ANALYTICAL APPLICATIONS OF RETINOID-CYCLODEXTRIN INCLUSION COMPLEXES .1. CHARACTERIZATION OF A RETINAL-BETA-CYCLODEXTRIN COMPLEX

In studies in these laboratories on the supramolecular chemistry of th e retinoids, it has been recently confirmed that inclusion of these su bstances within the cavity of cyclodextrins protects their excited sta tes, thus improving their photochemical stability. In the present pape r, the isolation is described of a crystalline stable complex between retinal and beta-cyclodextrin, which has been characterized by means o f several techniques including atomic force microscopy (AFM). The comp lex shows distinct spectroscopic differences from both retinal and bet a-cyclodextrin. Thus, it absorbs at lambda(max) = 380 nm in water wher eas retinal is insoluble; it shows room-temperature luminescence, whic h retinal does not; finally, it gives H-1-NMR and C-13-NMR spectra in d(6)-DMSO with clear differences in chemical shifts with respect to th ose of beta-cyclodextrin. Besides these studies in solution, the behav iour of the complex in the solid state has been compared with that of physical mixtures of retinal and beta-cyclodextrin. IR spectroscopy sh ows clear differences, particularly a shift in the retinal carbonyl ab sorption (1644-1672 cm(-1)). AFM studies reveal the existence of aggre gates; X-ray diffractometry also supports the formation of a cyclodext rin-retinal complex.