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-ethylhexyl) 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 n-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)=[H(2)O]/[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. Copyright 2001 Academic Press.