Reactions of reducing radicals with 2- and 3-nitroanilines in aqueous solutions: a pulse radiolysis study

Reactions of 2- and 3-nitro anilines (2- and 3-NA) with e_aq⁻, H-atoms and one-electron reductants have been studied using pulse radiolysis in aqueous solutions. Reactions of e_aq⁻ were found to be quite fast with both 2-NA and 3-NA resulting in their corresponding semi-reduced species which are reducing in nature. Reduction potentials for 2-NA/2-Na^•′ have been estimated to be approx. −0.56 Vvs. NHE and that for 3-NA/3-NA^•− was found to be between −0.185 V and −0.45 Vvs. NHE. Semi-reduced 2-NA has main absorption peak at 300 nm with a shoulder in the 350 nm region and a broad weak band in the 470–500 nm region, whereas semi-reduced 3-NA possesses an absorption peak at 520 nm. Reducing radicals such as (CH₃)₂ C^•OH and CO₂^•− reacted with 2-NA, producing semi-reduced species, whereas reactions of these radicals with 3-NA produced their corresponding radical-adduct species.     

A systematic theoretical investigation of the valence excited states of the diatomic molecules B2, C2, N2 and O2

A quantitative survey on the performance of multireference (MR), configuration interaction with all singles and doubles (CISD), MRCISD with the Davidson correction and MR-average quadratic coupled cluster (AQCC) methods for a wide range of excited states of the diatomic molecules B₂, C₂, N₂ and O₂ is presented. The spectroscopic constants r _e, ω_e, T _e and D _e for a total of 60 states have been evaluated and critically compared with available experimental data. Basis set extrapolations and size-extensivity corrections are essential for highly accurate results: MR-AQCC mean-errors of 0.001 ?, 10 cm⁻¹, 300 cm⁻¹ and 300 cm⁻¹ have been obtained for r _e, ω_e, T _e and D _e, respectively. Owing to the very systematic behavior of the results depending on the basis set and the choice of method, shortcomings of the calculations, such as Rydberg state coupling or insufficient configuration spaces, can be identified independently of experimental data. On the other hand, significant discrepancies with experiment for states which indicate no shortcomings whatsoever in the theoretical treatment suggest the re-evaluation of experimental results. The broad variety of states included in our survey and the uniform quality of the results indicate that the observed systematics is a general feature of the methods and, hence, is molecule-independent. Received: 12 June 2000 / Accepted: 1 September 2000 / Published online: 21 December 2000     

Use of Raman spectroscopy for the identification of radical-mediated damages in human serum albumin

Damages induced by free radicals on human serum albumin (HSA), the most prominent protein in plasma, were investigated by Raman spectroscopy. HSA underwent oxidative and reductive radical stress. Gamma-irradiation was used to simulate the endogenous formation of reactive radical species such as hydrogen atoms (^•H), solvated electrons (e_aq⁻) and hydroxyl radicals (^•OH). Raman spectroscopy was shown to be a useful tool in identifying conformational changes of the protein structure and specific damages occurring at sensitive amino acid sites. In particular, the analysis of the S–S stretching region suggested the radical species caused modifications in the 17 disulphide bridges of HSA. The concomitant action of e_aq⁻ and ^•H atoms caused the formation of cyclic disulphide bridges, showing how cystine pairs act as efficient interceptors of reducing species, by direct scavenging and electron transfer reactions within the protein. This conclusion was further confirmed by the modifications visible in the Raman bands due to Phe and Tyr residues. As regards to protein folding, both oxidative and reductive radical stresses were able to cause a loss in α-helix content, although the latter remains the most abundant secondary structure component. β-turns motifs significantly increased as a consequence of the synergic action of e_aq⁻ and ^•H atoms, whereas a larger increase in the β-sheet content was found following the exposure to ^•OH and/or ^•H attack.     

Prediction of 13C NMR chemical shifts in substituted naphthalenes

The prediction of the ¹³C NMR signals for derivatives of naphthalene has been investigated using mathematical modeling techniques. Two empirical multiple regression models which utilize the field, resonance, and Charton's steric parameters together with molar refractivity were developed, one for α- and the other for β-substituted naphthalene derivatives. In the α case the model had a correlation coefficient of observed versus predicted line positions of r = 0.973 with a standard deviation of 2.2 ppm while in the β case r = 0.979 with the standard deviation being 2.3 ppm. The database consisted of 3152 signals from 394 naphthalene derivatives. We also report the use of the Taft steric parameter in place of the Charton steric parameter in the above- mentioned prediction equations. Received: 19 June 1998 / Accepted: 20 October 1998 / Published online: 16 March 1999     

Corannulene derivatives as models for calculations of fullerene derivatives

Hypothetical derivatives of corannulene C₂₀H₁₀ (Cor), namely, CorX₅ ^• radicals, CorX₅ ⁻ anions (X=H, Cl, or Br), and their η⁵-π-complexes with SiCp, were calculated by the MNDO/PM3 method. The possibilities of using the results of these calculations for modeling the electronic structure and geometry of fragments of the analogous complexes of the fullerene derivatives C₆₀X₅ are discussed. Calculations of C₆₀X₅ ^• radicals and C₆₀X₅ ⁻ anions were also carried out. In all the compounds under study, the X atoms are attached to carbon atoms in α positions with respect to the same five-membered ring. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1268–1272, July, 1997.     

The role of triplet repulsion in alkyl radical addition to a 7π-C-O bond and alkoxy radical addition to a π-C-C bond

The experimental activation energies of the R^• + O = CR¹R² and RO^• + CH₂ = CHR¹ addition reactions are analyzed within the framework of the parabolic model of the bimolecular addition reaction. The activation energy also depends on the dissociation energy of the forming C-O bond and on the reaction enthalpy: the higher the dissociation energy, the higher the activation energy. The empirical relationshipr _e J..D _e = 0.97 x 10-¹³ m kJ.-¹ mol is found for H^•, Cl^•, Br^{• •} and RO^• radical addition to multiple C=C and C=O bonds (r_e is the distance between the peaks of the intersecting parabolic curves). This is due to the effect of the triplet repulsion on radical addition. The interaction of polar groups and the steric effect also influence the activation energy.     

Scavenging Effects of Aliphatic Alcohols and Acetone on H<Superscript>•</Superscript> Radicals in Anodic Contact Glow Discharge Electrolysis: Determination of the Primary Yield of H<Superscript>•</Superscript> Radicals

A study of scavenging effects of iso-propanol, n-propanol, n-butanol and acetone on H^• radicals, precursors for nonfaradaic products of anodic contact glow discharge electrolysis (CGDE) in aqueous media, has been carried out. A kinetic analysis of the competing reactions of H^• with different species in the system leads to a primary yield of 9.8 mol (mol electron)⁻¹ each of H^• and OH^• radicals in the liquid-phase reaction zone of anodic CGDE. The method is considerably more convenient to apply than those reported earlier based on the use of OH^• scavengers.