Rodolfo D. Porasso

Study of the Benzocaine Transfer from Aqueous Solution to the Interior of a Biological Membrane

The precise molecular mechanism of general anesthetics remains unknown. It is therefore important to understand where molecules with anesthetic properties localize within biological membranes. We have determined the free energy profile of a benzocaine molecule (BZC) across a biological membrane using molecular dynamics simulation. We use an asymmetric phospholipid bilayer with DPPS in one leaflet of a DPPC bilayer (Lopez Cascales et al. J. Phys. Chem. B 2006, 110, 2358-2363) to model a biological bilayer. From the free energy profile, we predict the zone of actuation of a benzocaine is located in the hydrocarbon region or at the end of the lipid head, depending of the presence of charged lipids (DPPS) in the leaflet. We observe a moderate increase in the disorder of the membrane and in particular an increase in the disorder of DPPS. Static and dynamic physicochemical properties of the benzocaine, such as its dipole orientation, translational diffusion coefficient, and rotational relaxation time were measured.

Complexation of heavy metals by humic acids: analysis of voltammetric data by polyelectrolyte theory

The complexation of heavy metals by humic acids has been studied by voltammetry for a number of different cases, including (a) different metal ions; (b) various concentrations of the supporting salt; (c) different total metal concentrations and (d) different charge densities of the humic acid. The voltammetric speciation data are compared with predictions of a theoretical model which, besides polyelectrolytic interactions, takes into account: (i) chemical binding of metal ions; (ii) ionic strength effects; (iii) entropic effects; and (iv) competitive interactions between counterions of different valences. All experimentally obtained speciation data agree well with theoretical predictions by considering a single value of the intrinsic free energy of binding, which appeared to be the same for the Cd and Zn metal ions. This means that under the presently applied experimental conditions the interactions between the humate ion and the metal ions is similar. It is furthermore shown that with the present experimental-theoretical procedure a very consistent and precise behavior of the stability constant of the complex is reached in the wide range of the physico-chemical solution variables reported in this work.

Analysis of potentiometric titrations of heterogeneous natural polyelectrolytes in terms of counterion condensation theory: application to humic acid

A model, developed within the framework of the counterion condensation theory of linear polyelectrolytes, is presented in this paper to describe the acid-base properties of linear polyelectrolytes, consisting of several types of functional ionizable groups. This formalism has been successfully applied to Fluka humic acid under salt-free conditions, as well as in the presence of supporting simple 1:1 salt (KNO3) at three different concentrations. As part of this approach, the charge density of the humic acid is obtained from the activity coefficient measurements of potassium counterions at different humic acid concentrations at a constant degree of dissociation of the polyelectrolyte. The humic acid average charge density was found to be 0.80 +/- 0.05. Using the present model, we are able to satisfactorily describe the experimental data obtained from acid-base potentiometric titrations. Four main functional groups making up the polymer are determined through their fractional abundances (Xi) and intrinsic pK (pK0i) values. The fractional abundances remained constant and independent of the ionic strength, indicating that the humic acid constitution does not depend on the concentration of excess salts. The pK0i values show a small change with ionic strength, which can be explained by the polyelectrolytic behavior of the solution.

Chemical bonding of divalent counterions to linear polyelectrolytes: Theoretical treatment within the counterion condensation theory

Chemical bonding of counterions to a linear polyelectrolyte is addressed within the framework of the counterion condensation theory of linear polyelectrolytes. The model allows for the proper identification of the kind of counterion association to a linear polyelectrolyte when it is in solution with two different types of counterions (of equal or different valences). Two extreme modes of counterion association to the polyelectrolyte are considered: loose territorial (“classical” condensation) and a coalent bonding at a specific site on the polyion. The model provides analytical expressions for calculating the fraction of counterions in each type of association. Any thermodynamic function of interest can also be readily computed through the appropriate function of the calculated excess Gibbs energy of the system. The concentrations of the different types of associated counterions, for a few particular solution conditions, are presented and discussed in order to provide tools not only for analyzing experimental results, but also for planning suitable experiments aimed at discriminating between the two types of counterion association proposed.

Proton /metal exchange processes in synthetic and natural polyelectrolyte solution systems

Proton–metal exchange processes that take place in polyelectrolyte solutions have been studied using previously reported potentiometric titration data for polyacrylic acid (PAA) and humic acid (HA) systems obtained at different degrees of polymer deprotonation, for various metals and at different added metal concentrations. It is shown that the extent of the exchange process, quantified by the parameter νexch, strongly depends on the way it has been determined, i.e. at constant polyelectrolyte characteristics or at constant pH, being significantly larger under constant pH conditions. In addition, it is found that the exchange process depends upon polyelectrolyte structural charge density, the degree of ionization, the type of metal and the total metal concentration in solution. For the experiments reported here, in general the larger exchange coefficients are found for PAA at the lowest reported degree of ionization (α=0.2) and low pH values, while the lower values correspond to HA at the highest studied degree of ionization (α=0.6) and high pH values. The ability of the heavy metal ions to induce H+ release increase following the order Ca≈Cd≈Ni<Pb≤Cu. The present analysis shows that counterion condensation theory of linear polyelectrolytes appropriately describes the experimental data, provided chemical bonding interactions are not too strong to disrupt the general polyelectrolytic behavior.