Substituent effects on binding constants of carotenoids to n-heptane/AOT reverse micelles

The absorption spectra of 6 ' -apo-beta -caroten-6 ' -ol (1), 6 ' -apo-beta -caroten-6 ' -oic acid (2), and ethyl 6 ' -apo-beta -caroten-6 ' -oate (3) were analyzed in homogeneous media and in reversed micelles of AOT (sodium 1,4-bis(2-ethythexyl) sulfosuccinate) in n-heptane. The possible solute-solvent interactions of these compounds were analyzed in pure solvents by Taft and Kamlet's solvatochromic comparison method. These carotenoids show sensitivity similar to that of medium polarity-polarizability as measured by pi*. Moreover, the absorption spectra of carotenoid 3 and to much less extent carotenoid 2 display broadening of the visible bands induced by polar solvents characteristic of carotenoids that contain a carbonyl functional group in conjugation with the carbon-carbon pi -electron system. They are also sensitive to the ability of the solvent to accept protons in a hydrogen bond interaction measured by beta. This sensitivity follows the expected order: 2 > 1 > 3. In the reverse micellar system, while the spectra for 3 remain unchanged, the intensity of the absorption band characteristic of it-heptane for 1 and 2 decreases as the AOT concentration increases, and a new band develops. This new band is attributed to the solute bound to the micelle interface. These changes allowed us to determine the binding constant (K-b) between these compounds and AOT. At W-0 = [H2O]/[AOT] = 0 the values of K-b of 326 +/- 5 and 6.2 +/- 0.3 were found for the acid 2 and the alcohol 1, respectively. The strength of binding is interpreted considering their hydrogen-bond donor ability and the solubility in the organic pseudophase. For 1 K-b decreases as W-0 is increased, while for 2 no variation was observed. These effects are discussed in terms of carotenoid-water competition for interfacial binding sites. (C) 2001 Academic Press.