Manuel López-López

Electron transfer reactions in micellar systems: Separation of the true (unimolecular) electron transfer rate constant in its components

Abstract The intramolecular electron transfer reaction within the binuclear complex [(en) 2 Co III (μ-2-pzCO 2 )Fe II (CN) 5 ] − has been studied in micellar (dodecylsulphate sodium salt (SDS) and hexadecyltrimethylammonium chloride (CTACl)) solutions. The true (unimolecular) electron transfer rate constant as well as the parameters controlling this rate constant, the reorganization free energy and the reaction free energy were obtained. This has permitted us to establish that, in spite of the similar behaviour observed for k et in both kinds of micellar solutions, in the case of SDS solutions the kinetics is controlled by both the driving force and the reorganization energy. In the case of CTACl, only the reorganization energy controls the k et variations.

ELECTRON TRANSFER REACTIONS IN MICELLAR SYSTEMS

The influence of micelles on electron transfer processes is reviewed. The micelles modify the rate of electron transfer reactions by producing changes in all the relevant parameters controlling this rate; that is, through modification of the reorganization free energy, the reaction free energy, the nuclear dynamics and the strength of the coupling between the donor and the acceptor. Applications of studies on electron transfer reactions in micellar systems in different fields are presented.

Estimation of electron transfer rate constants by static (optical and electrochemical) measurements

Abstract A kinetic study of the reduction of Co(NH 3 ) 5 (DMSO) 3+ (DMSO=dimethylsulfoxide) by Fe(CN) 6 4− has been carried out in several water–cosolvent mixtures at 298.2 K. The cosolvents used were ethylene glycol, acetonitrile, methanol and glucose. The free energies of activation for this thermal electron-transfer reaction have been calculated from a combination of spectroscopic and electrochemical data and compared with those obtained from the kinetic study. Quantitative agreement is found between both series of data. This shows the possibility of estimating activation free energies for electron transfer reactions from these (static) measurements.

Binding of 12-s-12 dimeric surfactants to calf thymus DNA: Evaluation of the spacer length influence.

Several cationic dimeric surfactants have shown high affinity towards DNA. Bis-quaternary ammonium salts (m-s-m) have been the most common type of dimeric surfactants investigated and it is generally admitted that those that posses a short spacer (s≤3) show better efficiency to bind or compact DNA. However, experimental results in this work show that 12-s-12 surfactants with long spacers make the surfactant/ctDNA complexation more favorable than those with short spacers. A larger contribution of the hydrophobic interactions, which control the binding Gibbs energy, as well as a higher average charge of the surfactant molecules bound to the nucleic acid, which favors the electrostatic attractions, could explain the experimental observations. Dimeric surfactants with intermediate spacer length seem to be the less efficient for DNA binding.

P-Sulfocalix[6]arene as Nanocarrier for Controlled Delivery of Doxorubicin

Given the high toxicity of the anthracycline antibiotic doxorubicin (DOX), it is relevant to search for nanocarriers that decrease the side effects of the drug and are able to transport it towards a therapeutic target Here, the encapsulation of DOX by p-sulfocalix[6]arene (calix) has been studied. The interaction of DOX with the macrocycle, as well as with DNA, has been investigated and the equilibrium constant for each binding process estimated. The results showed that the binding constant of DOX to DNA, KDNA , is three orders of magnitude higher than that to calix, Kcalix . The ability of calixarenes to encapsulate DOX molecules, as well as the capability of the DOX molecules included into the inner cavity of the macrocycle to bind with DNA have been examined. Cytotoxicity measurements were done in different cancer and normal cell lines to probe the decrease in the toxicity of the encapsulated DOX. The low toxicity of calixarenes has also been demonstrated for different cell lines.

Determination of reaction and reorganization free energies of electron transfer reactions under restricted geometry conditions

In this paper, different methods of obtaining the two parameters controlling the rate of electron transfer processes (reaction and reorganization free energies, Λ and ΔG0’, respectively) under restricted geometry conditions are considered. The main difficulty of accomplishing this comes from lack of knowledge of the properties in the interfacial region, where the reaction occurs. A general method has been presented and illustrated with the study of intermolecular processes in micelles. This method is optimized when the free energies for (at least) the three reactions required are quite different. For excited state electron transfer, the general approach is based on the appearance of the so-called Marcus inverted region: at the starting point of this region the value of ΔG0’ gives the value of Λ directly. These reaction free energies also present some uncertainties because in their calculation it is necessary to know the value of the local dielectric constant. Finally, it should be mentioned that some authors have suggested that the treatments for electron transfer reactions could not be applicable under restricted conditions. However, experiments do seem to show the applicability of the Marcus-Hush treatment under these conditions.

Importance of hydrophobic interactions in the single-chained cationic surfactant-DNA complexation

The goal of this work was to understand the key factors determining the DNA compacting capacity of single-chained cationic surfactants. Fluorescence, zeta potential, circular dichroism, gel electrophoresis and AFM measurements were carried out in order to study the condensation of the nucleic acid resulting from the formation of the surfactant-DNA complexes. The apparent equilibrium binding constant of the surfactants to the nucleic acid, Kapp, estimated from the experimental results obtained in the ethidium bromide competitive binding experiments, can be considered directly related to the ability of a given surfactant as a DNA compacting agent. The plot of ln(Kapp) vs. ln(cmc), cmc being the critical micelle concentration, for all the bromide and chloride surfactants studied, was found to be a reasonably good linear correlation. This result shows that hydrophobic interactions mainly control the surfactant DNA compaction efficiency.

Fluorescence quenching of 1-pyrene-carboxaldehyde by iodide ions in the presence of anionic (SDS) and cationic (CTAC) micelles: a quantitative treatment

The quenching of the fluorescence of 1-pyrene-carboxaldehyde by iodide ions has been studied in the presence of micellar solutions of CTAC and SDS. In the presence of the latter surfactant, the fluorophore accumulates at the micelles whereas the quencher is repelled from the aggregates. Consequently, a decrease of the quenching is observed when the concentration of micelles increases. In the CTAC case, both the 1-pyrene-carboxaldehyde and the iodide ions accumulate on the micelles, so that a maximum in the quenching is observed (see Fig. 3). These facts can be formally rationalized using the pseudophase model. However, the meaning of the parameters appearing in the equations of this model are, for photochemical quenching processes, different from those corresponding to ground state reactions. The quantitative analysis of the parameters has been carried out employing a formulation previously developed by us, which is extended here in order to take into account the particular characteristics of micellar solutions.

Reversibility of the interactions between a novel surfactant derived from lysine and biomolecules.

In this work the novel cationic surfactant derived from lysine (S)-5-acetamido-6-(dodecylamino)-N,N,N-trimethyl-6-oxohexan-1-ammonium chloride, LYCl, was prepared and the physicochemical characterization of its aqueous solutions was carried out. The binding of LYCl to bovine serum albumin, BSA, and to double stranded calf thymus DNA, ctDNA, was investigated using several techniques. Results show that LYCl binding to BSA is followed by a decrease in the α-helix content caused by the unfolding of the protein. LYCl association to ctDNA mainly occurs through groove binding and electrostatic interactions. These interactions cause morphological changes in the polynucleotide from an elongated coil structure to a more compact globular structure, resulting in the compaction of ctDNA. Addition of β-cyclodextrin, β-CD, to the BSA-LYCl and ctDNA-LYCl complexes is followed by the refolding of BSA and the decompaction of ctDNA. This can be explained by the ability of β-CD to hinder BSA-LYCl and ctDNA-LYCl interactions due to the stronger and more specific β-CD-LYCl hydrophobic interactions. The stoichiometry of the β-CD:LYCl inclusion complex and its formation equilibrium constant were determined in this work. The reported procedure using β-CD is an efficient way to refold proteins and to decompact DNA, after the morphological changes caused in the biomolecules by their interaction with cationic surfactants.

Stoppering/unstoppering of a rotaxane formed between an N-hetorycle ligand containing surfactant: β-cyclodextrin pseudorotaxane and pentacyanoferrate(II) ions

The assembly of a surfactant-based rotaxane by adding the labile aquopentacyanoferrate(II) ion to the previously formed pseudorotaxane between the surfactant 11-(isonicotinoyloxy)-N,N,N-triethyl-1-undecanaminium bromide and β-cyclodextrin was investigated by 1H NMR and kinetic measurements. NMR spectroscopy has showed that the rotaxane can be formed through two different mechanisms. The rotaxane can be unstoppered by using the pyridine ligand substitution reaction by the high-field cyanide ligand. In this work a new method is developed for the preparation of several new surfactant-based rotaxanes by changing the hydrophilic and hydrophobic regions of the surfactants and the nature of the macrocycle.