Halina Kozubek

Spectroscopic and semiempirical studies of gossypol complexes with Fe2+ and Fe3+ cations

Abstract The complexes of gossypol with Fe2+ and Fe3+ cations were studied in acetonitrile by UV–Vis and FT-IR spectroscopy. Very complex equilibria and structural changes were observed for various ratios of gossypol–iron cation mixtures. With the formation of complexes, the ketol–ketol tautomer form of gossypol was favored. With increasing concentration of iron cation in the mixture, the deprotonation of hydroxyl groups was indicated. The semiempirical studies show that the most stable complexes of gossypol with iron cations are 1:4, 1:1 and 2:1. The structures of these complexes are discussed.

Photooxidation of Methionine Derivatives by the 4-Carboxybenzophenone Triplet State in Aqueous Solution. Intracomplex Proton Transfer Involving the Amino Group

Abstract— Oxidation of the triplet state of 4‐carboxybenzophenone (CB) by a series of five substituted methionines and three methionine‐containing dipeptides was monitored under laser flash photolysis conditions in aqueous solution. Spectral resolution techniques were employed to follow the concentration profiles of the intermediates formed from the quenching events. From these concentration profiles, quantum yields for the intermediates were determined. Branching ratios were evaluated for the decay of the charge‐transfer complex by the competing processes of back electron transfer, proton transfer and escape of radical ions. The relative prominence of these processes was discussed in terms of the proton‐transfer tendencies of the nominal sulfur‐radical‐cationic species. A systematic decrease was observed in the quantum yields for the escape of radical ions along with a correlated increase in the proton‐transfer yields. The enhanced propensity of the sulfur radical cations to depro‐tonate is due to deprotonation at the carbons adjacent to the sulfur‐cationic site and at the unsubstituted amino groups when present. This scheme was supported by an observed decrease in the yields of dimeric sulfur radical cations with an increase in the electron‐withdrawing abilities of the substituents, making the radical‐cationic species stronger acids. The involvement of protons on the amino groups was implicated by the correlation of the quantum yields of ketyl radical formation in the photochemistry experiments with the rate constants for the reaction of the CB radical anion with the sulfur‐containing substrates in pulse radiolysis experiments.

Photo-oxidation of Methionine-containing Peptides by the 4-Carboxybenzophenone Triplet State in Aqueous Solution. Competition Between Intramolecular Two-centered Three-electron Bonded (S∴S)+ and (S∴N)+ Formation¶

Abstract Quantum yields for the formation of transients were measured following the quenching of triplet 4-carboxybenzophenone (3CB*) by methionine-containing peptides in aqueous solutions. Ketyl radicals (CBH·), ketyl radical anions (CB·−) and various sulfur radical cations were identified following the triplet-quenching events. The presence of these intermediates indicated that the triplet-quenching mechanism can be characterized as mainly electron-transfer in nature. The quenching rate constants were of the order of 2 × 109 M−1 s−1. There were small, but significant, differences in the triplet-quenching rate constants, and these trends indicate the existence of multiple sulfur targets in the quenchers. The absorption of the transient products was followed in detail by using spectral-resolution analysis. From the absorption data, quantum yields were estimated for the formation of the various transients. There were differences found in the yields of the transient products between the experiments, where the quenchers were the “mixed” stereoisomers of methionylmethionine (l,d and d,l) and experiments where the quenchers were l,l and d,d stereoisomers. Triplet-quenching data from several other methionine-containing small oligopeptides were analyzed in an analogous manner. Systematic variations were observed, and these patterns were discussed in terms of competitive donation of protons to the CB·− within the charge-transfer complex. The competition was between protons on carbons adjacent to the sulfur-radical center and protons on the protonated amino groups of the radical cation. In addition, there was a competition between the two intramolecular two-centered, three-electron bonded species (S∴S)+ and (S∴N)+ that play roles in the secondary kinetics.

Sensitized photooxidation of s-methylglutathione in aqueous solution: intramolecular (S∴O) and (S∴N) bonded species.

Nanosecond laser flash photolysis was used to generate sulfur radical cations of the thioether, S-methylglutathione (S-Me-Glu), via the one-electron oxidation of this thioether by triplet 4-carboxybenzophenone. The purpose of this investigation was to follow the neighboring group effects resulting from the interactions between the sulfur radical cationic sites and nearby lone-pair electrons on heteroatoms within the radical cation, especially the electron lone-pairs on heteroatoms in the peptide bonds. The tripeptide, S-Me-Glu, offers several possible competing neighboring group effects that are characterized in this work. Quantum yields of the various radicals and three-electron bonded (both intramolecular and intermolecular) species were determined. The pH dependence of photoinduced decarboxylation yields was used as evidence for the identification of a nine-membered ring, sulfur-nitrogen, three-electron bonded species. The mechanisms of the secondary reactions of the radicals and radical cations were characterized by resolving their overlapping transient-absorption spectra and following their kinetic behavior. In particular, sulfur-oxygen and sulfur-nitrogen three-electron bonded species were identified where the oxygen and nitrogen atoms were in the peptide bonds.

Spectroscopic and kinetic studies of the aldehyde–lactol tautomerization of gossypol in solution

1 H NMR spectra of gossypol in C2H5OD solution show that an equilibrium between aldehyde 1 and lactol 2 tautomers is formed and that the aldehyde tautomer is predominant, whereas in C2H5OD–DCl solution (pH ⩽ 1) only the lactol form is observed. UV–VIS absorption spectroscopy studies demonstrate that in ethanol and ethanol–HCl solutions gossypol shows different spectra which can be attributed to both tautomers 1 and 2. The influence of various solvents on the 1⇌2 equilibrium is studied by means of UV–VIS absorption spectroscopy and the results are compared with those of 1H NMR studies. Furthermore, the decrease of the absorbance at the long wavelength band in the spectrum of 1 is used to determine the kinetic parameters of aldehyde–lactol tautomerization in ethanol–HCl solution (pH ⩽ 3): k298=(2.14 ± 0.03) 10–2 dm3 mol–1 s–1; ΔH≠= 64.0 ± 3.4 kJ mol–1; ΔS≠=–63 ± 11 J mol–1 K–1; and the value of kinetic isotope effects, kH/kD= 5.6 at 55 °C. A mechanism for aldehyde–lactol tautomerization of gossypol is proposed.

Photophysical properties of some highly fluorescent derivatives of vitamin B1. Solvent effect

Abstract The influence of the solvent on the photophysical properties of some highly fluorescent vitamin B 1 derivatives (the products of the reaction of N-methylated vitamin B 1 with cytidine ( I ), adenosine ( II ) and 2-amino-4-methylpyridine ( III ) was studied using absorption and fluorescence techniques (both steady state and time resolved). For II and III , the photophysical properties do not change significantly in various solvents. For I , a large decrease in the fluorescence quantum yield and lifetime of the singlet state is observed on going from acetonitrile to alcohol. These large changes in the photophysical parameters of I occur in spite of the small changes in the absorption spectrum and the shape of the fluorescence spectrum. No evidence for a specific solute-solvent interaction, i.e. a complex of I with alcohols, is found in pure (including deuterated solvents) or mixed solvents. The measured values of the intersystem crossing (ISC) quantum yield of I in acetonitrile and methanol (in the region of 10 −2 ) indicate that the ISC process is not responsible for the observed solvent effect. Possible explanations for the influence of the solvent on the photophysical properties of I in its first excited singlet state are discussed.

Selective Esterification of Gossypol by Copper Acetate in Acetonitrile-Spectroscopic Studies

Abstract Gossypol and its 1: 1 complexes with copper acetate in acetonitrile were studied by FTIR and UV-visible spectroscopy. With the complexation with Cu cation the tautomeric equilibrium of gossypol is completely shifted from the aldehyde-aldehyde to the lactol-lactol tautomer. The acidic character of O11H group in the gossypol-Cu complex led to the selective esterification of the lactol form of gossypol by the acetate group.

UV-Visible Absorption Studies of Gossypol-Metal Cation Complexes in Acetonitrile Solution

Abstract The systems of gossypol-metal cation in acetonitrile have been studied by UV-visible absorption spectroscopy. The formation of 1:1 complexes between gossypol and Be+2, Cu+2, Dy+3, and Zn+2 has been proved and their formation constants, K, have been determined to be of an order of 104 M.