Photoisomerization of Azobenzenecarboxylic Acids and Their Potassium Salts: Evidence of Structural Volume Changes Associated with Hydrogen Bond Formation

The contractions determined by laser-induced optoacoustic spectroscopy (LIOAS) for the E Z photoisomerization of the three isomers (para, meta, and ortho) of azobenzenecarboxylates in 0.01 M potassium phosphate buffer (pH 8) are rationalized in terms of the difference in the chromophore-water hydrogen bonds strength between the photoisomer and the parent compound. The relatively high concentration of K+ is needed to solubilize the compounds by means of interrupting the intermolecular interactions. At this salt concentration a large portion of the anions is paired with potassium ions. Due to the reduced conjugation, a stronger interaction of the nitrogens' lone pairs and water is expected in the Z isomers, leading to the contraction upon isomerization. A higher K+ concentration reduces the structural volume change as a consequence of its perturbing the hydrogen bond network. No structural volume change was observed for the E Z photoisomerization of meta-azobenzenecarboxylic acid in a series of cyclic alkanes, consistent with the assignment of the changes to variations in the hydrogen bonds strengths, in view of the lack of those bonds in the cycloalkanes. Despite the relatively large error of the Z-E energy differences derived from LIOAS, they are near those reported for nonsubstituted azobenzene and larger than those calculated for both free acid and lithium salts. With the LIOAS-derived structural volume changes VR and the quantitative relationship SR = (cp/)ik VR/T [(Cp/)ik = 14 kJ cm-3, the ratio of thermoelastic parameters at the isokinetic temperature in water, ca. 300 K], a large entropic term, in turn determined by the specific solute-water interactions, is calculated for the Z E thermal isomerization

LASER-INDUCED OPTOACOUSTIC SPECTROSCOPY (OAS) OF PHYTOCHROME IN-VITRO

On étudie le phytochrome photorécepteur des plantes à l'aide d'une méthode de spectroscopie optoacoustique induite par laser. Une solution de phytochrome de (Pr) est irradiée à l'aide de pulses de 15 ns issus d'un laser à colorant accordable de 590 à 720 nm. On effectue une étude comparative à l'aide d'une solution de CuCl2 qui présente la même absorption. Les résultats conduisent à l'interprétation de la desexcitation non radiative du système phytochromique.Laser-induced optoacoustic spectroscopy (OAS) has been applied to the plant photoreceptor phytochrome. A solution of purified oat phytochrome (Pr) was irradiated at room temperature with a 15 ns dye laser pulse tunable from 590 to 720 nm. The first acoustic wave was studied, which is generated by fast heat-emitting processes in less than 0.5 µs. The Pr solution emitted about 70-90 % of the heat produced by a CuCl2 reference of the same absorbance at all wavelenghths except for 700 nm, where significantly more heat was produced by Pr. The results are interpreted in terms of a photochromic system consisting of Pr and the first intermediate in the Pr → Pfr pathway

Structural volume changes upon photoisomerization of the bilirubin-albumin complex: a laser-induced optoacoustic study

The Z --> E photoisomerization of the 1:1 complexes of human serum albumin (HSA) and several bilirubins (BR-IX alpha, -III alpha, meso-IX alpha and a mixture of -IX alpha, -III alpha and XIII alpha) affords in every case an almost negligible structural volume change (Delta V-R approximate to 0) within detection limits (i.e. less than 2-4 cm(3)/mol for this isomerization with very low quantum yield) as determined by laser-induced optoacoustic spectroscopy. Based on previous model studies of photoisomerizations in aqueous environment, this negligible small change is interpreted as indicating that the part of the molecule undergoing photoisomerization is not exposed to water but is located in a hydrophobic protein cavity that shields the molecule from the aqueous medium. The BR- protein interaction within this cavity seems to be very loose in view of the small structural volume change observed. The energy difference between the Z and E isomers of the BR-HSA complexes was estimated to be less than 4 kJ/mol, probably close to zero (Delta H approximate to 0)

Protein movement within the bacteriorhodopsin photocycle detected by beam deflection spectroscopy

Photothermal beam deflection of the bacteriorhodopsin photocycle in the time range from 1 to 200 µs resolved a new process with a time constant of ca. 20 µs at 20 °C in addition to the decays of the K and L intermediates with time constants of 1.3 and 90 µs. The latter time constants are identical to those measured by flash photolysis with optical detection on the same samples. The additional process may be attributed to a protein conformational change concomitant with the proton transfer

Molecular volume changes in cyanines. A laser-induced optoacoustic study

The molecular movements taking place during the E -Z photoisomerization of the carbocyanines 3,3' -diethyloxadicarbocyanine iodide (DODCI) and 3,3' -diethyloxacarbocyanine iodide (DOCI) were studied in ethanol-water mixtures using solvent- and temperature-dependent laser-induced optoacoustic spectroscopy (LIOAS) with nanosecond excitation and 1-µs heat integration time. The contraction measured for the DODCI photoisomerization is attributed to solvent rearrangement around the DODCI isomers with different dipole moments, as evaluated using force field calculations. No volume change was observed for the DOCI photoisomerization