Substituent Effects on Binding Constants of Carotenoids to n-Heptane/AOT Reverse Micelles

✍ Scribed by N.Mariano Correa; Edgardo N. Durantini; Juana J. Silber

• Publisher: Elsevier Science
• Year: 2001
• Tongue: English
• Weight: 95 KB
• Volume: 240
• Category: Article
• ISSN: 0021-9797
• DOI: 10.1006/jcis.2001.7640

✦ Synopsis

The absorption spectra of 6 -apo-β-caroten-6 -ol (1), 6 -apo-βcaroten-6 -oic acid (2), and ethyl 6 -apo-β-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 π * . 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 π -electron system. They are also sensitive to the ability of the solvent to accept protons in a hydrogen bond interaction measured by β. 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.