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The photochemical activity of the O-state was investigated in bacteriorhodopsin (BR) films containing wildtype BR at pH 6.5 in the presence of glycerol. The formation of a photoproduct of O with an absorption maximum at 490 nm and 9-cis-retinal configuration was found. This 490-nm product was named P and shows a slow thermal reaction into a compound with a maximal absorption at 380 nm which was named Q and contains free 9-cis-retinal in the proteins binding site. The photoproducts of O, i.e., P and Q, are very similar, or even identical, to those previously observed in blue membranes. Common to the O-state and blue membrane forms of bacteriorhodopsin is a protonated aspartic acid 85, and we suggest that it is the reduced negative charge around the Schiff base which is responsible for the 9-cis photoisomerization. The release of a proton from aspartic acid 85 is linked to the conversion of the O-state back to the initial state of BR. Therefore the conditions of low proton mobility in BR films containing glycerol favor the accumulation of the O-state. For optical and holographic applications such BR films are very attractive. It is possible to create photoproducts with red light which are thermally stable at room temperature and that can be photochemically erased. Dependent on the light composition both properties can be realized in the same sample material. This feature may bridge the gap between information processing and short-term and long-term storage of information with BR.

We present a systematic study on the influence of the miscut orientation on structural and electronic properties in the homoepitaxial growth on off-oriented β-Ga2O3 (100) substrates by metalorganic chemical vapour phase epitaxy. Layers grown on (100) substrates with 6° miscut toward the [001¯] direction show high electron mobilities of about 90 cm2 V−1 s−1 at electron concentrations in the range of 1–2 × 1018 cm−3, while layers grown under identical conditions but with 6° miscut toward the [001] direction exhibit low electron mobilities of around 10 cm2 V−1 s−1. By using high-resolution scanning transmission electron microscopy and atomic force microscopy, we find significant differences in the surface morphologies of the substrates after annealing and of the layers in dependence on their miscut direction. While substrates with miscuts toward [001¯] exhibit monolayer steps terminated by (2¯01) facets, mainly bilayer steps are found for miscuts toward [001]. Epitaxial growth on both substrates occurs in step-flow mode. However, while layers on substrates with a miscut toward [001¯] are free of structural defects, those on substrates with a miscut toward [001] are completely twinned with respect to the substrate and show stacking mismatch boundaries. This twinning is promoted at step edges by transformation of the (001)-B facets into (2¯01) facets. Density functional theory calculations of stoichiometric low index surfaces show that the (2¯01) facet has the lowest surface energy following the (100) surface. We conclude that facet transformation at the step edges is driven by surface energy minimization for the two kinds of crystallographically inequivalent miscut orientations in the monoclinic lattice of β-Ga2O3.

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Structural changes in the retinal chromophore during the formation of the bathorhodopsin intermediate (bathoRT) in the room-temperature rhodopsin (RhRT) photosequence (i.e., vision) are examined using picosecond time-resolved coherent anti-Stokes Raman scattering. Specifically, the retinal structure assignable to bathoRT following 8-ps excitation of RhRT is measured via vibrational Raman spectroscopy at a 200-ps time delay where the only intermediate present is bathoRT. Significant differences are observed between the C==C stretching frequencies of the retinal chromophore at low temperature where bathorhodopsin is stabilized and at room temperature where bathorhodopsin is a transient species in the RhRT photosequence. These vibrational data are discussed in terms of the formation of bathoRT, an important step in the energy storage/transduction mechanism of RhRT.It is well-recognized that the energy storage/transduction mechanism describing vision under physiological conditions occurs at room temperature in the transmembrane protein rhodopsin (RhRT) (1-3). After photon absorption, a series of at least seven intermediates making up the RhRT photosequence appears. These room-temperature intermediates have been identified primarily via transient absorption spectroscopy (3-9).The initial processes in the RhRT photosequence have been investigated by monitoring absorbance changes with time resolution of e 10-13 s and with an emphasis on excited electronic state lifetimes and the isomerization kinetics associated with the retinal chromophore (4, 5). Many characteristics of these processes, as well as their mechanistic interpretations, remain under discussion (4-7).The RhRT photosequence is well characterized kinetically over the 10-9-to 101-s regime (8,10,11 (14,15,(17)(18)(19)(20)(21)(22)(23). Lowering the protein temperature is thought to reduce the probability with which specific activation barriers along the reaction coordinate can be crossed. Since such an interruption of the Rh photosequence at low temperatures may also alter the protein environment, which is itself intimately involved in the energy storage/ transduction mechanism, it is unclear whether lowtemperature data, and the mechanistic models based on them (1,14,(18)(19)(20)(21)(22), are relevant to the RhRT photosequence and, thereby, to vision.The major experimental limitation to recording vibrational spectra from the RhRT photosequence has been its photoirreversibility. The efficient photodissociation of RhRT into free retinal and opsin after seconds ensures that the sample rapidly disappears after illumination, and, therefore, any spectroscopic data must be recorded efficiently and with an excellent signal-to-noise ratio (S/N). This is especially challenging in the case of vibrational data.CARS spectroscopy using picosecond pulsed excitation is used in this study to successfully address these experimental issues. The high S/N available from CARS means that highquality vibrational data can be obtained efficiently from small quantities of RhRT. T...