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À propos de : Time-Resolved Spectroscopy of the Excited Singlet States of Tirapazamine andDesoxytirapazamine        

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  • Time-Resolved Spectroscopy of the Excited Singlet States of Tirapazamine andDesoxytirapazamine
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  • Laser flash photolysis (LFP, 400 nm excitation) of the anti-cancer drug tirapazamine (TPZ) in acetonitrileproduces the singlet excited-state S1 with λmax = 544 nm. The lifetime of this state is 130 ps, in good agreementwith the reported fluorescence lifetime. The excited state is reduced to the corresponding radical anion byKSCN or KI. The spectrum of the radical anion is in good agreement with previously reported pulse radiolysisstudies and time-dependent density functional theory (TD-DFT) calculations. LFP of desoxytirapazamine(dTPZ) also produces the first excited singlet state, S1. The fluorescence quantum yield and lifetime (5.4 ns)of the dTPZ singlet excited state are both much greater than the corresponding values of TPZ. This is explainedby DFT calculations that predict that cyclization of TPZ to form an oxaziridine is thermodynamically facilebut that cyclization of dTPZ to form an oxadiaziridine is not. Thus, the S1 state of TPZ has a short lifetimeand low fluorescence quantum yield due to ready cyclization whereas the cyclization of the S1 state of dTPZis unimportant and does not limit either the fluorescence quantum yield or the fluorescence lifetime. Thisconclusion is confirmed by studies of dTPZ‘, an isomer of dTPZ containing the CN−O moiety which hasa low quantum yield and short fluorescence lifetime similar to that of TPZ.
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