The ultrafast, optically-heterodyne-detected optical Kerr effect has been used to investigate the dynamics of the dispersed oil phase of a novel oil-in-water microemulsion. The carbon disulfide-dodecyltrimethylammonium bromide-water (CS 2 -DTAB-H 2 O) system has been characterized by viscosity and conductivity measurements. The results of these measurements are discussed in terms of possible shape changes and interparticle interactions. Ultrafast measurements are reported for several different compositions of the microemulsion. The results are compared and contrasted with the dynamics of neat CS 2 and CS 2 dissolved in n-alkanes. The dynamics of CS 2 dispersed in the microemulsion do not approach those of the neat liquid in any case, even at the highest possible loading, implying that the dynamics in the oil phase are strongly perturbed by the surfactant. The slow (picosecond) diffusive reorientation of the CS 2 does not reflect the macroscopic viscosity of the microemulsion.

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Medium-chain acyl-CoA dehydrogenase (MCAD) catalyzes the flavin-dependent oxidation of fatty acyl-CoAs to the corresponding trans-2-enoyl-CoAs. The interaction of hexadienoyl-CoA (HD-CoA), a product analogue, with recombinant pig MCAD (pMCAD) has been studied using (13)C NMR and (1)H-(13)C HSQC spectroscopy. Upon binding to oxidized pMCAD, the chemical shifts of the C1, C2, and C3 HD carbons are shifted upfield by 12.8, 2.1, and 13.8 ppm, respectively. In addition, the (1)H chemical shift of the C3-H is also shifted upfield by 1.31 ppm while the chemical shift of the C4 HD-CoA carbon is unchanged upon binding. These changes in chemical shift are unexpected given the results of previous Raman studies which revealed that the C3=C2-C1=O HD enone fragment is polarized upon binding to MCAD such that the electron density at the C3 and C1 carbons is reduced, not increased (Pellet et al. Biochemistry 2000, 39, 13982-13992). To investigate the apparent discrepancy between the NMR and Raman data for HD-CoA bound to MCAD, (13)C NMR spectra have been obtained for HD-CoA bound to enoyl-CoA hydratase, an enzyme system that has also previously been studied using Raman spectroscopy. Significantly, binding to enoyl-CoA hydratase causes the chemical shifts of the C1 and C3 HD carbons to move downfield by 4.8 and 5.6 ppm, respectively, while the C2 resonance moves upfield by 2.2 ppm, in close agreement with the alterations in electron density at these carbons predicted from Raman spectroscopy (Bell, A. F.; Wu, J.; Feng, Y.; Tonge, P. J. Biochemistry 2001, 40, 1725-33). The large increase in shielding experienced by the C1 and C3 HD carbons in the HD-CoA/MCAD complex is proposed to arise from the ring current field from the isoalloxazine portion of the flavin cofactor. The flavin ring current, which is only present when the enzyme is placed in an external magnetic field, also explains the differences in (13)C NMR chemical shifts for acetoacetyl-CoA when bound as an enolate to MCAD and enoyl-CoA hydratase and is used to rationalize the observation that the line widths of the C1 and C3 resonances are narrower when the ligands are bound to MCAD than when they are free in the protein solution.