Photocycling of chromophoric structures

Photocycling of Chromophoric Structures during Irradiation of High-Yield Pulps... 

I Porsskahl and C Maunier. Photocycling of Chromophoric Structures during Irradiation of High-Yield Pulps. In C Heitner and JC Scaiano, eds. Photochemistry of Lignocellulosic Materials, 531. Washington, DC American Chemical Society, 1993, pp. 156-166. 

When bacteriorhodopsin was discovered in 1971, its similarity to the visual rhodopsin in the chromophore structure (retinal), the primary light-induced event (retinal isomerization) and the photocycle (short-wavelength intermediate formation) impelled the authors to suggest that this novel retinal protein is somehow involved in photoreception [1]. Such a suggestion seemed reasonable since halobacteria are known to change their direction of swimming in response to a light stimulus. 

Reduction of bacteriorhodopsin during illumination in the presence of borohydride produces a fluorescent species which absorbs at 360 nm [195,196]. The fine structure of the absorption band of this form is like that of retro-retinal, but the participation of such an isomer in the photocycle of bacteriorhodopsin [215,216] is unlikely. Rather, planarization of the chromophore by constraints of the retinal binding site might be responsible for the fine structure [110]. 

Van Thor JJ, Mackeen M, Kuprov I, Dwek RA, Wormald MR (2006) Chromophore structure in the photocycle of the cyanobacteiial phytochrome Cphl. Biophys J 91 1811-1822... 

Mroginski MA, Murgida DH, Hildebrand P (2007) The chromophore structural changes during the photocycle of phytochrome a combined resonance Raman and quantum chemical approach. Acc Chem Res 40 258-266... 

Photoreceptor Pigments. There have been several reviews on the structures, photochemistry, and functioning of the retinal-protein photoreceptor pigments involved in the processes of visionand in the purple membrane of Halobacteria (bacteriorhodopsin). ° ° In addition to the papers quoted earlier on the spectroscopy of these pigments, many other reports have appeareddealing with rhodopsin and intermediates in its photocycle, especially photochemistry, chromophore-protein conformation and binding, and reaction kinetics. Similar studies on bacteriorhodopsin have also been described." "-"" ... 

It has been proposed that the structural changes associated with the protonation of the chromophoric phenol are responsible for the signal transduction by the pB state. The pB—>pR conversion is photo reversible and its kinetics have been examined [11], Calculations [12] have shown that proton transfer is much more likely in the cis pR state than in the initial dark trans pG state. The PYP photocycle is completed by the pB state relaxing back to the pG dark state. This deprotonation and reisomerization is pH-dependent (r = 140 ms). It has been shown that isomerization of the deprotonated chromophore is faster than for the protonated form, therefore it has been suggested that protonation precedes isomerization. 

A similar study by Yamada et al. [13] concluded that the protein prevents the chromophore from adopting a completely planar structure. Based on their calculations they proposed that the efficiency of photoisomerization in PYP is due to the asymmetric protein-chromophore interaction that can serve as the initial accelerant for the light-induced photocycle. They also found that the C4—C7-C8-C9 dihedral always twists counterclockwise. 

The enhanced signal-to-noise ratio that is provided by resonance enhancement as well as the reduced complexity of the vibrational spectrum make it possible to perform a wide variety of time-resolved studies to determine the structure of the chromophore in the photocycle intermediates. These approaches are discussed in more detail elsewhere in this volume by Kincaid with emphasis on time-resolved Raman studies of heme proteins. Room-temperature flow methods have been extensively used to obtain time-resolved spectra with time resolution ranging from seconds to microseconds.The basic idea is to flow the sample and then introduce an optical pump beam upstream from the probe to initiate the photochemical cycle. Such experiments have been performed on the millisecond and microsecond time scales. For experiments with time resolution faster than microseconds, it is necessary to convert the setup to a two-pulse, pump-probe technique where the time resolution is established by the delay between the pump and probe laser pulses. The time resolution of this approach can be increased to around 1 psec beyond this point increased time resolution will be achieved only with reduced spectral resolution according to the uncertainty principle. 

Fig. 6 Comparison of retinal protonated Schiff base chemical shifts in rhodopsin different photocycle states. The chemical structure of retinal chromophore in 11-cw (a) and all-trans configuration (b). Chemical shifts of the retinal atoms in ground state, and Batho-, Meta I, and Meta II intermediate states (c) and a schematic drawing of the retinal binding pocket containing all residues within 4 A to the retinal and Lys296 (d). The chemical shifts are adapted from the following references [146-148] (black) [14] (blue) [149-151] (green) and [17, 145] (red), (d) is adapted from [188] with permission from the Elsevier B.V...

While the photocyllzation of 4a proceeded cleanly to afford only the desired dihydrobenzofuran products, irradiation of 4b-f did not always give complete conversion of each o-benzyloxybenzoyl chromophore. The best conversions were realized in low molecular weight oligomers. For example, photolysis of 4b resulted in complete photocyclization to afford 5b. H and NMR spectra of the photoproduct (Rgure 4) are consistent with proposed structure. A broad peak at 5.69 ppm in the... 

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