Chalk River Laboratories

About: Chalk River Laboratories is a based out in . It is known for research contribution in the topics: Neutron diffraction & Neutron scattering. The organization has 2297 authors who have published 2700 publications receiving 73287 citations.

Raman scattering from crystals of the diamond structure

Abstract: The theory of the Raman scattering of light from crystals of the diamond structure is discussed and detailed calculations made of the scattering by germanium, silicon and diamond. The anharmonicity between the normal modes of vibrations gives rise to a broadening of the one-phonon peak. The shape of this peak then depends upon the detailed behaviour of the lifetime and frequency of the long wavelength optic mode. The anharmonicity also introduces a coupling between the one and two-phonon scattering, part of which tends to shift the center of the one phonon Raman peak away from the frequency of the optic mode. Shell models which were fitted to the measured dispersion curves are used to calculate the frequencies of the normal modes of vibration. The one and two-phonon matrix elements for the Raman scattering are evaluated by means of the shell model formalism for these models and the spectra calculated including the effects of anharmonicity on the one phonon scattering. The results which are compared with the available experimental results show that the details of the spectra depend considerably on the polarization of the light and on the orientation of the crystal.

Cholesterol in Bilayers with PUFA Chains: Doping with DMPC or POPC Results in Sterol Reorientation and Membrane-Domain Formation

Abstract: Using neutron diffraction Harroun et al. [(2006) Biochemistry 45, 1227-1233; (2008) Biochemistry 47, 7090-7096] carried out studies that unequivocally demonstrated cholesterol preferentially sequestering in the middle of bilayers (i.e., flat orientation) made of lipids with polyunsaturated fatty acids (PUFA), in contrast to its "usual" position where its hydroxyl group locates near the lipid/water interface (i.e., upright orientation). Here we clearly show, using neutron diffraction, cholesterol's orientational preference in different lipid bilayers. For example, although it requires 50 mol % POPC (16:0-18:1 PC) in DAPC (di20:4 PC) bilayers to cause cholesterol to revert to its upright orientation, only 5 mol % DMPC (di14:0 PC) is needed to achieve the same effect. This result demonstrates not only cholesterol's affinity for saturated hydrocarbon chains, but also its aversion for PUFAs. Molecular dynamics (MD) simulations performed on similar systems show that in high PUFA content bilayers cholesterol is simultaneously capable of assuming different orientations within a bilayer. Although this result is known from previous MD studies by Marrink et al. [(2008) J. Am. Chem. Soc. 130, 10-11], it has yet to be confirmed experimentally. Importantly, MD simulations predict the formation of DMPC-rich domains, data corroborated by experiment (i.e., 10 mol % DMPC-doped DAPC bilayers), where cholesterol preferentially locates in its upright orientation, while in DMPC-depleted domains cholesterol is found mostly in the bilayer center (i.e., flat orientation). These results lend credence to DMPC's aversion for PUFAs, supporting the notion that domain formation is primarily driven by lipids.