Biomedical subjects

Baltzar Stevensson

Publications and source records attributed to Baltzar Stevensson.

5 recordsLinked to original sources

PubMed2006

Mesomorphism in columnar phases studied by solid-state nuclear magnetic resonance.

In this paper, we present 13C and 1H NMR investigations of 2, 3, 6, 7, 10, 11-hexahexyl-thiotriphenylene (HHTT). The measurements were carried out under both static and magic-angle spinning conditions. The phase diagram of HHTT is K<-->H<-->D(hd)<-->I , where H is a helical phase and D(hd) is a columnar liquid crystal. The motivation was to characterize the molecular order and dynamics and to investigate differences at the molecular level between the two mesophases: H and D(hd). It is shown that D(hd) is a conventional columnar liquid crystal, where the molecular core undergoes fast rotation about the symmetry axis. The orientational order in this mesophase is lower and the temperature dependence of the order parameter is steeper than in other triphenylene-based compounds. On the other hand, in the helical phase the core, similarly to the solid phase, is essentially rigid. The difference between the solid and helical phases is mainly manifested in an increased mobility of the aliphatic chains observed in the latter phase. In addition, the sample exhibits thermal history effects, which are observed in the different behavior upon cooling and heating.

Journal Article↗

PubMed2006

Molecular conformations of a disaccharide investigated using NMR spectroscopy.

The molecular structure of alpha-L-Rhap-(1--> 2)-alpha-L-Rhap-OMe has been investigated using conformation sensitive NMR parameters: cross-relaxation rates, scalar 3J(CH) couplings and residual dipolar couplings obtained in a dilute liquid crystalline phase. The order matrices of the two sugar residues are different, which indicates that the molecule cannot exist in a single conformation. The conformational distribution function, P(phi, psi) related to the two glycosidic linkage torsion angles phi and psi was constructed using the APME method, valid in the low orientational order limit. The APME approach is based on the additive potential (AP) and maximum entropy (ME) models. The analyses of the trajectories generated in molecular dynamics and Langevin dynamics (LD) computer simulations gave support to the distribution functions constructed from the experimental NMR parameters. It is shown that at least two conformational regions are populated on the Ramachandran map and that these regions exhibit very different molecular order.

Carbohydrate Conformation↗

PubMed2006

Efficient orientational averaging by the extension of Lebedev grids via regularized octahedral symmetry expansion.

Gaussian spherical quadrature methods in the guise of the Lebedev sampling grids are highly efficient for some orientational ("powder") averaging problems in solid state NMR. However, their applicability is currently restricted, as the sets of orientations are derived analytically and because they are not well adapted to simulate the broad peakshapes encountered, for example, in the NMR on static powders or on half-integer quadrupolar spins subject to second order quadrupolar interactions under magic-angle spinning conditions. We remedy these problems by (i) introducing the recursive procedure regularized octahedral symmetry expansion (ROSE), to which any existing Lebedev set may be subjected. Each recursive step gives a 9-fold enlarged set of orientations. (ii) We demonstrate that ROSE-expanded grids, in conjunction with spectral interpolation, is well suited for calculating broad peakshapes. These advances combine into the apparently most efficient general-purpose two-angle orientational averaging technique proposed to date for solid state NMR applications.

Journal Article↗

PubMed2005

Molecular structure extracted from residual dipolar couplings: diphenylmethane dissolved in a nematic liquid crystal.

This paper describes an analysis of 1H-1H residual dipolar couplings (RDCs) in diphenylmethane (DPM) dissolved in a nematic liquid crystal, reported by Celebre et al. [J. Chem. Phys. 118, 6417 (2003)]. In that article, the conformational distribution function for DPM was extracted from the RDCs, using the additive potential (AP) model which is based on the molecular-field theory. The AP approach is a powerful, and frequently used, tool for analysis of the nuclear-magnetic-resonance (NMR) parameters in liquid crystals. It requires, however, a priori knowledge of the functional form of the torsional potential, which may even for a simple molecule, such as DPM, be complicated to determine. Here, we analyze the same set of the RDCs using our APME procedure, which is a hybrid model based on the AP approach and maximum entropy (ME) theory. The APME procedure does not require any assumptions about the functional form of the torsional potential and, in contrast with the ME method, is applicable to weakly ordered systems. In the investigation reported in the present study, the results from the APME analysis are in good agreement with the AP interpretation, whereas the ME approach essentially fails in the extraction of the conformational distribution function for DPM.

Journal Article↗

PubMed2002

Conformational distribution function of a disaccharide in a liquid crystalline phase determined using NMR spectroscopy.

A new approach is proposed for analysis of NMR parameters in general, and residual dipolar couplings in particular. The method, which enables construction of the conformational distribution function, is applied to a disaccharide dissolved in a dilute liquid crystal. Our approach rests on two models that have been frequently used for interpretations of dipolar couplings in liquid crystals: (i) the additive potential model (AP) and (ii) the maximum entropy method (ME). These models suffer, however, from serious limitations: the AP model requires an a priori knowledge of the functional form of the torsional potential, while the ME approach gives the flattest possible distribution, which results in an incorrect description of systems with low orientational order. The procedure presented herein (APME) is valid in the low-order limit. The intraresidue dipolar couplings were used to determine the orientational order, while the conformational distribution function is constructed from the interresidue dipolar- and J couplings, together with NOEs. Very good agreement between experimental and calculated NMR parameters was observed. An attempt to analyze the experimental data using a single molecular conformation resulted in significantly larger errors. This study demonstrates that the APME method can be used as a tool in structure determination of flexible molecules in dilute liquid crystals.

Carbohydrate Conformation↗