3-Carboxybenzophenone (3-CB) as an efficient sensitizer in the photooxidation of methionyl-leucine in aqueous solutions: Spectral, kinetic and acid-base properties of 3-CB derived transients

3-Carboxybenzophenone (3-CB) was successfully employed in the photooxidation of a selected methionine derivative: methionyl-leucine (Met-Leu). It is also demonstrated that this photosensitizer has an advantage over its isomer, 4-carboxybenzophenone (4-CB), which is a commonly used water-soluble organic photosensitizer. The relative advantage of 3-CB over 4-CB lies in fact that the 3-CB derived transients have less absorption than the corresponding photoinduced transients from 4-CB. In particular ketyl radicals (3-CBH center dot) and ketyl radical anions (3-CB center dot-) are substantially less absorbing in the visible and near UV regions (i.e. 370-420 nm) than are the respective transients derived from 4-CB. This feature is very important since it allows a better sampling of the absorption spectra of three-electron-bonded radicals S therefore N and S therefore O resulting from the sensitized photooxidation of methionine derivatives by triplet 3-CB acting as an electron acceptor. This advantage of 3-CB over 4-CB was demonstrated during spectral resolutions of transient absorption spectra formed during the 3-CB-sensitized photooxidation of Met-Leu. Since the kinetic and spectral characteristics of free radicals and radical ions derived from 3-CB were not determined previously, pulse radiolysis was employed to determine molar absorption coefficients and selected rate constants for the 3-CB radical anion (3-CB center dot-) and its ketyl radical (3-CBH center dot). Rate constants were measured for the protonation of 3-CB center dot- by water (k(H2O) = 5.1 x 10(2) M-1 s(-1)) and by H+(k(H+) = 6.4 x 10(10) M-1 s(-1)). In addition, the acid-base equilibrium constant K-a of 3-CBH center dot was characterized by a pK(a) = 9.5 (+/- 0.1). This Ka value is more than one order of magnitude smaller than the literature value of K-a for 4-CBH center dot (pK(a) = 8.2). This observation was interpreted in terms of the difference in electron densities on the carbonyl oxygen atoms in these ketyl radicals, confirmed by DFT (PBE1PBE) calculations and based on a thermodynamic cycle of the protonation process. Using the newly determined pK(a) value, rate constants for the OH--assisted deprotonation (k((3-CBH center dot+HO-)) = 8.9 x 10(8) M-1 s(-1)) and the spontaneous dissociation (k((3-CBH center dot dissociation)) = 20 s(-1)) of 3-CBH center dot were calculated. Moreover, molar absorption coefficients (epsilon) for all of the intermediates derived from 3-CB were measured by pulse radiolysis and laser flash photolysis methods. They were found to be: epsilon(550)(3-CBH center dot) = 2700 (+/- 1200) M-1 cm(-1), epsilon(600)(3-CB center dot-)= 5200 (+/- 300) M-1 cm(-1), and epsilon(520)((3)3-CB*) = 5400 (+/- 300) M-1 cm(-1). (C) 2014 Elsevier B.V. All rights reserved.