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Abstract: The study is aimed at determining age-related changes in ocular tissues of crab-eating macaque (Macaca fascicularis). To this end, we measured the concentrations of a total of 71 major metabolites in aqueous humor, vitreous humor, and lens of two groups of animals, young (4-year-old, n= 6) and aged (21-year-old and 27-year-old, n= 2) macaques. Significant age-related changes were revealed for all three tissues. In the lens, the most significant changes were found for cytoprotective compounds – antioxidants, osmolytes, and molecular ultraviolet (UV) filters: the concentrations of these metabolites in the lenses of aged animals are much lower. The observed changes contribute to increased oxidative stress and predispose the lens to the development of cataracts. The majority of cytoprotective metabolites are synthetized in the lens epithelium. Findings of this work indicate that the observed age-related changes may be caused by the impairment of the lens epithelial cells leading to the increase of oxidative stress in the lens nucleus and developing of age-related cataracts.

A combination of time-resolved electron paramagnetic resonance (TREPR) and laser flash photolysis (LFP) studies of flexible acyl-containing biradicals over a wide temperature range is reported. In contrast to previous reports, it is shown that the main channel of intersystem crossing in these biradicals is the electron spin relaxation of the acyl moiety rather than spin−orbit interaction in the biradical. This relaxation determines the decay rate of the electron spin polarization at low temperatures and the biradical lifetime at high temperatures. The relaxation mechanism is attributed to the spin-rotation interaction, associated with the rotation of the carbonyl group about the neighboring C−C bond axis. From a model simulation of the time profile of the spin-polarized TREPR signal based on the numerical solution of the stochastic Liouville equation of the spin density matrix in frame of realistic model of biradical, the Arrhenius parameters for correlation times of spin rotation interaction and activation energies for molecular and spin dynamics were determined in two solvents, 2-propanol and hexane.

Time-resolved chemically induced dynamic nuclear polarization (CIDNP) and laser flash photolysis (LFP) techniques have been used to study the kinetics and mechanism of the photochemical reaction between 2,2‘-dipyridyl (DP) and the dipeptide l-tryptophan-l-tyrosine (TrpH-TyrOH) in acidic aqueous solution (pH = 3.8). Analysis of the geminate CIDNP pattern reveals that the quenching of the protonated triplet dipyridyl ^TDPH⁺ results in the formation of both tryptophan and tyrosine radicals from the dipeptide with comparative efficiency. The total quenching rate constant of triplet dipyridyl by TrpH-TyrOH was found to be (2.5 ± 0.3) × 10⁹ M^-1 s^-1. The radical transformation TrpH^+•→TyrO^• via intramolecular electron transfer (IET) leads to an increasing tyrosyl radical concentration, growth of the tyrosine CIDNP signals, fast decay of the CIDNP signal of TrpH, and inversion of the CIDNP sign from emission to enhanced absorption for DP. The nuclear spin−lattice relaxation times of the radicals formed in the reactions and the rate constant for IET (5 × 10⁵s^-1) were determined by quantitative analysis of the CIDNP kinetics at different dipeptide concentrations in D2O. A significant isotope effect (kH/kD = 1.5) was found for the IET rate constant by LFP measurements. In the presence of efficient IET, degenerate electron exchange between the TrpH^•+ radical of the dipeptide and the diamagnetic molecule makes a negligible contribution to the decay of tryptophan CIDNP signal.

The reaction mechanism and details of the formation of CIDNP (chemically induced dynamic nuclear polarization) in the photoreactions of 2,2‘-dipyridyl (DP) and N-acetyl histidine (HisH) in aqueous solution have been studied using laser flash photolysis and time-resolved CIDNP techniques. The triplet state ^TDP reacts with protonated HisH2⁺ via hydrogen atom transfer with a rate constant kH = 1.2 × 10⁸ M^-1 s^-1, and with deprotonated His^- via electron transfer with ke = 7.5 × 10⁹ M^-1 s^-1. No reaction occurs when the histidine imidazole ring is in its neutral state HisH, or when the dipyridyl triplet is protonated, ^TDPH⁺. The nuclear spin−lattice relaxation times in the radicals formed in these reactions have been determined from the CIDNP kinetics:  T1 = 44 ± 9 μs for all DP protons, T1 = 196 ± 25 μs for the β-CH2 protons of HisH, and T1 = 16 ± 5 μs for the H-2 and H-4 protons of HisH. Under strongly basic conditions the CIDNP is greatly affected by degenerate electron exchange between the neutral His· radical and His^- anion, with rate constant kex = 1.5 × 10⁸ M^-1 s^-1.

The mechanism of the reactions between photoexcited 2,2‘-dipyridyl and N-acetyl tryptophan has been studied by laser flash photolysis and time-resolved CIDNP (Chemically Induced Dynamic Nuclear Polarization). The transient absorption spectra obtained at different delays after the laser pulse are attributed to the triplet state of dipyridyl and to dipyridyl and tryptophan radicals. Depending on the pH of the solution, all three intermediates can be present in either protonated or deprotonated forms. It is shown that irrespective of pH the primary photochemical step is electron transfer from the tryptophan to triplet dipyridyl followed by protonation/deprotonation of the radicals so formed. The rate constant of the reaction of triplet dipyridyl with tryptophan is close to the diffusion-controlled limit and decreases slightly with increasing pH. The kinetics and the stationary value of the CIDNP are determined by the rates of radical termination, nuclear paramagnetic relaxation, and degenerate electron exchange. The last reaction is important for the protonated tryptophan radical and determines the CIDNP kinetics of tryptophan in acidic conditions. The nuclear relaxation times estimated from the CIDNP kinetics are 44 ± 9 μs for all protons in the dipyridyl radical, 91 ± 20 μs for the β-CH2, 44 ± 9 μs for H2,6, and 63 ± 12 μs for H4 aromatic protons in the tryptophan radical.

Low- and high-field ¹H chemically induced dynamic nuclear polarizations (CIDNP) have been applied to investigate geminate reactions of consecutive biradicals generated during the photolysis of 2,12-dihydroxy-2,12-dimethylcyclododecanone. The detailed mechanism of the reaction is established. It is shown that the fast chemical transformation (decarbonylation) of the primary acyl-ketyl biradical manifests itself in CIDNP effects in low and high magnetic fields:  the difference in the CIDNP field dependencies for the products of the primary and secondary biradicals is observed, and the net and multiplet effects for the products of the secondary bis-ketyl biradical develop in different time scales. The first observation of the CIDNP “memory effect” in biradical reactions is reported. Model calculations of the CIDNP kinetics based on the numerical solution of the stochastic Liouville equation for the Fourier transform of the biradical spin density matrix have been performed, and a good agreement between the theoretical calculations and experimental data has been found.

The transient absorption spectrum of the tert-butoxyl radical in the UV region was obtained by the laser flash photolysis technique. The rate constants for β-scission and self-termination reactions of tert-butoxyl radicals were measured in five solvents; the Arrhenius parameters of the rate constant for β-scission kβ were determined. It was shown that both the solvent polarity and ability for hydrogen bonding accelerate the reaction of β-scission. The solvent effect on the rate constant of the β-scission reaction is discussed in terms of a simple Onzager−Betcher model, a point dipole model, and a model of the H-bonded complex of the radical with the solvent molecule.

Several time-resolved spectroscopic techniques (¹H CIDNP, time-resolved EPR, and nanosecond laser flash photolysis) have been used to investigate geminate reactions of consecutive biradicals generated from photolysis of 2-hydroxy-2,12-dimethylcyclododecanone (1). It is shown that the primary photochemical process is Norrish type I cleavage, yielding an acyl−ketyl biradical, which is transformed to an alkyl−ketyl biradical via decarbonylation. The rate constant for decarbonylation, estimated by theoretical modeling of the TR-CIDNP kinetics, is (4.2 ± 1.0) × 10⁵ s^-1. The calculations are based on the numerical solution of the stochastic Liouville equation for the Fourier transform of the biradical spin density matrix. The lifetime of the acyl−ketyl biradical (τ = 67 ± 8 ns) has been measured using laser flash photolysis. The distribution of reaction products in the photolysis of 1, including recombination to the starting ketone, was determined from the CIDNP amplitude and kinetics. The reaction products include unstable compounds containing enol functional groups, which are transformed to the keto form during the dark reaction of tautomerization.

Laser flash photolysis (LFP), low-field chemically induced dynamic nuclear polarization (CIDNP), and time-resolved electron paramagnetic resonance (TREPR) techniques have been used for the comparative study of magnetic field and spin effects in acyl−ketyl and bis(ketyl)biradicals formed during the photolysis of 2-hydroxy-2,12-dimethylcyclododecanone (1-OH) and 2,12-dihydroxy-2,12-dimethylcyclododecanone (2-OH), respectively. The short biradical lifetime, the small magnetic field effect (MFE) on a biradical lifetime, and the low intensity of spin-correlated radical pair (SCRP) polarization observed during the photolysis of 1-OH indicate that the main channel of intersystem crossing in acyl−ketyl biradical is spin−orbit coupling (SOC). For bis(ketyl) biradicals observed during the photolysis of 2-OH, SOC is of minor importance, and both MFE and SCRP polarization are much larger. It is shown that the presence of significant SOC in biradicals can result in an increase of the CIDNP intensity at low magnetic fields. Calculations of biradical evolution, based on the numerical solution of the stochastic Liouville equation (SLE), were carried out by taking into account (1) the molecular dynamics of the polymethylene chain linking the radical centers, (2) the distance-dependent exchange interaction, (3) the chemical reactions of the biradical, (4) the spin relaxation processes, and (5) state-to-state transitions caused by hyperfine interaction and SOC. A common set of parameters was found which allows quantitative description of the field dependence of the biradical lifetime, the CIDNP field dependence, and the TREPR spectra and kinetics.

CIDNP effects arising in the geminate evolution of consecutive biradicals formed during the photolysis of 2,2,12,12-tetramethylcyclododecanone were investigated by the low- and high-field¹H and¹³C CIDNP and time-resolved¹H CIDNP methods. The¹H field dependence of nuclear polarization induced in the recombination and disproportionation products of the consecutive biradicals as well as the¹³C CIDNP spectra at low and high magnetic fields was analyzed. It was confirmed that two types of biradicals (acyl−alkyl and bisalkyl) of different chain lengths contribute to the spin polarization. Model calculations of the geminate recombination kinetics were based on the numerical solution of the stochastic Liouville equation for the Fourier transform of the biradical spin density matrix. A detailed kinetic analysis of the CIDNP effects allowed us to reveal all chemical transformations of the alkyl−acyl and bisalkyl biradicals and to estimate the probability of the “hidden” recombination of the biradicals to the starting ketone, as well as to determine the product distribution.