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| Author(s): |
Tereza Pereira de Souza
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Conjunto das Químicas (IQ e FCF) (CQ/DBDCQ) |
| Defense date: | 2006-06-12 |
| Examining board members: |
Iolanda Midea Cuccovia;
João Pedro Simon Farah;
Rosângela Itri;
Pier Luigi Luisi;
Tibor Rabóczkay
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| Advisor: | Iolanda Midea Cuccovia |
| Abstract | |
Membranes and organelles are structures present in biological systems. Such structures are interfaces between electrolytes. Addressing to the description, interpretation and comprehension of the ionic distribution around the structures an attempt is done is this work analyzing experimental results from the investigation of the following systems: 1) SDS micelles. \"pH\" measurements in the micellar surface neighborhood using salicylic acid probes show the pH values dependence with the SDS and added salt concentrations. The experiments aimed to infer the pH behavior in biological membranes where same phospholipids may dissociate and portions of the surface can acquire electrical charges. 2) The zwitterionic nature of biological membranes leads to the investigation of ion binding in zwitterionic micelles in electrolyte solutions with varied concentrations of Cl- or Br<SUP- salts with the cations Li+, Na+, K+, Rb+, Cs+ e tetramethylammonium (TMA+) and the bivalent cations Mg2+ e Ca2+. Halide concentration in the micellar vicinity was measured. Chemical trapping method shows there is a selectivity degree that does not depend only on the ionic charge. 3) Preliminary results in the determination of the inner dissociation degree of dioctadecyldimethylammonium chloride vesicles, DODAC, show that about 8% of the vesicle constituent monomers are dissociated in vesicles with 150 and 300 nm mean diameter. The theoretical description is based upon the thermodynamics equilibrium hypothesis about the systems and that the electrostatic interaction is the stronger interaction and also it is attributed to the mean electrostatic potential the role of the mean force potential acting on the ionic species. A further simplification in considering the ions as point charges with respect to the electrostatic interactions leads the model to the Poisson-Boltzmann equation and under linearization results in the Debye-Hückel alternative description. Additional hypothesis are necessary in order to have the model as a mathematical problem with boundary conditions and are discussed for each system. Hypothetical systems are analysed aiming the comparison between Poisson-Boltzmann and Debye- Hückel descriptions. Some conclusions derived in the analysis are: 1- The salicylic acid probes have the dissociable groups always near (~0,1nm) the micellar surface, even to the probes that have a long chain between the nitrogen group and the dissociable group. 2- In a zwitterionic micelle the ions on the surface have translational freedom and this is way the zwitterionic membranes in electrolyte solutions are conducting surfaces. 3- The ionic concentrations in the vesicle interior have uniformly value almost everywhere showing variations only in the vicinity of the electrically charged interior surface. The theoretical study of the three systems considered gives results in accord with experimental data. (AU) | |