Photochemical Isomerization of Colchicine and Thiocolchicine

Year: 2003

Authors: Bussotti L., Cacelli I., D\’Auria M., Foggi P., Lesma G., Silvani A., Villani V.

Autors Affiliation: LENS, Università di Firenze, Via N. Carrara 1, 50019 Sesto Fiorentino, Firenze, Italy; Dipartimento di Chimica, Università di Pisa, Via Risorgimento 35, 56126 Pisa, Italy; Dipartimento di Chimica, Università della Basilicata, Via N. Sauro 85, 85100 Potenza, Italy; Dipartimento di Chimica, Università di Perugia, via Elce di sotto 8, 06123 Perugia, Italy; Departimento di Chimica Organica, Università di Milano, Via Venezian 21, 20133 Milano, Italy

Abstract: The photochemical reactivity of colchicine and thiocolchicine is described. Although the irradiation of colchicine gave a well-known transposition reaction to ß- and ?-lumicolchicines, thiocolchicine did not react. Femtosecond transient spectroscopy of colchicine showed a strong band with maximum at 510 nm appearing at t = 0. It disappeared within few hundred femtoseconds, leaving a broad structureless band with a maximum around 470 nm. A second band is observed around 410 nm. The analysis in time showed that the 510-nm component appeared instantaneously and decayed following a biexponential low with time constants of 300 ± 100 fs and 40 ps. The kinetics at 420 nm has a measurable rise time of 300 ± 150 fs. Quantum mechanical calculations on colchicine showed that this absorption is due to a S 1 ? S11 transition. In thiocolchicine, the instantaneous formation of a structure with maxima out of the investigated spectral region was observed. A strong absorption around 650 nm indicated the presence of a band with a maximum at longer wavelengths (> 700 nm) and a peak around 380 nm, which partially coincides with the ground-state absorption and therefore strongly affected by its bleaching. The instantaneous formation of an absorption around 650 nm and its rapid (~500 fs) decay was observed. At shorter wavelengths (400 nm), the decay was fitted with a biexponential curve with the first time constant of about 80 ps. The second part of the decay had a very long tail up to 500 ps. Transient spectroscopy and configuration interaction calculations are in agreement with a mechanism involving a disrotatory cyclization of colchicine in its first excited singlet state. The lack of reactivity observed in thiocolchicine was explained by considering the presence of efficient ISC to the triplet state.

Journal/Review: JOURNAL OF PHYSICAL CHEMISTRY A

Volume: 107 (43)      Pages from: 9079  to: 9085

KeyWords: Absorption spectroscopy; Aromatic compounds; Electron transitions; Ground state; Isomerization; Photochemical reactions; Quantum theory; Reaction kinetics, Femtosecond transient spectroscopy; Photoisomerization, Drug products
DOI: 10.1021/jp035507l

Citations: 13
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