Mercedes Novo

Fluorescence Emission of Pyrene in Surfactant Solutions

The systematic description of the complex photophysical behaviour of pyrene in surfactant solutions in combination with a quantitative model for the surfactant concentrations reproduces with high accuracy the steady-state and the time resolved fluorescence intensity of pyrene in surfactant solutions near the cmc, both in the monomer and in the excimer emission bands. We present concise model equations that can be used for the analysis of the pyrene fluorescence intensity in order to estimate fundamental parameters of the pyrene-surfactant system, such as the binding equilibrium constant K of pyrene to a given surfactant micelle, the rate constant of excimer formation in micelles, and the equilibrium constant of pyrene-surfactant quenching. The values of the binding equilibrium constant K(TX100)=3300·10³ M⁻¹ and K(SDS)=190·10³ M⁻¹ for Triton X-100 (TX100) and SDS micelles, respectively, show that the partition of pyrene between bulk water and micelles cannot be ignored, even at relatively high surfactant concentrations above the cmc. We apply the model to the determination of the cmc from the pyrene fluorescence intensity, especially from the intensity ratio at two vibronic bands in the monomer emission or from the ratio of excimer to monomer emission intensity. We relate the finite width of the transition region below and above the cmc with the observed changes in the pyrene fluorescence in this region.

A model for monomer and micellar concentrations in surfactant solutions: application to conductivity, NMR, diffusion, and surface tension data.

An empirical model for the concentrations of monomeric and micellized surfactants in solution is presented as a consistent approach for the quantitative analysis of data obtained with different experimental techniques from surfactant solutions. The concentration model provides an objective definition of the critical micelle concentration (cmc) and yields precise and well defined values of derived physical parameters. The use of a general concentration model eliminates subjective graphical procedures, reduces methodological differences, and thus allows one to compare directly the results of different techniques or to perform global fits. The application and validity of the model are demonstrated with electrical conductivity, surface tension, NMR chemical shift, and self-diffusion coefficient data for the surfactants SDS, CTAB, DTAB, and LAS. In all cases, the derived models yield excellent fits of the data. It is also shown that there is no need to assume the existence of different premicellar species in order to explain the chemical shifts and self-diffusion coefficients of SDS as claimed recently by some authors.

Host-Guest Complexation Studied by Fluorescence Correlation Spectroscopy: Adamantane-Cyclodextrin Inclusion

The host-guest complexation between an Alexa 488 labelled adamantane derivative and β-cyclodextrin is studied by Fluorescence Correlation Spectroscopy (FCS). A 1:1 complex stoichiometry and a high association equilibrium constant of K = 5.2 × 104 M−1 are obtained in aqueous solution at 25 °C and pH = 6. The necessary experimental conditions are discussed. FCS proves to be an excellent method for the determination of stoichiometry and association equilibrium constant of this type of complexes, where both host and guest are nonfluorescent and which are therefore not easily amenable to standard fluorescence spectroscopic methods.

Dynamics of Supramolecular Association Monitored by Fluorescence Correlation Spectroscopy

Supramolecular binding is a key process in many biological systems and in newly developed supramolecular assemblies. Most of the scientific work on these systems is focused on their structural properties and on the thermodynamics of the association process. However, the underlying dynamics are usually much less known, in spite of the great importance they have during the binding process in these highly dynamic systems. Understanding supramolecular binding in biological systems and controlling the functionality of new synthetic supramolecular systems can only be achieved through knowledge of the structure-dynamics relationship. There is a strong need for suitable techniques which cover the typically wide time interval of the association dynamics and which do not need a perturbation of the system. We briefly review high-resolution fluorescence correlation spectroscopy (FCS) as a technique to monitor supramolecular dynamics and to give information on how structure determines the dynamics of host-guest association. The comparison of hosts and guests with different structures shows that geometrical and orientational requirements determine the association rate constant, whereas the dissociation is defined by the strength of specific interactions. As model hosts cyclodextrins and micelles are studied.

Neighborhood and Community Factors: Effects on Deviant Behavior and Social Competence

Socialization in a neighborhood and community at risk, defined in terms of violence, social alienation, school failure, and disruptive behavior, is a risk factor for the acquisition of antisocial and delinquent behavior. In order to test this hypothesis and examine the underlying mechanisms involved, 346 participants, 155 high-risk and 191 low-risk, aged 11 to 13, that is, under the age of criminal responsibility as established by the Spanish Law 5/2000 were selected. The results reveal that high-risk youngsters had higher rates of antisocial behavior and lower levels of social skills (i.e., greater tendency to externalize attribution of responsibility, fewer conflict resolution strategies, lower self esteem, and a lower degrees of emotional intelligence) in comparison to the lower-risk group. Finally, the results and implications of the study are discussed in the light of designing prevention programs.

Exchange-dynamics of a neutral hydrophobic dye in micellar solutions studied by Fluorescence Correlation Spectroscopy

The dynamics of the exchange of the moderately hydrophobic neutral dye Coumarine 152 between the aqueous phase and the phase formed by neutral Triton X-100 micelles is studied by Fluorescence Correlation Spectroscopy. The changes in the photophysical properties of the dye in presence of the micelles are discussed. The low quantum yield, the low saturation threshold and the necessary high energetic excitation of this dye requires a careful selection of the experimental conditions in order to obtain dynamic and diffusional properties with reasonable precision. It is shown that the contrast between the brightness of free and bound dye has a strong influence on the sensitivity of the FCS experiment. The entry rate constant of the dye to the micelles, k(+)=(0.8±0.3)×10(10) M(-1) s(-1), is very near to the diffusion controlled limit. The high association equilibrium constant of K=(129±3)×10(3) M(-1) is mainly determined by the low exit rate constant, k(-)=(0.6±0.2)×10(5) s(-1).

Influence of acidity on the fluorescence spectra of 2-pyridylbenzimidazoles in aqueous solution

The fluorescence spectra of 2-(3-pyridyl)benzimidazole (3PBI) and the quaternary salt 2-(1-methyl-4-pyridinio)-benzimidazole iodide (4PBIQS) in aqueous solutions covering a wide range of acidities have been studied to investigate their photophysical behaviour and the acid–base processes occurring in the first excited singlet states of these species. Depending on the acidity, 3PBI exists in the excited state as the dication, the monocation protonated at the benzimidazole N(3) atom (C*), the neutral species and the anion. Of all the possible processes of interconversion among these species, only the protonation of C* is fast enough to compete with its deactivation. 4PBIQS too has the same species in excited and ground states: the dication D*, the monocation with a positive charge on the pyridyl nitrogen (T*) and the zwitterion Z*. Deprotonation of D*, protonation of T* and deprotonation of T* by the dihydrogen phosphate anion were all detected. 3PBI and 4PBIQS thus allow separate study of the photophysical properties of the two 2-pyridylbenzimidazole monocations, C* and T*, with no interference between the two.

Host-Assisted Guest Self-Assembly : Enhancement of the Dimerization of Pyronines Y and B by γ-Cyclodextrin

Buckle up! The dimerization of small fluorescent guests is strongly enhanced in presence of a cyclodextrin host. The host cavity acts like a belt to assist the self-assembly of guests (see picture). Small variations in the guest structure have significant influence on the stability and geometry of the aggregates.The role of small variations in the structural properties of host and guest molecules on the stoichiometry and strength of supramolecular associations is analyzed. Earlier we found that a change in substituents from pyronine B to pyronine Y has a dramatic effect on both the stability and the dynamics of the association of these guests with beta-cyclodextrin as host. Now we study the association between these two pyronines and a cyclodextrin with a bigger cavity (gamma-cyclodextrin) using UV/Vis absorption and fluorescence spectroscopy. The absorption spectra of the pyronines show complex variations with cyclodextrin concentration indicating that pyronine dimerization is strongly enhanced inside the cavity of the cyclodextrin. A full model is proposed and the equilibrium constants of the involved processes and the absorption and emission spectra of the different species are estimated. The equilibrium constants of the formation of complexed dimers are much higher than those for free dimerization or for the inclusion of a single guest. The gamma-cyclodextrin host acts like a belt to assist the guest self-assembly. The differences in the stability of pyronine B and pyronine Y dimers are explained on the basis of their structure and geometry.

Complexation of methyl orange with ß-cyclodextrin: detailed analysis and application quantification of polymer-bound cyclodextrin

The formation of inclusion complexes between methyl orange (MO) and β-cyclodextrin (β-CD) was studied extensively using UV-visible absorption spectroscopy. In neutral-to-basic media, where MO is an anion, two complexes of stoichiometries 1:1 and 1:2 (guest:host) are formed, as determined by principal components analysis of the absorption data. Global analysis yields precise values of the corresponding association constants and the spectra of each species: free MO, complex MO · CD and complex MO · 2CD. In acid media, where MO is a zwitterion, only 1:1 complexes are formed, but the system becomes complicated due to tautomerization of MO. The microscopic equilibria among the two tautomers and their corresponding complexes were resolved and estimations of the microscopic association constants were obtained. Finally, the application of MO complexation to the quantification of bound β-CD in a polymer sample is presented, with a detailed description of the experimental conditions to be used and a critical discussion of the results obtained.

Assessment of the standard forensic procedure for the evaluation of psychological injury in intimate-partner violence

In judicial terms, a victim refers to any person who has suffered injury arising from an action or omission of an action that constitutes an offence, and the burden of proof lies with the prosecution. A review of Spanish judicial judgements underscored that the lack of evidence of psychological injury in cases of intimate-partner violence (IPV) accounted for approximately 40% of acquittals. Thus, the Spanish standard of proof for the forensic evaluation of psychological injury i.e., the MMPI-2 and the unstructured interview were assessed in order to determine if they met the statutory requirement for the assessment of psychological injury and the differential diagnosis of feigning. The results of the comparison of 51 women victims of IPV with firm convictions against their aggressors, and 54 women mock victims of IPV showed that the F, K, Fb, Fp and Ds scales, and the F-K index discriminated significantly and with medium and large effect sizes, between adjudicated and mock victims. However, the results did not provide a valid decision criterion for forensic settings i.e., false negatives (identifying feigner as honest protocols) were not classified correctly. In conclusion, the standard forensic procedure for the evaluation of psychological injury in cases of IPV did not constitute valid proof for judges who acquitted defendants on the grounds of not proven due to the lack of evidence of psychological injury.