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) and ketyl radical anions (3-CB−) 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∴N and S∴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−) and its ketyl radical (3-CBH). Rate constants were measured for the protonation of 3-CB− by water ( k H 2 O = 5.1 × 10 2  M − 1  s − 1 ) and by H+ ( k H + = 6.4 × 10 10  M − 1  s − 1 ). In addition, the acid–base equilibrium constant Ka of 3-CBH was characterized by a pKa = 9.5 (±0.1). This Ka value is more than one order of magnitude smaller than the literature value of Ka for 4-CBH (pKa = 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 pKa value, rate constants for the OH−-assisted deprotonation ( k ( 3 -CBH + HO − ) = 8.9 × 10 8  M − 1  s − 1 ) and the spontaneous dissociation ( k ( 3 -CBH   dissociation ) = 20  s − 1 ) of 3-CBH were calculated. Moreover, molar absorption coefficients (ɛ) for all of the intermediates derived from 3-CB were measured by pulse radiolysis and laser flash photolysis methods. They were found to be: ɛ550(3-CBH) = 2700 (±200) M−1 cm−1, ɛ600(3-CB−) = 5200 (±300) M−1 cm−1, and ɛ520(33-CB*) = 5400 (±300) M−1 cm−1.