Studies of all-trans-retinal as a photooxidizing agent

The photophysical properties of all-trans-retinal (RAL) have been extensively studied because of the importance of the retinoids in the visual process. However, little information is available regarding the participation of RAL in photochemical transformations such as photoxidation. RAL is one of several native chromophores that have been suggested to act as photosensitizers of damage in the human retina, and this damage would likely occur through oxidative pathways. Time-resolved and steady state techniques have been used to examine the photoreactivity of RAL toward several suitable substrates. The lifetime of the RAL triplet excited state is observed to decrease with increasing concentration of the well-known electron and hydrogen atom donors, 2,3,5,6-tetramethyl-1,4-phenylenediamine (DAD), hydroquinone (HQ), methylhydroquinone (MHQ), 2,3-dimethylhydroquinone (DMHQ) and trimethylhydroquinone (TMHQ), although the bimolecular rate constants for the reaction are much less than that of diffusion controlled (2.9 x 10(7) M-1 s-1, 1.2 x 10(5) M-1 s-1, 1.2 x 10(5) M-1 s-1, 1.5 x 10(5) M-1 s-1 and 1.6 x 10(6) M-1 s-1, for DAD, HQ, MHQ, DMHQ and TMHQ, respectively). In the presence of the donors, new absorptions grow concomitant with the decay of the triplet excited state, and for DAD and TMHQ, the observed spectra are similar to the spectra of p-phenylenediamine and TMHQ radicals. Irradiation of RAL in argon-saturated methanol results in fairly efficient photobleaching of RAL and in the formation of two new compounds having absorption spectra that are shifted below 300 nm. Irradiation of RAL in argon-saturated acetonitrile also results in photobleaching of RAL, but the reaction proceeds at a slower rate.     

SINGLET AND TRIPLET REACTIVITY IN THE PHOTOREDUCTION OF OXONINE(3,7-DIAMINOPHENOXAZIN-5-IUM CHLORIDE) BY IRON (II)

The oxazine dye, oxonine (3,7-diaminophenoxazin-5-ium chloride), 1, is photoreduced by Fe (II) sulfate in dilute sulfuric acid. The reaction mechanism is analogous to that for the photo-reduction of thiazine dyes by Fe (II), the most important difference being that reduction of oxonine occurs predominantly from its excited singlet state, S1, rather than from the triplet state, T1. The latter is formed with an intersystem crossing (isc) quantum yield of ca 1.7 x 10(-3). The quenching of S1 by Fe (II) has a rate constant kSQ = 2.2 +/- 0.1 x 10(9) M-1 s-1 and affords the one electron reduced product, semioxonine (R), with a limiting quantum yield, phi SR, of 0.26 +/- 0.02. In contrast, quenching of T1, generated by bromide ion quenching of S1 or by diacetyl sensitization, occurs with KTQ approximately 1.2 x 10(6) M-1 s-1, extrapolated to zero ionic strength, and affords R with a limiting probability, phi TR = 1.1 +/- 0.2. Three possible reasons for the lower quantum yield of the more exothermic S1 reduction are discussed. These are energy transfer from S1 to Fe (II), different rates of escape of R from the encounter complex as a consequence of the different states of protonation of R as initially formed from S1 and T1, and spin allowed back electron transfer in an exciplex formed between S1 and Fe (II). Evidence is also presented for a very low probability (ca 1%) induced isc from the encounter of S1 with paramagnetic Fe (II). Rate parameters for other processes important to the overall reduction mechanism such as disproportionation of R to leucooxonine L and oxonine, k(R)DIS = 1.7 +/- 0.2 x 10(9) M-1 s-1, oxidation of R by Fe (III), k(R)OX = 1.5 +/- 0.1 x 10(5) M-1 s-1, and oxidation of L by Fe (III), kLOX = 1.1 +/- 0.1 x 10(3) M-1 s-1, have also been measured. These results are contrasted with those for the closely related thionine/Fe(II) photoredox reaction, the most well understood system for photogalvanic energy conversion.     

Environmental effects on the photochemistry of A2-E, a component of human retinal lipofuscin

Several retinal dystrophies are associated with the accumulation of lipofuscin, a pigment mixture, in the retinal pigment epithelium (RPE). One of the major fluorophores of this mixture has been identified as the bis-retinoid pyridinium compound, A2-E. Because this compound absorbs incident radiation that is transmitted by the anterior segment of the human eye, photophysical and photochemical studies were performed to determine if A2-E could photosensitize potentially damaging reactions. Steady-state fluorescence measurements indicate that the fluorescence emission maximum and quantum yield are very sensitive to the chemical environment and a correlation between these two parameters and the solvent dielectric constant is observed. Time-resolved absorption experiments of A2-E in pure organic solvents showed no formation of transient species on the timescale of our experiments. However, when these measurements were repeated for A2-E in Triton X-100 micelles, a short-lived (tau approximately 14 microseconds), weak absorption was observed. This species is quenched by oxygen (k = 2 x 10(9) M-1 s-1) and by the addition of the antioxidants, cysteine and N,N,N',N'-tetramethylphenylenediamine. Quenching of this species by 2,3,5-trimethylhydroquinone results in the formation of the 2,3,5-trimethylsemiquinone free radical and an increase in yield of the A2-E-derived species. Sensitization of the A2-E triplet excited state indicates that the species observed in micelles upon direct excitation is not consistent with the triplet excited state. Based on these data we tentatively assign this absorption to a free radical. In the RPE these initial processes can ultimately lead to damage to the tissue through the formation of peroxides and other oxidized species.     

Reactions of 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonate) dianion, ABTS2-, with *OH, (SCN)2*-, and glycine or valine peroxyl radicals

ABTS2-, 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonate) dianion, was used as a reference to compare the reactivity of peroxyl radicals of two amino acids, glycine and valine, in aqueous solutions at natural pH. Peroxyl radicals were produced by pulse radiolysis and the product of their reaction with ABTS2- the ABTS*- radical was observed spectrophotometrically. Experimental kinetic traces were fitted using chemical simulation. The rate constants of reactions of glycine and valine peroxyl radicals with ABTS2- were (6.0+/-0.2)x10(6) and (1.3+/-0.1)x10(5) M-1.s-1, respectively. Moreover, it was found that only 60% of glycine radicals formed upon its reaction with *OH radicals reacted with molecular oxygen to yield peroxyl radicals. Comparison of experimental data with simulations of chemical reactions in irradiated ABTS and ABTS/NaSCN solutions showed that ABTS*- forms in the reaction with *OH with a yield of 43% and rate constant of (5.4+/-0.2)x10(9) M-1.s-1 and in the reaction with (SCN)2*- with a yield of 57% and rate constant of (8.0+/-0.2)x10(8) M-1.s-1.     

Kinetic and thermodynamic properties of the aminoxyl (NH2O*) radical

The product of one-electron oxidation of (or H-atom abstraction from) hydroxylamine is the H2NO* radical. H2NO* is a weak acid and deprotonates to form HNO-*; the pKa(H2NO*) value is 12.6+/-0.3. Irrespective of the protonation state, the second-order recombination of the aminoxyl radical yields N2 as the sole nitrogen-containing product. The following rate constants were determined: kr(2H2NO*)=1.4x10(8) M-1 s-1, kr(H2NO*+HNO-*)=2.5x10(9) M-1 s-1, and kr(2HNO-*)=4.5x10(8) M-1 s-1. The HNO-* radical reacts with O2 in an electron-transfer reaction to yield nitroxyl (HNO) and superoxide (O2-*), with a rate constant of ke(HNO-*+O2-->HNO+O2-*)=2.2x10(8) M-1 s-1. Both O2 and O2-* seem to react with deprotonated hydroxylamine (H2NO-) to set up an autoxidative chain reaction. However, closer analysis indicates that these reactions might not occur directly but are probably mediated by transition-metal ions, even in the presence of chelators, such as ethylenediamine tetraacetic acid (EDTA) or diethylenetriamine pentaacetic acid (DTPA). The following standard aqueous reduction potentials were derived: E degrees (H2NO*,2H+/H3NOH+)=1.25+/-0.01 V; E degrees (H2NO*,H+/H2NOH)=0.90+/-0.01 V; and E degrees (H2NO*/H2NO-)=0.09+/-0.01 V. In addition, we estimate the following: E degrees (H2NOH+*/H2NOH)=1.3+/-0.1 V, E degrees (HNO, H+/H2NO*)=0.52+/-0.05 V, and E degrees (HNO/HNO-*)=-0.22+/-0.05 V. From the data, we also estimate the gaseous O-H and N-H bond dissociation enthalpy (BDE) values in H2NOH, with BDE(H2NO-H)=75-77 kcal/mol and BDE(H-NHOH)=81-82 kcal/mol. These values are in good agreement with quantum chemical computations.     

Kinetic origin of the chelate effect. Base hydrolysis, H-exchange reactivity, and structures of syn,anti-[Co(cyclen)(NH3)2]3+ and syn,anti-[Co(cyclen)(diamine)]3+ ions (diamine = H2N(CH2)2NH2, H2N(CH2)3NH2)

The synthesis of syn,anti-[Co(cyclen)en](ClO4)3 (1(ClO4)3) and syn,anti-[Co(cyclen)tn](ClO4)3 (2(ClO4)3) is reported, as are single-crystal X-ray structures for syn,anti-[Co(cyclen)(NH3)2](ClO4)3 (3(ClO4)3). 3(ClO4)3: orthorhombic, Pnma, a = 17.805(4) A, b = 12.123(3) A, c = 9.493(2) A, alpha = beta = gamma = 90 degrees, Z = 4, R1 = 0.030. 1(ClO4)3: monoclinic, P2(1)/n, a = 8.892(2) A, b = 15.285(3) A, c = 15.466(3) A, alpha = 90 degrees, beta = 91.05(3) degrees, gamma = 90 degrees, Z = 4, R1 = 0.0657. 2Br3: orthorhombic, Pca2(1) a = 14.170(4) A, b = 10.623(3) A, c = 12.362(4) A, alpha = beta = gamma = 90 degrees, Z = 4, R1 = 0.0289. Rate constants for H/D exchange (D2O, I = 1.0 M, NaClO4, 25 degrees C) of the syn and anti NH protons (rate law: kobs = ko + kH[OD-]) and the apical NH, and the NH3 and NH2 protons (rate law: kobs = kH[OD-]) in the 1, 2, and 3 cations are reported. Deprotonation constants (K = [Co(cyclen-H)(diamine)2+]/[Co(cyclen)(diamine)3+][OH-]) were determined for 1 (5.5 +/- 0.5 M-1) and 2 (28 +/- 3 M-1). In alkaline solution 1, 2, and 3 hydrolyze to [Co(cyclen)(OH)2]+ via [Co(cyclen)(amine)OH)]2+ monodentates. Hydrolysis of 3 is two step: kobs(1) = kOH(1)[OH-], kobs(2) = ko + kOH(2)[OH-] (kOH(1) = (2.2 +/- 0.4) x 10(4) M-1 s-1, ko = (5.1 +/- 1.2) x 10(-4) s-1, kOH(2) = 1.0 +/- 0.1 M-1 s-1). Hydrolysis of 2 is biphasic: kobs(1) = k1K[OH-]/(1 + K[OH-] (k1 = 5.0 +/- 0.2 s-1, K = 28 M-1), kobs(2) = k2K2[OH-]/(1 + K2[OH-]) (k2 = 3.5 +/- 1.2 s-1, K2 = 1.2 +/- 0.8 M-1). Hydrolysis of 1 is monophasic: kobs = k1k2KK2[OH-]2/(1 + K[OH-1])(k-1 + k2K2[OH-]) (k1 = 0.035 +/- 0.004 s-1, k-1 = 2.9 +/- 0.6 s-1, K = 5.5 M-1, k2K2 = 4.0 M-1 s-1). The much slower rate of chelate ring-opening in 1, compared to loss of NH3 from 3, is rationalized in terms of a reduced ability of the former system to allow the bond angle expansion required to produce the SN1CB trigonal bipyramidal intermediate.     

Time-Resolved Studies of the Excited-State Dynamics of meso-Tetra(hydroxylphenyl)chlorin in Solution

Meso-tetra(hydroxyphenyl)chlorin (m-THPC) is a new photosensitizer developed for potential use in photodynamic therapy (PDT) for cancer treatment. In PDT, the accepted mechanism of tumor destruction involves the formation of excited singlet oxygen via intermolecular energy transfer from the excited triplet-state dye to the ground triplet-state oxygen. Femtosecond transient absorption measurements are reported here for the excited singlet state dynamics of m-THPC in solution. The observed early time kinetics were best fit using a triple exponential function with time constants of 350 fs, 80 ps and > or = 3.3 ns. The fastest decay (350 fs) was attributed to either internal conversion from S2 to S1 or vibrational relaxation in S2. Multichannel time-resolved absorption and emission spectroscopies were also used to characterize the excited singlet and triplet states of the dye on nanosecond to microsecond time scales at varying concentrations of oxygen. The nanosecond time-resolved absorption data were fit with a double exponential with time constants of 14 ns and 250 ns in ambient air, corresponding to lifetimes of the S1 and T1 states, respectively. The decay of the T1 state varied linearly with oxygen concentration, from which the intrinsic decay rate constant, ki, of 1.5 x 10(6) s-1 and the biomolecular collisional quenching constant, kc, of 1.7 x 10(9) M-1 s-1 were determined. The lifetime of the S1 state of 10 ns was confirmed by fluorescence measurements. It was found to be independent of oxygen concentration and longer than lifetimes of other photosensitizers.     

Equilibrium and kinetic studies of the aquation of the dinuclear platinum complex [[trans-PtCl(NH3)2]2(mu-NH2(CH2)6NH2)]2+: pKa determinations of aqua ligands via [1H,15N] NMR spectroscopy

By the use of [1H,15N] heteronuclear single quantum coherence (HSQC) 2D NMR spectroscopy and electrochemical methods we have determined the hydrolysis profile of the bifunctional dinuclear platinum complex [[trans-PtCl(15NH3)2]2(mu-15NH2(CH2)(6)15NH2)]2+ (1,1/t,t (n = 6), 15N-1), the prototype of a novel class of potential antitumor complexes. Reported are estimates for the rate and equilibrium constants for the first and second aquation steps, together with the acid dissociation constant (pKa1 approximately pKa2 approximately pKa3). The equilibrium constants determined by NMR at 25 and 37 degrees C (I = 0.1 M) were similar, pK1 approximately pK2 = 3.9 +/- 0.2, and from a chloride release experiment at 37 degrees C the values were found to be pK1 = 4.11 +/- 0.05 and pK2 = 4.2 +/- 0.5. The forward and reverse rate constants for aquation determined from this chloride release experiment were k1 = (8.5 +/- 0.3) x 10(-5) s-1 and k-1 = 0.91 +/- 0.06 M-1 s-1, where the model assumed that all the liberated chloride came from 1. When the second aquation step was also taken into account, the rate constants were k1 = (7.9 +/- 0.2) x 10(-5) s-1, k-1 = 1.18 +/- 0.06 M-1 s-1, k2 = (10.6 +/- 3.0) x 10(-4) s-1, k-2 = 1.5 +/- 0.6 M-1 s-1. The rate constants compare favorably with other complexes with the [PtCl(am(m)ine)3]+ moiety and indicate that the equilibrium of all these species favors the chloro form. A pKa value of 5.62 was determined for the diaquated species [[trans-Pt(15NH3)2(H2O)]2(mu-15NH2(CH2)(6)15NH2)]4+ (3) using [1H,15N] HSQC NMR spectroscopy. The speciation profile of 1 and its hydrolysis products under physiological conditions is explored.     

Photochromic dibenzobarrlenes: long-lived triplet biradical intermediates

Upon exposure to UV light, the disubstituted dibenzobarrelene derivative 1a turns green in the solid phase and reverts back to its original pale-yellow color within several hours in the dark. The lifetime of the colored species in degassed benzene at room temperature is 37 +/- 2 s (Ea for decoloration is 14.5 +/- 0.7 kcal mol-1 and log A is 8.92 +/- 0.5 s-1) and highly sensitive to molecular oxygen; the Stern-Volmer quenching constant is 6.9 +/- 0.2 x 108 M-1 s-1. Similarly, the disubstituted dibenzobarrelenes 1b and 1c exhibited pink coloration when exposed to UV light in the solid phase. On the basis of combined experimental and theoretical evidence, it is proposed that upon photoexcitation the excited singlet state of 1a undergoes rapid intersystem crossing to its triplet state, followed by intramolecular delta-H abstraction, to yield the triplet biradical intermediate (3)2. Upon prolonged irradiation, 2 undergoes cyclization to the alcohol 3, which affords the enone 4 as the final photoproduct. The delta-H abstraction on the triplet-state potential energy surface, calculated at the B3LYP/6-31G* level of density functional theory (DFT), has an activation energy of 18.5 kcal/mol. Further, the absorption spectrum of the triplet biradical (3)2, obtained from time-dependent DFT calculations, displays an intense absorption maximum at 670 nm, which is in good agreement with the observed absorption peak at 700 nm. The molecular-orbital analysis of the triplet diradical (3)2 suggests that its long-wavelength absorption involves the transition of the unpaired electron from the comparatively localized benzyl-type HOMO to the extensively conjugated benzoyl-type LUMO. The present experimental and theoretical results strongly support the intervention of a long-lived triplet biradical (3)2 in the photochromism of appropriately substituted dibenzobarrelenes.     

Rate coefficients for the reaction of OH with (E)-2-pentenal, (E)-2-hexenal, and (E)-2-heptenal

Rate coefficients for the gas-phase reaction of the OH radical with (E)-2-pentenal (CH(3)CH(2)CH[double bond]CHCHO), (E)-2-hexenal (CH(3)(CH(2))(2)CH[double bond]CHCHO), and (E)-2-heptenal (CH(3)(CH(2))(3)CH[double bond]CHCHO), a series of unsaturated aldehydes, over the temperature range 244-374 K at pressures between 23 and 150 Torr (He, N(2)) are reported. Rate coefficients were measured under pseudo-first-order conditions in OH with OH radicals produced via pulsed laser photolysis of HNO(3) or H(2)O(2) at 248 nm and detected by pulsed laser-induced fluorescence. The rate coefficients were independent of pressure and the room temperature rate coefficients and Arrhenius expressions obtained are (cm(3) molecule(-1) s(-1) units): k(1)(297 K)=(4.3 +/- 0.6)x 10(-11), k(1)(T)=(7.9 +/- 1.2)x 10(-12) exp[(510 +/- 20)/T]; k(2)(297 K)=(4.4 +/- 0.5)x 10(-11), k(2)(T)=(7.5 +/- 1.1)x 10(-12) exp[(520 +/- 30)/T]; and k(3)(297 K)=(4.4 +/- 0.7)x 10(-11), k(3)(T)=(9.7 +/- 1.5)x 10(-12) exp[(450 +/- 20)/T] for (E)-2-pentenal, (E)-2-hexenal and (E)-2-heptenal, respectively. The quoted uncertainties are 2sigma(95% confidence level) and include estimated systematic errors. Rate coefficients are compared with previously published room temperature values and the discrepancies are discussed. The atmospheric degradation of unsaturated aldehydes is also discussed.     

Mechanisms of photoinitiated cleavage of DNA by 1,8-naphthalimide derivatives.

Using water-soluble 1,8-naphthalimide derivatives, the mechanisms of photosensitized DNA damage have been elucidated. Specifically, a comparison of rate constants for the photoinduced relaxation of supercoiled to circular DNA, as a function of dissolved halide, oxygen and naphthalimide concentration, has been carried out. The singlet excited states of the naphthalimide derivatives were quenched by chloride, bromide and iodide. In all cases the quenching products were naphthalimide triplet states, produced by induced intersystem crossing within the collision complex. Similarly, the halides were found to quench the triplet excited state of the 1,8-naphthalimide derivatives by an electron transfer mechanism. Bimolecular rate constants were < 10(5) M-1 s-1 for quenching by bromide and chloride. As expected from thermodynamic considerations quenching by iodide was 6.7 x 10(9) and 8.8 x 10(9) M-1 s-1 for the two 1,8-naphthalimide derivatives employed. At sufficiently high ground-state concentration self-quenching of the naphthalimide triplet excited state also occurs. The photosensitized conversion of supercoiled to circular DNA is fastest when self-quenching reactions are favored. The results suggest that, in the case of 1,8-naphthalimide derivatives, radicals derived from quenching of the triplet state by ground-state chromophores are more effective in cleaving DNA than reactive oxygen species or radicals derived from halogen atoms.     

Thermodynamics and kinetics of the nickel(II)-salicylhydroxamic acid system. Phenol rotation induced by metal ion binding

The kinetics and the equilibria of Ni(II) binding to p-hydroxybenzohydroxamic acid (PHBHA) and salicylhydroxamic acid (SHA) have been investigated in an aqueous solution at 25 degrees C and I=0.2 M by the stopped-flow method. Two reaction paths involving metal binding to the neutral acid and to its anion have been observed. Concerning PHBHA, the rate constants of the forward and reverse steps are k1=(1.9+/-0.1)x10(3) M-1 s-1 and k-1=(1.1+/-0.1)x10(2) s-1 for the step involving the undissociated PHBHA and k2=(3.2+/-0.2)x10(4) M-1 s-1 and k-2=1.2+/-0.2 s-1 for the step involving the anion. Concerning SHA, the analogous rate constants are k1=(2.6+/-0.1)x10(3) M-1 s-1, k-1=(1.3+/-0.1)x10(3) s-1, k2=(5.4+/-0.2)x10(3) M-1 s-1, and k-2=6.3+/-0.5 s-1. These values indicate that metal binding to the anions of the two acids concurs with the Eigen-Wilkins mechanism and that the phenol oxygen is not involved in the chelation. Moreover, a slow effect was observed in the SHA-Ni(II) system, which has been put down to rotation of the benzene ring around the C-C bond. Quantum mechanical calculations at the B3LYP/lanL2DZ level reveal that the phenol group in the most stable form of the Ni(II) chelate is in trans position relative to the carbonyl oxygen, contrary to the free SHA structure, where the phenol and carbonyl oxygen atoms both have cis configuration. These results bear out the idea that the complex formation is coupled with phenol rotation around the C-C bond.     

General method for determination of the activation,deactivation, and initiation rate constants in transition metal-catalyzed atom transfer radical processes

A general method for the determination of the activation (ka), deactivation (kd), and initiation (ki) rate constants in atom transfer radical processes is reported. The method involves the monomer trapping techniques and the analytical solution of the persistent radical effect. For tert-butyl 2-bromopropionate, using ATRP catalyst [CuI(dNbpy)2][Br] and methyl methacrylate in CH3CN at 22 degrees C, the values of ka, kd, and ki were determined to be (9.4 +/- 0.6) x 10-3 M-1 s-1, (8.5 +/- 1.2) x 106 M-1 s-1 and (5.5 +/- 0.9) x 104 M-1 s-1, respectively. The determined initiation rate constant was in good agreement with the literature value (6.0 x 104 M-1 s-1), confirming the validity of the proposed approach. For methyl 2-bromopropionate, under the same conditions, ka, kd, and ki values were found to be (26 +/- 5.9) x 10-3 M-1 s-1, (29 +/- 7.3) x 106 M-1 s-1, and (5.7 +/- 1.6) x 104 M-1 s-1, respectively.     

Photochemical studies of A2-E.

Lipofuscin is thought to be involved in age-related macular degeneration as is one of its proposed components, an amphiphillic pyridinium-based bis-retinoid with a quaternary nitrogen atom, known as A2-E. We report the triplet state spectra obtained from photosensitisation using anthracene and 1-nitronaphthalene in benzene and methanol. The triplet state of A2-E has lambda(max) at 550 nm and a lifetime of approximately 30 micros, it is efficiently quenched by molecular oxygen with a second-order quenching rate constant of approximately 1 x 10(9) dm(3) mol(-1) s(-1). There is no significant triplet state formation from direct laser excitation of A2-E and hence its quantum yield of triplet state formation must be <0.01.     

Fluorescence properties of pyrylretinol

A fluorescent analog of retinol, 3,7-dimethyl-9-(1-pyryl)-2E,4E,6E,8E-nonatetr aene-1-ol (referred to as pyrylretinol, or 1) has been synthesized. The fluorescence properties (e.g. quantum yield, lifetime, steady-state anisotropy, and excitation/emission spectra) of this compound in various organic solvents and in dimyristoylphosphatidylcholine (DMPC) liposomes have been studied, and the results are compared with those obtained from 3-methyl-5-(1-pyryl)-2E,4E-pentadiene-1-ol (2), which has the same fused aromatic ring system but a much shorter acyclic chain. 1 and 2 form excimer in aqueous media and fluorescence anisotropies of both 1 and 2 in DMPC liposomes exhibit an abrupt decrease at approximately 21-23 degrees C, which coincides with the main phase transition temperature of DMPC liposomes, indicating that both compounds may be a useful membrane probe. In addition, the binding and quenching capability of pyrylretinol (1) to bovine serum albumin has been investigated. Pyrylretinol (1) binds with BSA with a binding constant of 3.6 x 10(4) M-1, although the value is somewhat lower than that obtained for retinol (3.06 x 10(5) M-1). Pyrylretinol (1) also quenches the BSA intrinsic fluorescence with the quenching rate constant of 1.67 x 10(13) M-1 s-1 and the value is lower than that obtained for retinol (4.06 x 10(13) M-1 s-1). The binding and quenching studies suggest that pyrylretinol (1) may serve as a useful fluorescence probe for structure/function studies of different retinoid binding proteins.     

Type I and type II photosensitization by the antibacterial drug nalidixic acid. A laser flash photolysis study.

The 355 nm laser flash photolysis of nalidixic acid at pH 9.2 leads to the formation of the nalidixate anion triplet state (absorption lambda max = 620 nm; 5700 less than or equal to epsilon T less than or equal to 9000 M-1cm-1; 0.6 less than or equal to phi T less than or equal to 1). The first order triplet state decay (kT = 7.7 x 10(3) s-1) is accompanied by a diffusion controlled triplet-triplet annihilation. Oxygen efficiently quenches the triplet state (k = 3.2 x 10(9) M-1s-1). The nalidixate radical dianion (absorption lambda max = 650 nm; epsilon = 3000 M-1cm-1) is produced by the diffusion controlled reductive quenching of the triplet state by tryptophan and tyrosine. The superoxide anion (O2-.) is produced by diffusion controlled reaction of the radical dianion with oxygen. The O2-. is characterized by its reactions with ferricytochrome c and superoxide dismutase. The physiological form of nalidixic acid is thus a good Type I and Type II photosensitizer.     

Gas-phase rate coefficients for the reactions of nitrate radicals with (Z)-pent-2-ene, (E)-pent-2-ene, (Z)-hex-2-ene, (E)-hex-2-ene, (Z)-hex-3-ene, (E)-hex-3-ene and (E)-3-methylpent-2-ene at room temperature

Rate coefficients for reactions of nitrate radicals (NO3) with (Z)-pent-2-ene, (E)-pent-2-ene, (Z)-hex-2-ene, (E)-hex-2-ene, (Z)-hex-3-ene, (E)-hex-3-ene and (E)-3-methylpent-2-ene were determined to be (6.55 +/- 0.78)x 10(-13) cm3 molecule(-1) s(-1), (3.78 +/- 0.45)x 10(-13) cm3 molecule(-1) s(-1), (5.30 +/- 0.73)x 10(-13) cm(3) molecule(-1) s(-1), (3.83 +/- 0.47)x 10(-13) cm(3) molecule(-1) s(-1), (4.37 +/- 0.49)x 10(-13) cm(3) molecule(-1) s(-1), (3.61 +/- 0.40)x 10(-13) cm3 molecule(-1) s(-1) and (8.9 +/- 1.5)x 10(-12) cm3 molecule(-1) s(-1), respectively. We performed kinetic experiments at room temperature and atmospheric pressure using a relative-rate technique with GC-FID analysis. The experimental results demonstrate a surprisingly large cis-trans(Z-E) effect, particularly in the case of the pent-2-enes, where the ratio of rate coefficients is ca. 1.7. Rate coefficients are discussed in terms of electronic and steric influences, and our results give some insight into the effects of chain length and position of the double bond on the reaction of NO3 with unsaturated hydrocarbons. Atmospheric lifetimes were calculated with respect to important oxidants in the troposphere for the alkenes studied, and NO3-initiated oxidation is found to be the dominant degradation route for (Z)-pent-2-ene, (Z)-hex-3-ene and (E)-3-methylpent-2-ene.     

The photophysics of monomeric bacteriochlorophylls c and d and their derivatives: properties of the triplet state and singlet oxygen photogeneration and quenching

Measurements of pigment triplet-triplet absorption, pigment phosphorescence and photosensitized singlet oxygen luminescence were carried out on solutions containing monomeric bacteriochlorophylls (Bchl) c and d, isolated from green photosynthetic bacteria, and their magnesium-free and farnesyl-free analogs. The energies of the pigment triplet states fell in the range 1.29-1.34 eV. The triplet lifetimes in aerobic solutions were 200-250 ns; they increased to 280 +/- 70 microseconds after nitrogen purging in liquid solutions and to 0.7-2.1 ms in a solid matrix at ambient or liquid nitrogen temperatures. Rate constants for quenching of the pigment triplet state by oxygen were (2.0-2.5) x 10(9) M-1 s-1, which is close to 1/9 of the rate constant for diffusion-controlled reactions. This quenching was accompanied by singlet oxygen formation. The quantum yields for the triplet state formation and singlet oxygen production were 55-75% in air-saturated solutions. Singlet oxygen quenching by ground-state pigment molecules was observed. Quenching was the most efficient for magnesium-containing pigments, kq = (0.31-1.2) x 10(9) M-1 s-1. It is caused mainly by a physical process of singlet oxygen (1O2) deactivation. Thus, Bchl c and d and their derivatives, as well as chlorophyll and Bchl a, combine a high efficiency of singlet oxygen production with the ability to protect photochemical and photobiological systems against damage by singlet oxygen.     

Atmospheric chemistry of the Z and E isomers of CF3CF=CHF; kinetics, mechanisms, and products of gas-phase reactions with Cl atoms, OH radicals, and O3

Smog chamber/FTIR techniques were used to study the atmospheric chemistry of the Z and E isomers of CF3CF=CHF, which we refer to as CF3CF=CHF(Z) and CF3CF=CHF(E). The rate constants k(Cl + CF3CF=CHF(Z)) = (4.36 +/- 0.48) x 10-11, k(OH + CF3CF=CHF(Z)) = (1.22 +/- 0.14) x 10-12, and k(O3 + CF3CF=CHF(Z)) = (1.45 +/- 0.15) x 10-21 cm3 molecule-1 s-1 were determined for the Z isomer of CF3CF=CHF in 700 Torr air diluent at 296 +/- 2 K. The rate constants k(Cl + CF3CF=CHF(E)) = (5.00 +/- 0.56) x 10-11, k(OH + CF3CF=CHF(E)) = (2.15 +/- 0.23) x 10-12, and k(O3 + CF3CF=CHF(E)) = (1.98 +/- 0.15) x 10-20 cm3 molecule-1 s-1 were determined for the E isomer of CF3CF=CHF in 700 Torr air diluent at 296 +/- 2 K. Both the Cl-atom and OH-radical-initiated atmospheric oxidation of CF3CF=CHF give CF3C(O)F and HC(O)F in molar yields indistinguishable from 100% for both the Z and E isomer. CF3CF=CHF(Z) has an atmospheric lifetime of approximately 18 days and a global warming potential (100 year time horizon) of approximately 6. CF3CF=CHF(E) has an atmospheric lifetime of approximately 10 days and a global warming potential (100 year time horizon) of approximately 3. CF3CF=CHF has a negligible global warming potential and will not make any significant contribution to radiative forcing of climate change.     

Absolute rate constants for the quenching of reactive excited states by melanin and related 5,6-dihydroxyindole metabolites: implications for their antioxidant activity

The triplet-excited state of benzophenone and the singlet-excited state of 2,3-diazabicyclo[2.2.2]oct-2-ene (Fluorazophore-P) have been employed as kinetic probes to obtain information on the antioxidant activity of the skin and eye pigment melanin and its biogenetic precursors 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA). The excited states were generated by the laser-flash photolysis technique and their reaction kinetics was examined by time-resolved transient absorption or fluorescence spectroscopy, respectively. The reaction between triplet benzophenone and DHI produced with unit efficiency the corresponding 6O-centered semiquinone radical, which was characterized by its characteristic transient absorption. The quenching rate constants for DHI (3.1-8.4 x 10(9) M-1 s-1) and DHICA (3.3-5.5 x 10(9) M-1 s-1) were near the diffusion-controlled limit, indicating excellent antioxidant properties. Kinetic solvent effects were observed. The reactivity of synthetic melanin, assessed through the quenching rate constant of Fluorazophore-P and normalized to the number of monomer units, was more than one order of magnitude lower (2.7 x 10(8) M-1 s-1) than that of its precursors. The trend of the quenching rate constants, i.e. DHI > DHICA approximately alpha-tocopherol > melanin, along with the preferential solubility of DHICA in aqueous environments, serves to account for several experimental results from biochemical studies on the inhibition of lipid peroxidation by these natural antioxidants.