1. Spectroscopic and structural investigation of the confinement of d and l dimethyl tartrate in lecithin reverse micelles.
Angela Ruggirello, Vincenzo Turco Liveri, France Lebon, Franca Castiglione, Walter Panzeri, Andrea Mele, Alessandro Longo, Giovanna Longhi, Sergio Abbate. J Phys Chem B. 2009 Mar 12; 113(10): 3024-33. DOI: 10.1021/jp809793u. PMID: 19708163.
The confinement of D and L dimethyl tartrate in lecithin reverse micelles dispersed in cyclohexane has been investigated by FT-IR, polarimetry, electronic and vibrational circular dichroism (ECD and VCD), 1H NMR, and small-angle X-ray scattering (SAXS). Measurements have been performed at room temperature as a function of the solubilizate-to-surfactant molar ratio (R) at fixed lecithin concentration. The analysis of experimental data indicates that the dimethyl tartrate molecules are solubilized within reverse micelles in proximity to the surfactant head groups in the same way for the D and L forms. The encapsulation of dimethyl tatrate within lecithin reverse micelles involves changes in its H-bonds, from what is observed in the pure solid or in CCl4 solutions; this is a consequence of the establishment of specific solute-surfactant headgroup interactions and of confinement effects. In the 0 < or = R < or = 1.7 range, SAXS profiles of dimethyl tartrate/lecithin/ cyclohexane micellar solutions are well-described by a model of interacting polydisperse spherical micellar cores whose mean radius does not change appreciably with R (i.e., it changes from about 18 to 20 angstroms). 1H NMR diffusion measurements of both dimethyl tartrates and lecithin were rationalized in terms of collective translational motions of the entire micellar aggregate and of their molecular diffusion among clusters of reverse micelles. The association of optically active lecithin with D and L dimethyl tartrate leads to the formation of self-organized supramolecular aggregates whose interesting chiroptical features are evidenced by polarimetry and CD.
2. Kramers-kronig transformation of experimental electronic circular dichroism: application to the analysis of optical rotatory dispersion in dimethyl-l-tartrate.
Prasad L Polavarapu, Peng Zhang, Ana G Petrovic. Chirality. 2006 Sep; 18(9): 723-32. DOI: 10.1002/chir.20310. PMID: 16856171.
When a limited region of the experimental electronic circular dichroism (ECD) spectrum is subjected to Kramers-Kronig (KK) transformation, the resulting optical rotatory dispersion (ORD) may or may not reproduce the experimentally measured ORD in the long-wavelength nonresonant region. If the KK transform of experimentally measured ECD in a limited wavelength region reproduces the experimentally measured ORD in the long-wavelength nonresonant region, then that observation indicates that the ORD in the long-wavelength nonresonant region should be satisfactorily predicted from the correspondingly limited number of electronic transitions in a reliable quantum mechanical calculation. On the other hand, if the KK transform of experimentally measured ECD in a limited region does not reproduce the experimentally measured ORD in the long-wavelength nonresonant region, then it should be possible to identify the ECD bands in the shorter wavelength region that are responsible for the differences between experimentally observed ORD and KK-transformed ECD. This approach helps to identify the role of ECD associated with higher energy-excited states in the nature of ORD in the long-wavelength nonresonant region. These concepts are demonstrated here by measuring the experimental ECD and ORD for dimethyl-L-tartrate in different solvents. While ECD spectra of dimethyl-L-tartrate in different solvents show little variation, ORD spectra in the long-wavelength nonresonant region show marked solvent dependence. These observations are explained using the difference between experimental ORD and KK-transformed ECD. Quantum mechanical predictions of ECD and ORD are also presented for isolated (R, R)-dimethyl tartrate at the B3LYP/aug-cc-pVDZ level.
