Chiroptical properties of lecithin reverse micelles and organogels

The ultraviolet absorbance and circular dichroism (CD) spectra of lecithin reverse micelles and gels were investigated in order to establish whether the formation of these noncovalent macromolecular aggregates, which was induced by the addition of water to solutions of lecithin in organic solvents, was accompanied by specific spectroscopic changes. Systems containing the synthetic short‐chain lecithins, 1,2‐hexanoyl‐, 1,2‐diheptanoyl‐, 1,2‐dioctanoyl‐, and 1,2‐dinonaoyl‐sn‐glycero‐3‐phosphatidylcholines were used as models for the long‐chain lecithins, soybean phosphatidylcholine and palmitoyl‐oleoyl‐phosphatidylcholine. All the molecules studied had asymmetric centres, formed reverse micelles under appropriate conditions, and, while both the long‐chain lecithins also formed gels, none of the short‐chain molecules did. As well as having CD spectra that were simpler to interpret, spectroscopic observations on solutions of the short‐chain lecithins could be carried out over a large water content range. The ester chromophore of these compounds was shown to be highly sensitive to variation in both the solvent environment and the temperature, and components of both direct solvent effects and conformational change upon the addition of water were detected in the spectra. The spectra of the longer chain lecithins were complicated by the presence of double bonds although, here again, it was found that significant changes occurred as the water content increased, as monitored by the ester chromophore. However, no specific effect that could be ascribed to gelation alone was detected. The overall picture that emerged was that the ester chromophore of anhydrous micelles gave rise to a specific negative band in the CD spectrum (λmax ≈ 210 nm) whereas a positive CD signal (λmax ≈ 233 nm) was associated with the same chromophore in filled (i.e., hydrated) micelles. The two signals correspond to two different conformational states of the lecithin molecule, the hydrated state being not only more conformationally restricted but also providing a less polar environment for the ester groups, while the addition of water to the system shifts the conformational equilibrium. These observations have been interpreted as showing that only a limited range of lecithin conformation is compatible with the formation of the micellar structure and that it is this constraint, together with those introduced by the overall geometry of the aggregated state, that gives rise to the changes observed in the CD spectrum. Copyright © 1991 Wiley‐Liss, Inc.