Aldo Gamba

The binding of bilirubin to albumin a study using spin-labelled bilirubin

Binding between human serum albumin and a spin-labelled derivative of bilirubin was investigated by circular dichroism, fluorescence quenching, electron spin resonance and visible spectroscopy. The orders of magnitude of the binding constants obtained by flurorescence quenching and electron spin resonance spectroscopies were 10(7) and 10(3) 1 . mol-1, respectively. These data suggest that most spin-labelled bilirubin interacts with human serum albumin at the side not holding the spin-labelled side-arm. CD measurements showed the presence of at least two sites, associated with opposite Cotton effects. It is worthy of note that the Cotton sign of the first site is inverted with respect to the corresponding one of bilirubin. CD measurements on mixed systems (spin-labelled bilirubin/human serum albumin/bilirubin) were also performed. The decomposition of the ternary curves shows that the rotatory power of bilirubin bound to human serum albumin is higher in the ternary system than in the binary (bilirubin/human serum albumin). The corresponding CD measurements for the binding between spin-labelled bilirubin and bovine serum albumin are also reported and discussed.

Theoretical investigation on hydroformylation reactions : I. Structures and reactivities of cobalt carbonyls and hydrocarbonyls

Abstract Extended Huckel Theory calculations have been carried out in a study of the most important cobalt carbonyls and hydrocarbonyls involved in the hydroformylation reaction. The geometries of the stable isomers of Co 2 (CO) 8 , Co 2 (CO) 7 , Co(CO) 4 , Co(CO) 3 have been calculated and used to interpret the changes in the IR spectrum of Co 2 (CO) 8 observed on varying the temperature. The reaction paths for the interconversions of the stable isomers have also been investigated. The optimized geometry of HCo(CO) 4 agrees well with the experimental structure. The C s symmetry found for the most stable isomer of HCo(CO) 3 is of much interest, serves to explain the formation of the complex with olefins.

Theoretical investigation on hydroformylation reactions: II. Hydrocarbonyliron intermediates in reppe synthesis

Abstract Extended Huckel theory calculations have been carried out to investigate the possible formation of stable π and σ complexes between olefins and some hydrocarbonylirons postulated as active catalysts in Reppe synthesis. It was found that H 2 Fe(CO) 4 and HFe(CO) 4 - do not coordinate ethylene, unlike the corresponding CO deficient species H 2 Fe(CO) 3 and HFe(CO) 3 - , which interact with the olefin to give stable π complexes. Moreover ethylene—H 2 Fe(CO) 3 is more stable than ethylene—HFe(CO) 3 - , in line with the conclusions based on the experimental results. Stable alkyl intermediates are predicted starting from the coordinatively saturated hydrocarbonyls.

Influence of the relative position of nitro groups on intermolecular sodium exchange processes in dinitrobenzene ion pairs, investigated by EPR spectroscopy

Abstract The EPR spectra of the anion radicals of nitrobenzene isomers in aprotic solvents with added sodium tetraphenylborate show an intermolecular cation-exchange reaction in which the incoming cation goes to the uncomplexed nitro group. The activation energy barriers ΔH ‡ are almost the same for the m and p isomers, while the activation free energy ΔG ‡ is dependent on the relative position of the two nitro groups, probably due to different solvation effects. The transport properties of the systems were also investigated. The spin distribution was computed by the McLachlan method, modified according to McClelland, both for the ion pairs and for the triple ions which are the intermediates of the intermolecular cation exchange.

Analysis of restricted rotation of the acetyl group in m- and p-nitroacetophenone anion radicals

Abstract The internal rotation in the free anion radicals and the ion pairs from 3-nitroacetophenone and 4-nitroacetophenone is studied on the basis of their dynamic EPR spectra in solution. Energy barriers and related thermodynamic parameters which govern internal rotation and rotational diffusion of the studied systems have been provided together with a discussion about the accuracy and the reliability of their determination.

Calculation of hfs coupling constants by the roothaan “Plus” configuration interaction method within the INDO approximation

A new set of parameters, relating hfs coupling constants to the spin densities calculated by the RHF+CI method, within the INDO approximation, is given for1H, 13C, 14N, 17O, 19F. On the whole the results, obtained for a homogeneous sample of organic radicals, are in better agreement with experimental ones, as compared with UHF method.

The influence of substituents on the structure of dicobalt octacarbonyl. A theoretical investigation

Abstract The effect of the substituents on the flap angle of the inner ring in Co 2 (CO) 8 was investigated using EHT calculations. Ethylene, cis -butadiene and cyclopentadienyl anion were considered as replacements for the terminal carbonyls. The results show the trans isomers to be planar, unlike the cis isomers in which the ring puckering is dependent on the nature of the substituents. It is predicted that the interconversion of cis—trans tautomers takes place through ring opening.

Theoretical investigation on hydroformylation reactions. III. Hydrocarbonyliron-propylene π-intermediates☆

Abstract The formation of possible intermediates from a hydrocarbonyliron and propylene, the simplest model system of an olefin withan asymmetric double bond, was investigated. Extended Huckel calculations predict four stable isomers of the π-complex H2Fe(CO)3-propylene generated by reciprocal rotation of the component moieties. Their optimized geometries were calculated, and their stabilization curves turned out to have no barriers. The minimum energies of the conformers are strictly comparable, and their interconversion path was determined. Two of these isomers seem apter to turn into a linear alkyl and the other ones are more favourable to a branched alkyl.

ESR study of γ-irradiated solid tetracyclohexylcyclotetraphosphine

Abstract ESR spectra of γ-irradiated single crystals of tetracyclohexylcyclotetraphosphine show generation of oriented radicals due to cleavage of a cyclohexyl fragment. Observed couplings assigned to 31P nuclei suggest that “phosphinyl” radicals are the species responsible of the dark red-violet colour of the crystals following irradiation.