Re: In 13 and P56
From: Bobby Redmomd
Dear Dr. Gál,
Thank you for the comments and questions (P56) regarding our paper (IN13). It is refreshing to note that it stimulated a response! We will try to answer the questions you listed in your poster below.
(1) These free radical sources do not fall under Foote's type I definition, which deals with bimolecular sensitization reactions. The initiation rate for these compounds, i.e. the primary photolysis of the N-O bond, is very rapid, within the duration of the 10 ns laser pulse of our flash photolysis apparatus. The quantum yield of N-O cleavage is ~ 0.5 for the esters and 0.35 for the thione. Additionally, for the Barton esters 2a-2c, the decarboxylation rate of the acyloxy radicals to give the corresponding carbon-centered radicals is such that it cannot be resolved on our apparatus. Thus, for all intents and purposes the carbon-centered radical is produced "immediately". Compound 2d is the only one where the acyloxy radical is sufficiently long-lived for detection (300 ns lifetime) and in this case the potential reactivity of both R and RCOO radicals must be taken into consideration.
(2) You are right that the R radicals react efficiently with oxygen to give peroxyl radicals. We discussed this point in detail in the paper (see equations 4 and 5 and related text). We also discussed the potential reactivity of R and ROO radicals in the cellular system. The ultimate radical reactivity pattern (i.e. how much is due to R and how much is due to ROO) will be determined by the LOCAL concentration of oxygen and other substrates (e.g. unsaturated lipids) in proximity to the localized precursor. As we mentioned in the paper, we plan to carry out laser flash photolysis studies to investigate the kinetics of R generated in cells under the same irradiation conditions used for the phototoxicity studies. Kinetics will be determined as a function of equilibrated oxygen concentration to answer the question of which radicals are responsible for the biological response.
(3) Triplet is only seen for compound 1 and not for any of the esters. The quantum yield is 0.02. The N-O bond cleavage results in hydroxyl radical generation with a yield of 0.35. The triplet absorption is weak and in the same spectral region as the thiopyridyl radical which is concomitantly produced. The overlapping absorption complicates analysis but it is clear that the triplet lifetime is short, 0.5 - 1 µs. We have no data as to whether the triplet is quenched by the radical (as in your triplet-doublet mechanism) as both species are always produced together. Given the short lifetime of the triplet and the low concentrations involved I would think it might be unlikely. It might be possible to probe the possibility of the reaction generate the triplet of 1 by triplet photosensitization and compare the lifetimes obtained in this manner with that from direct photolysis of 1, providing the triplet of 1 does not react with the photosensitizer used.
The data given above are from recent publications on the photochemistry of these compounds, given below. Waldemar Adam's group in Germany also have very interesting results using free radical sources.
I thank you again for your interesting response and we am now off to the library to pick up your publications on the type III mechanism!!
Bobby Redmond Béatrice Aveline
B.M. Aveline, I.E. Kochevar, R.W. Redmond, Photochemistry of N-hydroxypyridine-2-thione derivatives: Involvement of the 2-pyridylthiyl radical in the radical chain reaction mechanism, J. Am. Chem. Soc. 117 (1995) 9699-9708. B.M. Aveline, I.E. Kochevar, R.W. Redmond, N-hydroxypyridine-2(1H)-thione: Not a selective generator of hydroxyl radicals in aqueous solution, J. Am. Chem. Soc. 118 (1996) 289-290. B.M. Aveline, I.E. Kochevar, R.W. Redmond, Photochemistry of the nonspecific hydroxyl radical generator, N-hydroxypyridine-2(1H)-thione, J. Am. Chem. Soc. 118 (1996) 10113-10123. B.M. Aveline, I.E. Kochevar, R.W. Redmond, Photochemistry of N-hydroxy-2(1H)-pyridone, a more selective source of hydroxyl radicals than N-hydroxypyridine-2(1H)-thione, J. Am. Chem. Soc. 118 (1996) 10124-10133.