We present a compilation of spectral parameters associated with triplet–triplet absorption of organic molecules in condensed media. The wavelengths of maximum absorbance and the corresponding extinction coefficients, where known, have been critically evaluated. Other data, for example, lifetimes, energies and energy transfer rates, relevant to the triplet states of these molecules are included by way of comments but have not been subjected to a similar scrutiny. Work in the gas phase has been omitted, as have theoretical studies. We provide an introduction to triplet state processes in solution and solids, developing the conceptual background and offering an historical perspective on the detection and measurement of triplet state absorption. Techniques employed to populate the triplet state are reviewed and the various approaches to the estimation of the extinction coefficient of triplet–triplet absorption are critically discussed. A statistical analysis of the available data is presented and recommendations for a hierarchical choice of extinction coefficients are made. Data collection is expected to be complete through the end of 1984. Compound name, molecular formula and author indexes are appended.

Radiation chemical methods were used to investigate the reactions of glycine anions, H2NCH2CO2 - (Gly-), with •OH, (CH3)2C•OH, and •CH3 radicals. A major and most significant product from all of these processes is CO2. Pulse-radiolysis revealed that the initial step in the •OH-induced mechanism is oxidation of the amino group, producing +H2N•-CH2-CO2 - and HN•-CH2-CO2 - with yields of 63% and 37%, respectively. The amino radical cation, +H2N•-CH2-CO2 -, suffers fast (≤100 ns) fragmentation into CO2 + •CH2NH2. The other primary radical, HN•-CH2-CO2 -, can also be converted into the decarboxylating +H2N•-CH2-CO2 - by reaction with proton donors such as phosphate (H2PO4 -/k = 7.4 × 107 M-1 s-1, and HPO4 2-/k = 2.5 × 105 M-1 s-1) or the glycine zwitterion, Gly± (k = 3.9 × 105 M-1 s-1), but only on a much longer (typically μs to ms) time scale (k ≈ 4 × 105 M-1 s-1). Competitively, the HN•-CH2-CO2 - transforms into a carbon-centered radical H2N-C•H-CO2 - either by an intramolecular 1,2-H-atom shift (k = (1.2 ± 1.0) × 103 s-1) or by bimolecular reaction with Gly- (k = (3.0 ± 0.2) × 104 M-1 s-1). Both C-centered radicals, H2N-C•H-CO2 - and •CH2NH2, are reductants as verified through their reactions with Fe(CN)6 3- and methyl viologen (MV2+) in pulse-radiolysis experiments (k ≈ 4 × 109 M-1 s-1). The eventual complete transformation of all primary radicals into H2N-C•H-CO2 - and •CH2NH2 was further substantiated by γ-radiolytic reduction of Fe(CN)6 3-. In the presence of suitable electron donors, the HN•-CH2-CO2 - radical acts as an oxidant. This was demonstrated through its reaction with hydroquinone (k = (7.4 ± 0.5) × 107 M-1 s-1). Although the C-centered H2N-C•H-CO2 - radical is not generated in a direct H-atom abstraction by •OH, this radical appears to be the exclusive product in the reaction of Gly- with (CH3)2C•OH, •CH2NH2, and •CH3 (k ≈ 102 M-1 s-1). A most significant finding is that H2N-C•H-CO2 - can be converted into the decarboxylating N-centered radical cation +H2N•-CH2-CO2 - by reaction with proton donors such as Gly± (k ≈ 3 × 103 M-1 s-1) or phosphate and thus also becomes a source of CO2. The •CH2NH2-induced route establishes, in fact, a chain mechanism which could be proven through dose rate effect experiments and suppression of the chain upon addition of Fe(CN)6 3- or MV2+ as a scavenger for the reducing precursor radicals. The possible initiation of amino acid decarboxylation by C-centered radicals and the assistance of proton donors at various stages within the overall mechanism are considered to be of general significance and interest in chemical and biological systems.

The pathogenesis of Alzheimer's disease is strongly associated with the formation and deposition of beta-amyloid peptide (beta AP) in the brain. This peptide contains a methionine (Met) residue in the C-terminal domain, which is important for its neurotoxicity and its propensity to reduce transition metals and to form reactive oxygen species. Theoretical studies have proposed the formation of beta AP Met radical cations as intermediates, but no experimental evidence with regard to formation and reactivity of these species in beta AP is available, largely due to the insolubility of the peptide. To define the potential reactions of Met radical cations in beta AP, we have performed time-resolved UV spectroscopic and conductivity studies with small model peptides, which show for the first time that (i) Met radical cations in peptides can be stabilized through bond formation with either the oxygen or the nitrogen atoms of adjacent peptide bonds; (ii) the formation of sulfur-oxygen bonds is kinetically preferred, but on longer time scales, sulfur-oxygen bonds convert into sulfur-nitrogen bonds in a pH-dependent manner; and (iii) ultimately, sulfur-nitrogen bonded radicals may transform intramolecularly into carbon-centered radicals located on the (alpha)C moiety of the peptide backbone.