No abstract

Proton decoupled deuterium NMR spectra of oriented bilayers made of DMPC and 30 mol % deuterated cholesterol acquired at 76.8 MHz (30 degreesC) have provided a set of very accurate quadrupolar splitting for eight C-D bonds of cholesterol. Due to the new precision of the experimental data, the original analysis by. Biochemistry. 23:6062-6071) had to be reconsidered. We performed a systematic study of the influence on the precision and uniqueness of the data-fitting procedure of: (i) the coordinates derived from x-ray, neutron scattering, or force field-minimized structures, (ii) internal mobility, (iii) the axial symmetry hypothesis, and (iv) the knowledge of some quadrupolar splitting assignments. Good agreement between experiment and theory could be obtained only with the neutron scattering structure, for which both axial symmetry hypothesis and full order parameter matrix analysis gave satisfactory results. Finally, this work revealed an average orientation of cholesterol slightly different from those previously published and, most importantly, a molecular order parameter equal to 0.95 +/- 0.01, instead of 0.79 +/- 0.03 previously found for the same system at 30 degreesC. Temperature dependence in the 20-50 degreesC range shows a constant average orientation and a monotonous decrease of cholesterol Smol, with a slope of -0.0016 K-1. A molecular order parameter of 0.89 +/- 0.01 at 30 degreesC was determined for a DMPC/16 mol % of cholesterol.

No abstract

We have measured by means of photoluminescence the energy of crystal-field peaks for RbCdF3: Mn2+ and KZnF3: Mn2+ where the value of the Mn2+–F− distance, R, derived by EXAFS is R=2.13±0.01 Å and R=2.08±0.01 Å, respectively. From these data and those for RbMnF3 and KMnF3 we have studied the dependence on R of the B, C, and 10 Dq parameters for the MnF4−6 complex. This analysis reveals that within the experimental errors, B and C are constant in the range 2.07<R<2.14 Å, in agreement with recent self-consistent calculations for MnF4−6, which also predict that 10 Dq=KR−n, where K and n are constant. The present study confirms this dependence, n being 4.4 which is also in accord to the theoretical predictions. The best values of R derived from optical spectra are found to be R=2.141±0.004 Å (for RbCdF3: Mn2+) and R=2.075±0.004 Å (for KZnF3: Mn2+). The present analysis also points out that by measuring the changes induced on the optical spectrum of MnF4−6 in a given lattice we can detect changes in the Mn2+–F− distance down to 10−3 Å. In this way we have derived the difference, ΔR, between R at room temperature and at 77 K for KZnF3: Mn2+. The obtained value ΔR=(9±1)10−3 Å is in agreement with the one ΔR=(10±3.5)10−3 Å derived previously from the variations undergone by the isotropic superhyperfine constant As. Finally the present results are compared to those for some complexes of Eu2+, Co2+, Ni2+, and Cr3+.