. The DNA has been sequenced, revealing an open reading frame of 1032 base pairs encoding 344 amino acids. The sequence of 60 nucleotides in the 5′‐flanking region has been determined by primer extension analysis. The encoded protein has been expressed in E. coliDH5 as detected by immunoblotting of protein extracts with antibodies raised against the native enzyme.

Incoherent optical processing with films made from the biological photochrome bacteriorhodopsin (BR) is accomplished by use of the photoinducible anisotropy of polymeric BR films. BR has two spectrally well separated states, B and M, both of which show a high degree of optical anisotropy. Whereas with green and blue light the BR molecule can be switched between the two states, the accompanying changes in the refractive index may be nondestructively read out by wavelengths in the red, e.g., at 676 nm. As the sensitivity of CCD arrays is quite high for such wavelengths, even low-power light sources are sufficient for detection. We describe a BR-based XOR computing module for incoherent optical data processing.

Reflection-type polarization holograms with phase-conjugated readout are very useful for low-light recording with bacteriorhodopsin (BR) films. The dependence of reflection-type holograms with parallel and orthogonal recording beams on their intensity ratio (1:1–1:20) was investigated. It was found that for orthogonally polarized beams the phase-conjugated signal depends significantly less on the beam intensity ratio than predicted by coupled-wave theory. This finding is of particular relevance for recording of very low object-beam intensities with BR films, e.g., in interferometry, where signals with a high signal-to-noise ratio, owing to the different polarizations of the scattered light and the signal, and with low dependence of the diffraction efficiency on the ratio between the reference and object-beam intensities are obtained. With this asymmetric recording process, holograms were recorded successfully in BR films with a good signal-to-noise ratio at exposures (from the object side) as low as 50 μJ/cm^2. These exposures are in the range of those typically used for silver halide films.

First experimental results from a direct-view display made from bacteriorhodopsin (BR) are presented. A dielectric mirror in direct contact with the photoactive BR layer forms the core of the BR display. The dielectric layer both decreases the light intensity necessary for writing and protects the observer from transmitted laser light. By illuminating the BR display with a suitably filtered light source from the rear we achieve the result that the information appears to the observer with an intensity contrast of more than 70:1, accompanied by a significant color shift. The combination of both enhances the visibility of and the ability to discern the information significantly. On the BR display the information appears in yellow on a dark purple-red background.

A dual-axis joint-Fourier-transform correlator is described with two liquid-crystal television screens as input devices and a bacteriorhodopsin film as the active holographic material in the Fourier plane. The experimental data presented demonstrate that this system is capable of processing two independent video signals in real time with a signal-to-noise ratio of 45 dB.

A self-pumped, phase-conjugating mirror (diameter 17 mm) is described that contains the bacteriorhodopsin variant D96N as the photoactive material. Resonant reflectivities up to 30% were obtained with an intensity of 25 mW/cm^2 at 647 nm. Rise times of less than 1ms were observed with an intensity of 500 mW/cm^2 at 532 nm. Wave-front restoration with spatial light modulators as the dynamic input device was demonstrated, and the dependence of the reflectivity on the intensity and polarization was analyzed. The reflectivity shows no significant decrease with increasing angle between the pump and signal beams as a result of the high resolution of the bacteriorhodopsin films.

An adaptive correlator system is presented that uses a bacteriorhodopsin film as the active holographic material. Because M-type holography with bacteriorhodopsin films is used, the TV frame rate is met at fairly low intensities. No limitations arising from the heterodyne operation of the dual-axis joint-Fourier-transform correlator were observed. The system’s capability to monitor autonomously the speed of an object moving randomly in the whole viewing field of a video camera is demonstrated.

The application of dry purple membrane films for nonlinear-optical filtering is described. The biological photo-chrome bacteriorhodopsin (BR) is contained in the purple membrane (PM) from Halobacterium halobium. BR has two dominant photoactive states, B and M, which have well-separated absorption bands with maxima at 570 nm (B) and 412 nm (M). Since the local transmission of a PM film depends on the ratio between the forward (B ? M) and the backward (M ? B) photoreactions, PM films can be used as light-controlled absorptive spatial light modulators. A model describing the nonlinear transmission of PM films containing the wild-type form of BR or one of its mutated variants, e.g., BR_D96N, and examples of their application in spatial filtering, e.g., edge enhancement, are presented.