M.A. Martin

Analytical applications of retinoid-cyclodextrin inclusion complexes. 1. Characterization of a retinal-β-cyclodextrin complex

In studies in these laboratories on the supramolecular chemistry of the retinoids, it has been recently confirmed that inclusion of these substances within the cavity of cyclodextrins protects their excited states, thus improving their photochemical stability. In the present paper, the isolation is described of a crystalline stable complex between retinal and beta-cyclodextrin, which has been characterized by means of several techniques including atomic force microscopy (AFM). The complex shows distinct spectroscopic differences from both retinal and beta-cyclodextrin. Thus, it absorbs at lambda(max) = 380 nm in water whereas retinal is insoluble; it shows room-temperature luminescence, which retinal does not; finally, it give 1H-NMR and 13C-NMR spectra in d6-DMSO with clear differences in chemical shifts with respect to those of beta-cyclodextrin. Besides these studies in solution, the behaviour of the complex in the solid state has been compared with that of physical mixtures of retinal and beta-cyclodextrin. IR spectroscopy shows clear differences, particularly a shift in the retinal carbonyl absorption (1644-1672 cm-1). AFM studies reveal the existence of aggregates; X-ray diffractometry also supports the formation of a cyclodextrin-retinal complex.

Spectrofluorimetric determination of stoichiometry and association constants of the complexes of harmane and harmine with β-cyclodextrin and chemically modified β-cyclodextrins ☆

The association characteristics of the inclusion complexes of the beta-carboline alkaloids harmane and harmine with beta-cyclodextrin (beta-CD) and chemically modified beta-cyclodextrins such as hydroxypropyl-beta-cyclodextrin (HPbeta-CD), 2,3-di-O-methyl-beta-cyclodextrin (DMbeta-CD) and 2,3,6-tri-O-methyl-beta-cyclodextrin (TMbeta-CD) are described. The association constants vary from 112 for harmine/DMbeta-CD to 418 for harmane/HPbeta-CD. The magnitude of the interactions between the host and the guest molecules depends on the chemical and geometrical characteristics of the guest molecules and therefore the association constants vary for the different cyclodextrin complexes. The steric hindrance is higher in the case of harmine due to the presence of methoxy group on the beta-carboline ring. The association obtained for the harmane complexes is stronger than the one observed for harmine complexes except in the case of harmine/TMbeta-CD. Important differences in the association constants were observed depending on the experimental variable used in the calculations (absolute value of fluorescence intensity or the ratio between the fluorescence intensities corresponding to the neutral and cationic forms). When fluorescence intensity values were considered, the association constants were higher than when the ratio of the emission intensity for the cationic and neutral species was used. These differences are a consequence of the co-existence of acid-base equilibria in the ground and in excited states together with the complexation equilibria. The existence of a proton transfer reaction in the excited states of harmane or harmine implies the need for the experimental dialysis procedure for separation of the complexes from free harmane or harmine. Such methodology allows quantitative results for stoichiometry determinations to be obtained, which show the existence of both 1:1 and 1:2 beta-carboline alkaloid:CD complexes with different solubility properties.

Modified β-cylclodextrins as enhancers of fluorescence emission of carbazole alkaloid derivatives

Experimental conditions for formation of inclusion complexes between carbazole and ellipticine and several cyclodextrins (CDs): β-cyclodextrin (β-CD), hydroxypropyl β-cyclodextrin (HPβ-CD), (2,6-di-O-methyl)-β-cyclodextrin (DMβ-CD), (2,3,6-tri-O-methyl)-β-cyclodextrin (TMβ-CD) and γ-cyclodextrin (γ-CD), are described. A notable increase of fluorescence intensity of the host molecules is observed upon complexation, attributed to protection of the excited state within the cyclodextrin cavity. This allows the quantitation of both compounds in aqueous media in concentration levels of 10−7 M to 10−6 M. Inclusion also affects the deprotonation equilibrium of the pyrrolic nitrogen of both compounds. Thus, β-CD facilitates the deprotonation of carbazole while HPβ-CD, DMβ-CD, TMβ-CD and γ-CD make it more difficult. Similar observations were made for ellipticine, although its behaviour was more complex because of the coexistence of two ionization equilibria owing to the presence of a basic pyridine-like nitrogen. Finally, the effect of cyclodextrins on fluorescence quenching caused by heavy halogen atoms was also studied. In the case of carbazole, a static quenching was observed, which is prevented by modified β-CDs. In the case of ellipticine, the quenching effect was related to acid-base equilibria, and therefore it has been studied under acidic (λex = 305 nm, λem = 520 nm) and basic (λex = 292 nm, λem = 430 nm) conditions.

Solvent effects on the fluorescent emission of some new benzimidazole derivatives

Abstract The benzimidazole moiety is present in a great variety of pharmaceutical compounds with anthelmintic action and other applications. These compounds exhibit native fluorescence which often depends on the substituents on the benzimidazole ring and the solvents. The fluorescence emission also changes with pH. In this article the solvatochromic effect is described in different solvents (polar protic and aprotic, alkanes, pyridine and acetic acid) of a series of new benzimidazole derivatives, namely 1-methyl-6-nitro-benzimidazole (6-NBI), 1-methyl-6-aminobenzimidazole (6-ABI) and 1-methyl-6-(p-tolylamino)benzimidazole (6-TABI). The fluorescence intensity is weaker for 6-NBI than for the other derivatives in all solvents tested due to quenching of fluorescence emission by the nitro group. The excitation and emission wavelengths are red shifted for 6-TABI with respect to those observed for 6-NBI and 6-ABI, due to the presence of the tolyl group. The fluorescence emission maximum is shifted for 6-ABI from λem = 362 nm in formic acid to λem = 470 nm in pyridine; this behaviour is explained in terms of the acid-base characteristics of these compounds. A remarkable shift in the emission wave-length can be also observed for 6-ABI with changing solvent polarity from 354 nm in alcohols to 420 nm in dimethyl sulphoxide. This behaviour could be related to the different equilibria that take place in the excited state, i.e., acid-base equilibria and also charge transfer processes, depending on the solvents.

Use of micellar media for the fluorimetric determination of ellipticine in aqueous solutions.

Ellipticine is a pyridocarbazole alkaloid with interesting antitumour activity. Use of neutral ellipticine is hampered by its very low water solubility and therefore this compound has been administered as a salt; however, nitrogen quaternization modifies the antitumour properties of ellipticine. Potential alternatives to quaternization include the use of cyclodextrins, and also the use of micellar media. The latter possibility is explored in this work as an analytical tool. The results obtained with model anionic (SDS), cationic (CTAB) and neutral (Brij-35) surfactants are described. Fluorimetric analysis shows that ellipticine solubilizes completely in the presence of all these compounds, as a result of its aromatic, planar structure. The use of micellar media considerably increases the slopes of the calibration curves with improved correlation coefficients (e.g. 0.8904 in water and 0.9982 with SDS). Micellar media also modify proton transfer processes, as a consequence of the apolar environment of the micellar phase. Deprotonation of ellipticine is hampered in SDS because of the relationship between this process and the surface charge of the micelles. Finally, fluorescence quenching in micellar media has been studied, it being found that surfactants provide protection towards this phenomenon.

Detection and characterization of cyclodextrin complexes with β-carboline derivatives by spectroscopic techniques

beta-Carboline alkaloids exhibit a great variety of pharmacological activities. The solid inclusion complexes of harmane and harmine with beta-cyclodextrin and also with hydroxypropyl-beta-cyclodextrin, have been prepared following different procedures. IR and NMR spectroscopies were employed to verify the interaction of the guest molecules with the cyclodextrin cavities. The differences observed in the IR and NMR spectra are in agreement with those described in the literature for other guest molecules. The shifts in the 13C- and 1H-NMR spectra confirm the existence of the inclusion complexes. The fluorescence emission spectra of these complexes dissolved in buffered aqueous solution (pH 7.3) exhibit the characteristic peaks of the cationic form for harmane alkaloids. The neutral bands are not present for the free alkaloids in aqueous solutions. Fluorescence quenching emission of the complexes is compared to that of the corresponding free alkaloids.

The use of fluorescent probes in pharmaceutical analysis.

Even though many pharmaceuticals show native fluorescence, there is also an important group of compounds which is not fluorescent. A main object of studies is to make them fluorescent, principally by using fluorescent probes through derivatisation reactions. An account of the fluorescent probes more widely used for the determination of drugs and related compounds is presented in this review paper. A wide variety of fluorescent probes is described on the basis of their ability to react specifically with various functional groups. Attention is focused on derivatisation reactions used in spectrofluorimetry and chromatographic techniques (HPLC, TLC) with fluorimetric detection. The review covers only those fluorescent probes whose use involves a chemical reaction with the analyte, and not those methods that involve physico-chemical interactions such as sensitised or charge transfer processes. Many of these derivatisation reactions have been widely used in the detection of primary and secondary amines. Reagents such as dansyl chloride, fluorescamine, o-phtalaldehyde are very well known. Other reagents have also been developed for other functional groups, for example dansyl hydrazine for compounds with a carbonyl function or 4-bromomethyl-7-methoxy-coumarin for acidic compounds . Acid chlorides such as dansyl chloride may also react with different functional groups carrying active hydrogens as do phenols. The use and development of new fluorescent probes in pharmaceutical analysis is a subject for further studies.

Study of the lumiescence properties of a new series of quinolizinium salts and their interaction with DNA

The spectrofluorimetric characteristic of a new group of benzo- and methyl- quinolizinium salts at room temperature and 77 LK are reported. At room temperature, linear calibration is wide; 10-9 M 9-cyanobenzo [a] phenathro [9,10-g] quinolizinium chloride can be detected in methanolic solution and 10-7 M in aqueous solution. The polynuclear compounds show the most intense luminescence bands, and a significant hypsochromic shift of the fluorenscence emission maximum was observed at 77 K compared with room temperature. For the 2,3-diphenyl derivatives, the presence of a methoxy substituent produces a marked Stokes shift, because it causes a decrease in the planarity of the molecule. The benzo compounds are similar in structure to the alkaloid coralyne, which has significant antileuikemic activity. The fused planar aromatic compounds are shown to bind with DNA.

Spectrofluorimetric study of the effects of cyclodextrins on the acid-base equilibria of harmine and harmane

β-Carboline alkaloids are important compounds because they exhibit a variety of pharmacological actions. Their acid–base behaviour can be studied by spectrofluorimetry since these molecules present a remarkable native luminescence. Acid–base equilibria depend on the environment of the molecules and inclusion into cyclodextrin (CD) cavities shifts the acid–base equilibria and alters the apparent pK a values. The influence of CDs on the acid–base equilibria of the model β-carbolines harmine and harmane is described. β-CD and γ-CD and the modified β-CDs hydroxypropyl-β-CD (HPβ-CD), 2,6-di-O-methyl-β-CD (DMβ-CD) and 2,3,6-tri-O-methyl-β-CD (TMβ-CD) were used to form the corresponding complexes with harmine and harmane in the pH range 7.8–8.0. In these buffered solutions the complexes with the different CDs exhibit an emission band with resolved peaks at 360 and 380 nm corresponding to the neutral form of harmane and with a remarkable enhancement in the emission intensity compared with aqueous solution. In the case of the complexes with β-CD and γ-CD, both the cationic and the neutral emission bands appear. However, for γ-CD the cationic band is more intense than the neutral band, the inverse being true for β-CD. In homogeneous aqueous solution at this pH value the cationic band is the only one observed and therefore the presence of the neutral band indicates the formation of inclusion complexes. In the harmane–HPβ-CD complexes, the emission bands ascribed to the anionic form are observed after addition of NaOH. This emission is only observed in homogeneous aqueous solution in strongly alkaline media outside the normal pH range.

The influence of solvent polarity and viscosity on fluorescence of quinolizinium salts

Abstract The effects of dielectric constant, viscosity and polarity of the solvents used on the relative fluorescent emission of 2,3-diphenylquinolizinium bromide and 2(p-dimethyl-amino)styryl-3-methylquinolizinium bromide are described. These salts were entirely dissolved in ten different solvents. The 2(p-dimethylamino)styryl derivative produced different ionic species, and was a fluorescent acid-base indicator. The fluorescence of these quinolizinium salts is shown to increase with greater viscosity of the medium.