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PHYTOCHROME MODELS, PART 9. CONFORMATION SELECTIVITY OF THE PHOTOCYCLIZATION OF THE BILIVERDIN IX γ and IX δ DIMETHYL ESTERS *
Author(s) -
Braslavsky Silvia E.,
AlEkabi Hussain,
Petrier Christian,
Schaffner Kurt
Publication year - 1985
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1985.tb03480.x
Subject(s) - chemistry , photochemistry , biliverdin , isomerization , solvent , conformational isomerism , absorption (acoustics) , absorption band , kinetic isotope effect , fluorescence , selectivity , irradiation , organic chemistry , molecule , deuterium , catalysis , heme , physics , heme oxygenase , quantum mechanics , acoustics , nuclear physics , optics , enzyme
Abstract— The photocyclization of the biliverdin IX γ and IX δ dimethyl esters to phorcabilin and neobiliverdin IX δ dimethyl ester, respectively, markedly depends on the irradiation wavelength within the first electronic absorption band. The quantum yields of reaction are highest on irradiation at575–600 nm. As judged from fluorescence excitation studies, absorption by the stretched conformations of the biliverdins is relatively important at these wavelengths. At longer wavelengths, where the reaction efficiency sharply declines, absorption by the coiled form predominates. Based on these findings and on supporting evidence from the solvent and temperature influence on the reaction, the photocyclization is proposed to occur selectively from a stretched conformation (an example of the principle of nonequilibration of excited rotamers), competing with E ‐Z isomerization of the centralC–10 double bond. A kinetic hydrogen isotope effect on the photochemical reaction rate of N,N,N‐trideuteriated biliverdin IX δ dimethyl ester indicates that the primary photoproduct in the cyclization reaction is labile and reacts via two competing processes: reopening to the starting material and a solvent‐mediated proton shift. The latter process leads to the phorcabilin and neobiliverdin IX δ esters. The results suggest, as a general rule, that the stretching of coiled biliverdins is confined to thermal and photochemical transformations within the B/C partial structure, while the Z‐syn geometry of the A/B and C/D moieties is retained.