Estudio estructural y dinámico de procesos de asociación supramolecular

En el desarrollo de esta tesis doctoral se han estudiado diversos sistemas supramoleculares formados por la agregación de tensioactivos mediante el uso de distintos huéspedes fluorescentes con el fin de analizar el proceso de formación de las estructuras micelares y su interacción con moléculas de distinta hidrofobicidad. Se ha propuesto un modelo, denominado modelo APN, para describir la formación de las micelas que permite realizar un análisis cuantitativo de cada una de las especies que son responsables de la formación de las mismas. Este modelo permite calcular la concentración de cada especie en función de la concentración total de surfactante y obtener el valor de la cmc para cualquier tensioactivo independientemente de la técnica empleada. La cmc es uno de los parámetros necesarios para la utilización de los surfactantes en sus diversas aplicaciones. Las expresiones utilizadas para estudiar la formación de las micelas mediante el modelo APN son precisas y sencillas, y evitan la discontinuidad en la cmc que presentan los modelos tradicionales; para cada concentración de surfactante todas las especies, monómeros y agregados, coexisten. Para dar cuenta del paso de estado monomérico a micelar se define un parámetro de transición r, cuyo valor refleja la anchura de dicho intervalo de transición y da información acerca de la estructura y de las fuerzas de interacción de las micelas. La validez del modelo para diversos tensioactivos con diferentes características estructurales se ha demostrado tanto con datos obtenidos en nuestro grupo mediante técnicas de fluorescencia y de conductividad eléctrica, como con datos de la bibliografía medidos con otras técnicas. Aprovechando las posibilidades que ofrece el modelo APN estudiamos el comportamiento del pireno, que es el fluoróforo más utilizado en la caracterización de sistemas micelares, pero que presenta una elevada complejidad dadas sus características fotofísicas especiales. Con el modelo APN es posible explicar todos los procesos implicados conjuntamente, como son la formación de excímero, el quenching del pireno por parte del tensioactivo o el equilibrio de partición entre el pireno y las micelas. Hemos obtenido excelentes ajustes de los datos de fluorescencia de estado estacionario que hemos corroborado mediante medidas de fluorescencia resuelta en el tiempo. La utilización de la relación de intensidades en 2 picos del espectro de emisión del pireno es un procedimiento habitual para la caracterización de los sistemas micelares. Sin embargo, nuestro trabajo demuestra que éste es un método poco preciso y que el valor obtenido para la cmc está influido la elevada correlación entre los parámetros de ajuste, especialmente entre la cmc y la constante de equilibrio de interacción entre el pireno y las micelas. No obstante, los estudios realizados con diversos tipos de surfactantes nos han permitido establecer tendencias generales de cómo la constante de equilibrio cambia dependiendo de la estructura de los tensioactivos que forman las micelas, avanzando hacia una mejor comprensión de la interacción de moléculas hidrofóbicas con las micelas de tensioactivos

Fluorescence Emission of Pyrene in Surfactant Solutions

This is the accepted manuscript of the following article: Piñeiro, L., Novo, M., & Al-Soufi, W. (2015). Fluorescence emission of pyrene in surfactant solutions. Advances In Colloid And Interface Science, 215, 1-12. doi: 10.1016/j.cis.2014.10.010. © 2015 Elsevier B.V. This manuscript version is made available under the CC-BY-NC-ND 4.0 license (http://creativecommons.org/licenses/by-nc-nd/4.0/)The systematic description of the complex photophysical behavior 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 KTX100 = 3300 103 M-1 and KSDS = 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 regionL.P. thanks the Xunta de Galicia for his research scholarships. M.N. and W.A. thank the Ministerio de Ciencia e Innovación and the Xunta de Galicia for their financial support (INCITE09262304PR, CTQ2010-21369, CN2012/314, GPC2013/052)S

Evidence of cyclodextrin aggregation obtained with Fluorescence Correlation Spectroscopy

The 14th International Electronic Conference on Synthetic Organic Chemistry session Polymer ChemistryWe have studied the complexation between an adamantane derivative labelled with the fluorescent probe Alexa 488 and the three natural cyclodextrins (-, - and -CD) by Fluorescence Correlation Spectroscopy (FCS), demonstrating the ability of this technique to detect association and to determine the corresponding equilibrium constants with no need for changes in the fluorescence properties of the guest. At low CD concentrations the observed increase of the diffusion time of the probe is mainly due to the complexation of the adamantyl moiety, but further changes are observed when increasing CD concentration that are attributed to the formation of CD aggregates. These aggregates appear at quite low CD concentrations and seem to be formed by a small number of CD molecules. These results show the potential of FCS for the study of CD self-assembly, a recently-recognized phenomenon that could be used to improve certain applications of CDsThe authors thank the Ministerio de Ciencia e Innovación and the Xunta de Galicia for financial support (CTQ2007-68057-C02-02/BQU, INCITE09E2R209064ES, INCITE09262304PR, 2009/029). J. Bordello and S. Freire thank Ministerio de Educación and Xunta de Galicia for scholarship

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

This is the accepted manuscript of the following article: Al-Soufi, W., Piñeiro, L., & Novo, M. (2012). A model for monomer and micellar concentrations in surfactant solutions: Application to conductivity, NMR, diffusion, and surface tension data. Journal Of Colloid And Interface Science, 370(1), 102-110. doi: 10.1016/j.jcis.2011.12.037. © 2012 Elsevier B.V. This manuscript version is made available under the CC-BY-NC-ND 4.0 license (http://creativecommons.org/licenses/by-nc-nd/4.0/)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 authorsL.P. thanks the Gil Dávila Foundation for a scholarship. M.N. and W.A. thank the Ministerio de Ciencia e Innovación and the Xunta de Galicia for their financial support (CTQ2010-21369, INCITE09262304PR, INCITE09E2R209064ES, IN845B-2010/094)S

High PEEP with recruitment maneuvers versus Low PEEP During General Anesthesia for Surgery -a Bayesian individual patient data meta-analysis of three randomized clinical trials

Background: The influence of high positive end-expiratory pressure (PEEP) with recruitment maneuvers on the occurrence of postoperative pulmonary complications after surgery is still not definitively established. Bayesian analysis can help to gain further insights from the available data and provide a probabilistic framework that is easier to interpret. Our objective was to estimate the posterior probability that the use of high PEEP with recruitment maneuvers is associated with reduced postoperative pulmonary complications in patients with intermediate-to-high risk under neutral, pessimistic, and optimistic expectations regarding the treatment effect. Methods: Multilevel Bayesian logistic regression analysis on individual patient data from three randomized clinical trials carried out on surgical patients at Intermediate-to-High Risk for postoperative pulmonary complications. The main outcome was the occurrence of postoperative pulmonary complications in the early postoperative period. We studied the effect of high PEEP with recruitment maneuvers versus Low PEEP Ventilation. Priors were chosen to reflect neutral, pessimistic, and optimistic expectations of the treatment effect. Results: Using a neutral, pessimistic, or optimistic prior, the posterior mean odds ratio (OR) for High PEEP with recruitment maneuvers compared to Low PEEP was 0.85 (95% Credible Interval [CrI] 0.71 to 1.02), 0.87 (0.72 to 1.04), and 0.86 (0.71 to 1.02), respectively. Regardless of prior beliefs, the posterior probability of experiencing a beneficial effect exceeded 90%. Subgroup analysis indicated a more pronounced effect in patients who underwent laparoscopy (OR: 0.67 [0.50 to 0.87]) and those at high risk for PPCs (OR: 0.80 [0.53 to 1.13]). Sensitivity analysis, considering severe postoperative pulmonary complications only or applying a different heterogeneity prior, yielded consistent results. Conclusion: High PEEP with recruitment maneuvers demonstrated a moderate reduction in the probability of PPC occurrence, with a high posterior probability of benefit observed consistently across various prior beliefs, particularly among patients who underwent laparoscopy