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Does the Photochemical Conversion of Colchicine into Lumicolchicines Involve Triplet Transients? A Solvent Dependence Study ¶
Author(s) -
Nery Ana L. P.,
Quina Frank H.,
Moreira Paulo F.,
Medeiros Carlos E. R.,
Baader Wilhelm J.,
Shimizu Karina,
Catalani Luiz H.,
Bechara Etelvino J. H.
Publication year - 2001
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 - 10.1562/0031-8655(2001)0730213dtpcoc2.0.co2
Subject(s) - photochemistry , chemistry , triplet state , flash photolysis , tropolone , singlet oxygen , photoisomerization , quantum yield , solvent , excited state , cycloisomerization , quenching (fluorescence) , acetone , singlet state , oxygen , fluorescence , kinetics , organic chemistry , isomerization , catalysis , molecule , physics , quantum mechanics , nuclear physics , reaction rate constant
ABSTRACT β‐ and γ‐lumicolchicines are photoproducts formed by the cycloisomerization of the tropolone ring of colchicine (COL) alkaloids. The mechanism of the photoconversion, suggested to involve the triplet state, is examined here by studying the effect of the solvent polarity on the lumicolchicine photoisomer ratio. Triplet COL, detected by laser flash photolysis, is quenched by oxygen, but not by trans ‐stilbene or 1‐methylnaphtalene. Neither the quantum yield of conversion of COL nor the photoproduct ratio was altered by the presence of oxygen. Likewise, energy transfer to COL from triplet acetone produced by either isobutanal/horseradish peroxidase system or tetramethyldioxetane thermolysis failed to provoke photoreaction of COL. Our data argue against the intermediacy of a COL triplet state in the photoisomerization and stress on the role of specific solvent–solute interactions in determining the partitioning of excited singlet state into the β‐ and γ‐isomer formation.