Time-Resolved FT EPR and Optical Spectroscopy Study on Photooxidation of Aliphatic r-Amino Acids in Aqueous Solutions; Electron Transfer from Amino vs Carboxylate Functional Group Peter Tara ´ bek, † Marija Bonifac ˇ ic ´ ,* ,‡ and Dieter Beckert* ,† Interdisciplinary Research Group “Time-ResolVed Spectroscopy”, Faculty of Chemistry and Mineralogy, UniVersity of Leipzig, Permoserstrasse 15, Leipzig, D-04318, Germany, and Department of Physical Chemistry, Rud - er Bos ˇkoVic ´ Institute, Bijenic ˇ ka c. 54, HR-10 000 Zagreb, Croatia ReceiVed: February 6, 2006; In Final Form: March 23, 2006 Using time-resolved Fourier transform electron paramagnetic resonance, FT EPR, and optical spectroscopy, the photooxidation of glycine, R-alanine, R-aminoisobutyric acid, and model compounds -alanine, methylamine and sodium acetate, by excited triplets of anthraquinone-2,6-disulfonate dianion was studied in aqueous solutions in the pH range 5-13. Anthraquinone radical trianions showing strong emissive spin-polarization (CIDEP) were formed, indicating fast electron transfer from the quenchers to the spin-polarized quinone triplet as the primary reaction. None of the primary radicals formed upon one-electron oxidation of quenchers could be detected at the nanosecond time scale of FT EPR measurements because of their very fast transformation into secondary products. The latter were identified to be decarboxylated R-aminoalkyl radicals for R-amino acids anions and zwitterions, -aminoalkyl radicals for -alanine zwitterions, and methyl radicals for acetate anions; corresponding aminyl radicals were the first EPR detectable products from -alanine anions and methylamine. Thus, anthraquinone-2,6-disulfonate triplet can take an electron from both NH 2 - and -CO 2 - functional groups forming aminium ( +• NH 2 -) and acyloxyl (-CO 2 • ) radicals, respectively. Aminium radicals derived from -alanine anions and CH 3 -NH 2 stabilize by deprotonation into aminyl radicals, whereas these derived from R-amino acids anions are known to suffer ultrafast decarboxylation (τ ∼ 10 ps). Analysis of the polarization patterns revealed that decarboxylation from acyloxyl radicals are considerably slower (ns < τ < 0.1 µs). Therefore, in the case of R-amino acids, the isoelectronic structures NH 2 -CR 2 -CO 2 • and +• NH 2 - CR 2 -CO 2 - probably do not constitute resonance mesomeric forms of one and the same species and the decarboxylation of aminium radicals is not preceded by the intramolecular carboxylate to amino group electron transfer. Absolute triplet quenching rate constants at zero ionic strength were in the range of 2 × 10 8 to 2 × 10 9 M -1 s -1 for R-NH 2 and 2 × 10 7 to 10 8 M -1 s -1 for R-CO 2 - type of electron donors, reflecting in principle their standard reduction potentials. The strengths of acids: + NH 3 - • CH 2 , + NH 3 - • C(CH 3 )H, and + NH 3 - • C(CH 3 ) 2 ,pK a <4, >6, and >7, respectively, were found to be remarkably strongly dependent on R-C substitution. The conjugate bases of these R-aminoalkyl radicals reduce anthraquinone-2,6-disulfonate dianion ground state with k sec ) 3 × 10 9 M -1 s -1 . 1. Introduction In the last years a rapid increase of interest is observable in oxidative damage of proteins and its relevance to pathological disorders and aging (for recent reviews see, for example, refs 1-3). Amino acid side chains are very important sites of oxidation agents attack at proteins. Therefore, detailed investiga- tions of reaction mechanisms and properties of amino acid and peptide derived radicals are imperative for better understanding of processes leading to the damage. Radicals of simple aliphatic R-amino acids and small peptides in aqueous solutions have been produced under various conditions and characterized by steady-state and time-resolved EPR and pulse radiolysis tech- niques, including initially produced radicals and these formed by the primary radicals subsequent reactions. 4-19 The main reactions and radical products of amino acid anions are exemplary shown for glycine in Scheme 1. The actual mech- anism and the fraction of the three possible primary products, aminium radical zwitterion +• NH 2 -CH 2 -CO 2 - , aminyl radical anion • NH-CH 2 -CO 2 - , and glycyl radical anion NH 2 - • CH- CO 2 - , are dependent on the nature of the attacking species. The one-electron transfer from the nonbonding electron pair on the nitrogen under formation of +• NH 2 -CH 2 -CO 2 - is a favorable mechanism for the reaction with excited molecules, 14,16 SO 4 •- , 6 and hydroxyl radicals, the latter being also able to directly abstract H atom from N-H and C-H groups, Scheme 1, to an appreciable extent. 11;15;17;18 The aminium radical, which is isoelectronic with the acyloxyl radical NH 2 -CH 2 -CO 2 • , was observed only in the X-ray irradiated single-crystal of glycine at low temperature 20;21 and was decomposing on warming by decarboxylation. In aqueous solutions, the decarboxylation, definitely the most dramatic consequence of an oxidative attack, occurs by an ultrafast rate constant of about 10 11 s -1 14 under formation of the reducing R-aminomethyl radical, NH 2 - • CH 2 . Deprotonation of the aminium radical to the aminyl form could be considered competitive with decarboxylation, at least for * Corresponding authors. E-mail: (D.B.) beckert@mpgag.uni-leipzig.de; (M.B.) bonifacic@irb.hr. † Interdisciplinary Research Group “Time-Resolved Spectroscopy”, Faculty of Chemistry and Mineralogy, University of Leipzig. ‡ Department of Physical Chemistry, Rud - er Bos ˇkovic ´ Institute. 7293 J. Phys. Chem. A 2006, 110, 7293-7302 10.1021/jp060764m CCC: $33.50 © 2006 American Chemical Society Published on Web 05/06/2006