The photocycle kinetics of bacteriorhodopsin were analyzed from 0 to 40 degrees C at 101 wavelengths (330-730 nm). The data can be satisfactorily approximated by eight exponents. The slowest component (half-time 20 ms at 20 degrees C) belongs to the 13-cis cycle. The residual seven exponentials that are sufficient to describe the all-trans photocycle indicate that at least seven intermediates of the all-trans cycle must exist, although only five spectrally distinct species (K, L, M, N, and O) have been identified. These seven exponentials and their spectra at different temperatures provide the basis for the discussion of various kinetic schemes of the relaxation. The simplest model of irreversible sequential transitions includes after the first K--> L step the quasiequilibria of L<-->M, M<-->N, and N<-->O intermediates. These quasiequilibria are controlled by rate-limiting dynamics of the protein and/or proton transfer steps outside the chromophore region. Thus there exists an apparent kinetic paradox (i.e., why is the number of exponents of relaxation (at least seven) higher than the number of distinct spectral intermediates (only five)), which can be explained by assuming that some of the transitions correspond to changes in the quasiequilibria between spectrally distinct intermediates (i.e., are spectrally silent).

The temperature and pH dependencies of the O(640) intermediate of the photocycle of bacteriorhodopsin (bR) were investigated by flash photolysis and T-jump experiments. The maximal concentration of the O(640) intermediate was found to be dependent on the temperature, which is described by a sigmoidal relationship. With increasing pH the midpoint of the sigmoidal curves shifts to higher temperatures. The Van't Hoff equation provides enthalpy and entropy values of the observed states. These results indicate that, in the investigated temperature (0-60 degrees C) and pH (pH 4.0-10.0) range, the sequence of the principal intermediates in the pathway "M-N-O-bR" does not change. The observations of the O(640) intermediate at pH < 8.0 and of the N(550) intermediate at pH > 8.0 are most probably due only to changes of the intrinsic rate constants of the bR photocycle, not to a different mechanism.

A model of the last parts of the bacteriorhodopsin (bR) photocycle is proposed on the basis of experimental data for the kinetic behavior of the 'O' intermediate during a temperature pulse in distilled water suspension. The model includes the previously proposed (but not well characterized) intermediate 'N' between the 'M' and 'O' states of bR. This intermediate exists in fast temperature-dependent quasi-stationary equilibrium with the red-shifted intermediate 'O' and has a maximum of absorption close to the bR spectrum.

Abstract:In view of its potential for dental diagnostics, noninvasive optical methods have been the object of research for the last years. Different techniques (spectroscopy, microscopy, and tomography) are used to discriminate between caries and sound enamel. The main aim of this paper is to determine a simple way to see incipient caries with the naked eye. Relative parties R of fluorescence intensities from the sound enamel (squares) and white spots (rings) as a function of excitation wavelength λexc

Magnesium binding to cation#x2010;depleted blue bacteriorhodopsin (b‐bR) was studied spectrophotometrically as well as by following stopped‐flow kinetics. There exist three kinetically different steps in the binding process, yielding purple bacteriorhodopsin (p‐bR). Since only the firtst step is dependent on the concentration of the reactants, the reaction scheme can be proposed as the simplest model, with MgbR being the first intermediate and ΣI denoting a set of successive intermediates. According to this model k 1, k −1 and k 2 are calculated to be 2.8 × 104 M−1 · s−1, 5.0 × 10 s−1 and 1 × 10−2 s−1, respectively.