Folding and hydrodynamic forces in J-aggregates of 5-phenyl-10,15,20-tris(4-sulfophenyl)porphyrin.
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Biomedical subjects
Joaquim Crusats
Publications and source records attributed to Joaquim Crusats.
Homochirality as a consequence of thermodynamic equilibrium?
Recent results on the crystallisation/dissolution equilibrium of enantiomorphic crystals of NaClO3 lead to the conclusion that liquid-phase systems composed of achiral or fast racemising compounds yielding enantiomorphic solid phases (racemic conglomerates) can derive spontaneously to a single homochiral solid phase. This is a thermodynamically driven total resolution, which can only occur when the system is so perturbed that chiral recognition between the species of the system becomes feasible.
Chlorates↗
Self-organized propagation patterns from dynamic self-assembly in monolayers.
Propagation of localized orientational waves, as imaged by Brewster angle microscopy, is induced by low intensity linearly polarized light inside axisymmetric smectic-C confined domains in a photosensitive molecular thin film at the air/water interface (Langmuir monolayer). Results from numerical simulations of a model that couples photoreorientational effects and long-range elastic forces are presented. Differences are stressed between our scenario and the paradigmatic wave phenomena in excitable chemical media.
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Photoinduced molecular reorientation dynamics in confined domains of Langmuir monolayers.
In a photoresponsive Langmuir monolayer comprised of smectic-C-like domains of mesogenic trans-azobenzene derivative embedded within an isotropic matrix of its cis isomer counterpart, several structurally differing circular droplets were irradiated with linearly polarized light. This report describes the structural rearrangements that occurred in these droplets upon illumination followed by Brewster angle microscopy analysis. Starting from initial well-characterized and symmetric states, final photoaligned situations were reached in which the azimuth angles of the rod-shaped elongated molecules were found to be perpendicular to the electric component of the excitation light. The dynamical aspects of the photoalignments, including their transient patterns, are captured by a theoretical model that couples a relaxational principle incorporating long-range elastic forces with a kinetic formalism presenting an anisotropic rate law.
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Photoswitchable orientational patterns of confined domains in monolayers.
We report here a photoswitch process that involves collective molecular reorientation in a monolayer of an azobenzene derivative. Using polarized light we force the transition between two clearly distinguishable orientational mesoscopic configurations that can be monitored by reflection optical microscopy. A model that combines thermodynamic and kinetic arguments is proposed, and it is able to reproduce both the two states and the mechanism involved in the transition. We conclude that the phenomenon reported here is essentially different from the usual electric-field-induced molecular alignment often found in liquid crystalline materials and devices. Instead, it involves a photoexcitation concomitant with an H-aggregation process.
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Texture changes inside smectic-C droplets in azobenzene Langmuir monolayers.
Preparation of Langmuir monolayers of a mixture of trans- and cis-isomers of an azobenzene derivative, 4-[4-[(4-octylphenyl)azo]phenoxy]butanoic acid, results in the segregation of birefringent trans-isomer domains embedded in an isotropic medium of cis-isomers. Brewster angle microscopy observations allow us to identify different textures inside the domains depending on surface pressure, temperature, and domain size. The evolution of the monolayer in the dark, from initial droplets formed after spreading to a stable stripe texture, is described. The dynamics of domain coalescence and some morphological transitions induced by temperature and surface pressure changes are also discussed. A simple theoretical model is included to supplement some of these experimental observations.
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Influence of temperature and composition on the mesoscopic textures of azobenzene Langmuir monolayers.
The monolayer behavior of the azobenzene derivative 8Az3COOH is shown to depend significatively on temperature and on the isomeric (trans-cis) composition. For pure trans monolayers, important temperature effects in the mesoscopic organization (as revealed by means of Brewster angle microscopy, BAM) are observed for the low-pressure phase in the studied temperature range (10 degrees C < T < 40 degrees C). Mixed trans-cis monolayers show that both isomers are virtually immiscible, leading to a phase segregation into birefringent, nearly pure trans droplets surrounded by an isotropic, nearly pure cis matrix. The existence of well-defined anchoring conditions at the droplet boundaries leads to highly symmetric textures, amenable of quantitative BAM image analysis, which helps to better visualize mesoscopic changes induced by variations in the control parameters (temperature, surface pressure or irradiation).
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Modulation of the porphyrin inner proton exchange rates by the steric effects of bridge substitution.
The tautomeric equilibria and H-N proton transfer taking place in the nonsymmetrically substituted water-soluble 2-sulfonato-5,15-bis(4-sulfonatophenyl)porphyrin (1) and in its 10-bromo-substituted derivative (2) were analyzed by NMR methods: 1H and 13C spectroscopies and heteronuclear multiple bond correlation (HMBC) and heteronuclear single quantum coherence (HSQC) 1H-13C and 1H-15N techniques. The existence of preferred pathways of H-N transfer was detected. The conclusions are rationalized by taking into account the effect partial meso-substitution exerts on the relative energies of the different cis-tautomer intermediates involved in the tautomerism. These results underline the experimental consequences stemming from the nonequivalence in porphyrins between the 'tautomeric interconversion' and 'proton transfer' terms, when observed by NMR techniques, as a consequence of the existence of two pairs of degenerate tautomers and transformation pathways.
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Regioselective Porphyrin Bridge Cleavage Controlled by Electronic Effects. Coupled Oxidation of 3-Demethyl-3-(trifluoromethyl)mesohemin IX and Identification of Its Four Biliverdin Derivatives.
This report describes the nonenzymatic oxidative cleavage of the title porphyrin (2) performed with oxygen and ascorbic acid in aqueous pyridine at 37 degrees C (coupled oxidation), via hydrolysis of the corresponding verdoheme intermediates, followed by esterification of the resulting free acid mesobiliverdin analogues to their dimethyl esters 4 (alpha isomer), 5 (beta isomer), 6 (gamma isomer), and 7 (delta isomer). The four biliverdin derivatives were purified by HPLC, and their structures were confirmed by FAB MS and also by UV-vis and (1)H NMR spectroscopies. The purity of each compound was checked by (19)F NMR, and the four regioisomers were assigned through their 2D-NMR ROESY spectra and confirmed by UV-vis spectroscopy. The ratio of regioisomers was determined by (19)F NMR spectroscopy before any purification of single compounds was attempted: alpha:beta:gamma:delta 11:6:26:57 (%). This unusually high regioselectivity was attributed to the electron-withdrawing effect of the CF(3) group on the electronic structure of porphyrin as shown considering the ab initio calculations of an iron(II) beta-substituted (trifluoromethyl)porphyrin used as a model compound. In porphyrin 2, the oxidation clearly takes place at the electron richest meso positions, the order of reactivity strictly following that of electron density, pointing out that the regiospecificity of the bridge cleavage can be effectively controlled by the electronic effects of some strategic substituents in the chromophore. The relevance of all these results in the study of the mechanism of the reactions involved in the natural catabolism of heme, catalyzed by heme oxygenase, is discussed. The advantages of 2 derived from this work, which make it a suitable model compound for the enzymatic reaction, are also discussed.
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