One-electron-reduced and -oxidized stages of donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes of different molecular architectures.

A series of monomeric and oligomeric donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs) with various topologies have been synthesized by means of thermal [2+2] cycloaddition between tetracyanoethylene (TCNE) and donor-substituted alkynes, followed by retro-electrocyclization. One-electron-reduced and -oxidized stages of the donor-substituted TCBDs were generated by chemical methods. The obtained radical anions and radical cations were studied by using electron paramagnetic resonance/electron nuclear double resonance (EPR/ENDOR) spectroscopy, supported by density functional theory (DFT) calculations. The extent of pi-electron delocalization in the paramagnetic species was investigated in terms of the EPR parameters. Despite favorable molecular orbital (MO) coefficients, the EPR results suggest that in radical anions the spin and charge are confined to the electron-withdrawing TCBD moieties on the hyperfine EPR timescale. The observed spin localization is presumably caused by an interplay between the nonplanarity of the studied pi systems, limited pi-electron conjugation, and very likely counterion effects. In radical cations, an analogous spin and charge localization confined to the electron-donating N,N-dialkylaniline moieties was found. In this case, an efficient electron delocalization is disabled by small MO coefficients at the joints between the donor and acceptor portions of the studied TCBDs.

Conjugates of gonadotropin releasing hormone (GnRH) with carminic acid: Synthesis, generation of reactive oxygen species (ROS) and biological evaluation.

We synthesized two carminic acid (7-alpha-d-glucopyranosyl-9,10-dihydro-3,5,6,8-tetrahydroxy-1-methyl-9,10-dioxo-2-anthracene carboxlic acid, CA)-GnRH conjugates to be used as a model for potential photoactive targeted compounds. CA was conjugated to the epsilon-amino group of [d-Lys(6)]GnRH through its carboxylic moiety or via a beta-alanine spacer (beta-ala). Redox potentials of CA and its conjugates were determined. We used electron spin resonance (ESR) and spin trapping techniques to study the light-stimulated redox properties of CA and its CA-GnRH conjugates. Upon irradiation, the compounds stimulated the formation of reactive oxygen species (ROS), that is, singlet oxygen ((1)O(2)) and oxygen radicals (O(2)(-*) and OH(*)). Both conjugates exhibited higher ROS production than the non-conjugated CA. The bioactivity properties of the CA conjugates and the parent peptide, [d-Lys(6)]GnRH, were tested on primary rat pituitary cells. We found that the conjugates preserved the bioactivity of GnRH as illustrated by their capability to induce ERK phosphorylation and LH release.

Mechanistic insights into stereoselective catalysis-the effects of counterions in a CuII-bissulfoximine-catalyzed Diels-Alder reaction.

The initial steps of an enantioselective Diels-Alder reaction catalyzed by a CuII-bissulfoximine complex were followed by EXAFS (EXAFS=extended X-ray absorption fine structure), EPR (EPR=electron paramagnetic resonance) spectroscopy (CW-EPR, FID-detected EPR, pulse ENDOR, HYSCORE; CW=continuous wave; ENDOR=electron nuclear double resonance; HYSCORE=hyperfine sublevel correlation; FID=free induction decay), and UV-visible spectroscopy. The complexes formed between the parent CuX2 (X=Cl-, Br-, TfO-, SbF6-) salts, the chiral bissulfoximine ligand (S,S)-1, and N-(1-oxoprop-2-en-1-yl)oxazolidin-2-one (2) as the substrate in CH2Cl2 were investigated in frozen and fluid solution. In all cases, penta- or hexacoordinated CuII centers were established. The complexes with counterions indicating high stereoselectivity (TfO- and SbF6-) reveal one unique species in which substrate 2 binds to pseudoequatorial positions (via O atoms), shifting the counterions to axial locations. On the other hand, those lacking stereoselectivity (X=Cl- and Br-) form two species in which the parent halogen anions remain at equatorial positions preventing the formation of geometries compatible with those found for X=TfO- and SbF6-.

An investigation into the initial degradation steps of four major dye chromophores: study of their one-electron oxidation and reduction by EPR, ENDOR, cyclic voltammetry, and theoretical calculations.

The degradation of dyes is frequently initiated by one-electron oxidation or reduction; however, relatively little is known about the initially formed radicals. Acid Green 25 (AG25), Crystal Violet (CVI), Methylene Blue (MB), and Acid Orange 7 (AO7), representing paradigms of four types of commercial organic dyes, were therefore investigated in terms of their redox behavior. Their redox potentials in MeCN and buffered aqueous solutions were determined by cyclic voltammetry. The structures of the one-electron reduced and oxidized dyes were established by EPR spectroscopy and by theoretical calculations on the density functional level of theory.