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-so
lvent interactions of these compounds were analyzed in pure solvents by Taf
t and Kamlet's solvatochromic comparison method. These carotenoids show sen
sitivity similar to that of medium polarity-polarizability as measured by p
i*. Moreover, the absorption spectra of carotenoid 3 and to much less exten
t carotenoid 2 display broadening of the visible bands induced by polar sol
vents characteristic of carotenoids that contain a carbonyl functional grou
p 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 bon
d 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-hepta
ne 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 i
nterface. 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 hydr
ogen-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 i
nterfacial binding sites. (C) 2001 Academic Press.