Physical hydrogels of poly(vinyl alcohol) with different syndiotacticity prepared in the presence of lactosylated chitosan derivative

Poly(vinyl alcohol) (PVA) physical hydrogels were prepared by repeated freeze-thawing cycles using aqueous solutions of two PVA samples having different degrees of syndiotacticity, a-PVA and s-PVA with 55% and 61% of syndiotactic diads, respectively. The hydrogels were prepared in the presence of different amounts of lactosilated chitosan derivatives (LC) of different molecular weight. The PVA stereoregularity was found to have a dramatic effect on the amount of PVA incorporated into the hydrogels, leading to remarkable differences in the swelling degree and porosity of a-PVA and s-PVA hydrogels. A significant amount of LC was retained in the hydrogels after equilibrium swelling. The swelling of the a-PVA hydrogels was found to increase significantly by increasing the amount of LC while it was only slightly increased in the case of s-PVA hydrogels. The amount of LC released after equilibrium swelling was lower when chitosan derivatives with higher molecular weights were used. Increased initial concentrations of LC resulted in much higher porosity of the hydrogels. TGA and DSC studies showed that LC is stabilized by the incorporation in the PVA hydrogels. The mellting temperature of the crystalline regions of PVA was not significantly influ need by LC. Conversely, the extension of the crystalline domains increased in the presence of LC. The retention of a chitosan derivative bearing β-D-galactose side chain residues makes these hydrogels potentially useful as scaffolds for hepatocytes culture

Influence of cardiolipin on the functionality of the Q(A) site of the bacterial photosynthetic reaction center

The effect of cardiolipin on the functionality of the QA site of a photosynthetic reaction center (RC) was studied in RCs from the purple non-sulfur bacterium Rhodobacter sphaeroides by means of time-resolved absorbance measurements. The binding of the ubiquinone-10 to the QA site of the RC embedded in cardiolipin or lecithin liposomes has been followed at different temperatures and phospholipid loading. A global fit of the experimental data allowed us to get quite reliable values of the thermodynamic parameters joined to the binding process. The presence of cardiolipin does not affect the affinity of the QA site for ubiquinone but has a marked influence on the rate of P+QA - f PQA electron transfer. The P+QA - charge recombination kinetics has been examined in liposomes made of cardiolipin/lecithin mixtures and in detergent (DDAO) micelles doped with cardiolipin. The electron-transfer rate constant increases upon cardiolipin loading. It appears that the main effect of cardiolipin on the electron transfer can be ascribed to a destabilization of the chargeseparated state. Results obtained in micelles and vesicles follow the same titration curve when cardiolipin concentration evaluated with respect to the apolar phase is used as a relevant variable. The dependence of the P+QA - recombination rate on cardiolipin loading suggests two classes of binding sites. In addition to a highaffinity site (compatible with previous crystallographic studies), a cooperative binding, involving about four cardiolipin molecules, takes place at high cardiolipin loading