Publications

Abstract A square wave voltammetric (SWV) method and a flow injection analysis system with electrochemical detection (FIA-EC) using a glassy carbon electrode were evaluated for the determination of codeine in pharmaceutical preparations. The interference of several compounds, such as acetaminophen, guaiacol, parabens, ephedrine, acetylsalicylic acid and caffeine, that usually appear associated with codeine pharmaceutical preparations was studied. It was verified that these electroanalytical methods could not be used with acetaminophen present in the formulations and that with guaiacol, parabens or ephedrine present the use of the FIA-EC ystem was impracticable. A detection limit of 5 mumol L-1 and a linear calibration range from 40 to 140 mumol L-1 was obtained with the SWV method. For the flow injection analysis procedure a linear calibration range was obtained from 7 to 50 mumol L-1 with a detection limit of 3 mumol L-1 and the FIA-EC system allowed a sampling rate of 115 samples per hour. The results obtained by the two methods, SWV and FIA-EC, were compared with those obtained using reference methods and demonstrated good agreement, with relative deviations lower than 4%.

Abstract A detailed study of the oxidative behaviour of apomorphine in aqueous media is reported. Resorting to the synthesis of apomorphine derivatives it was possible to identify all the anodic oxidation peaks of apomorphine, which are related to the oxidation of the catechol and tertiary amine groups. These findings were revealed to be important since they could lead to a better understanding of the biological interactions of apomorphine and gain insight into its metabolic pathways. During the voltammetric studies, it was also found that apomorphine forms a complex with borate through the catechol group leading to an increase of its oxidation potential. This property could be very useful with regard to the stabilization of apomorphine solutions since it could drastically reduce its autoxidation.

Abstract The native DNA duplex may be viewed as a cooperatively-ordered H bonded structure, including well localized Watson-Crick base pairs and H-bounded networks of hydration parts of the DNA-solvent interface in the grooves of the helix. In this paper, we present the effect of different metal ions (Li+, Mg2+ and Cu2+) on the denaturational heat capacity increment (DeltaC(p)) for calf thymus DNA. Since the contribution from the ordered hydration water fraction disruption energy to the total enthalpy and heat capacity increment values of double helix melting is significant, it must be possible to detect the effect of metal ions on the structural ordering/disordering of the H-bounded network in the hydration shell of the DNA duplex. Experimental results suggest that Li+, which is preferentially adsorbed in the minor groove of B-DNA and should contribute significantly to the stabilization of B-form, has a pronounced influence on the value of DeltaC(p). For the system Mg2+-DNA, the values of DeltaC(p) are also significant, which can be explained by the formation of inner hydration sphere complexes and immobilization of structural water by the grooves of the duplex, stabilizing the helix. The effect of Cu2+ ions is much more pronounced. The satellite peaks of calf thymus DNA become lower as the concentration of Cu2+ increases and for concentrations of Cu2+ higher than 0.010M, they disappear and DeltaC(p) = 0. This is indicative of the known preference of Cu2+ for purins and GC rich sites of DNA, binding to the N7 of guanine. As a result, disruption of the base stacking and hydrogen bounded water networks in the grooves of the double helix takes place.

Abstract The standard (p(o) = 0.1 MPa) molar enthalpies of formation for 2-, 3-, and 4-transmethoxycinnamic acids in the gaseous phase were derived from the standard molar enthalpies of combustion in oxygen of the crystalline compounds determined by static bomb combustion calorimetry at T = 298.15 K and from the literature values for the respective enthalpies of sublimation.

Abstract Dilute solutions of catanionic vesicles formed by the mixed single-chained (sodium dodecylsulphate, SDS) and double-chained (didodecyldimethylammonium bromide, DDAB) surfactants have been investigated by differential scanning calorimetry. It is for the first time reported a gel-to-liquid crystal phase transition temperature, T-m, in this type of mixed vesicles. The SDS-rich vesicles (at X-SDS = 0.71) show a concentration-dependent T-m in the range 9-16 degreesC. Addition of salt is seen to have an effect on T-m similar to that observed with increasing surfactant concentration, both inducing a decrease in T-m. These results differ from those obtained for neat DDAB vesicles. The observed effects in the two types of vesicles are rationalised in terms of headgroup electrostatic interactions which may have influence on the chain packing and phase transition temperature.

Abstract The phase behavior and aggregate structure of the didodecyldimethylammonium bromide (DDAB)-sodium taurodeoxycholate-water mixed surfactant system have been investigated in the dilute (above 95 wt% water) cationic-rich area, at 25 degreesC. A combination of techniques has been used, including polarizing and video-enhanced light microscopy, cryogenic transmission electron microscopy and water self-diffusion NMR. Between the dilute lamellar phase (approximate to4-30 wt% DDAB) and the very narrow isotropic solution (below 0.5 wt% DDAB), the lamellar dispersions formed by DDAB contain different types of vesicle structures (vesicles, microtubules and multilamellar structures). On addition of bile salt to the DDAB dispersions, strong electrostatic headgroup interactions and geometric packing effects (owing to the unusual molecular structure of the anionic surfactant) are present. The vesicle aggregates, however, are able to incorporate an amount of bile salt up to roughly 20 mol%. Further addition induces a macroscopic phase separation, with the formation of a strong white dispersion. The light and electron micrographs show that the vesicles are spherical and integral, undergoing an increase in size when the bile salt concentration is varied between 0 and 10 mol%. At higher bile salt concentration, smaller vesicles, tubular structures and submicron-sized dispersion droplets are observed. Water self-diffusion NMR measurements give further information regarding vesicle size and polydispersity.

Abstract The standard (pdegrees = 0.1 MPa) molar enthalpies of formation Delta(f)H(m)degrees, at the temperature 298.15 K, for crystalline 2,3,4-, 2,4,5-, 2,4,6- and 3,4,5-trichloroaniline were derived from the molar enthalpies of combustion Delta(c)H(m)degrees in oxygen using rotating bomb combustion calorimetry. The reaction products were CO2(g), NI(g), and HCl . 600H(2)O(l). The standard molar enthalpies of sublimation Delta(cr)(g)H(m)degrees at T = 298.15 K were measured by Calvet microcalorimetry. The results are as follows: [GRAPHICS] The derived standard molar enthalpies of formation of the gaseous compounds were compared with values estimated by assuming the enthalpy increment for substitution of chlorine in aniline to be the same as for substitution into benzene.

Abstract A cellulase from Trichoderma reesei was immobilized on Eudragit L-100, a reversibly soluble polymer depending on the pH of the medium. The solubility of the modified cellulase was studied at different pH values. By changing the pH, the adsorption equilibrium of the derivatized proteins is switched towards the liquid phase, thus making recycling possible. This method allows for improved stability, without major loss of specific activity. The adsorption of cellulase on Eudragit lowers the enthalpy of denaturation, but affects only slightly the denaturation temperature. The use of carbodiimide was ineffective on linking the enzymes covalently to the polymer, since the immobilization process was found to be only mediated by non-covalent forces.