Terms Of Orientation Distribution Research Articles

Nanostructural Alteration in Bone Quantified in Terms of Orientation Distribution of Mineral Crystals: A Possible Tool for Fracture Risk Assessment

There may be different causes of failures in bone; however, their origin generally lies at the lowest level of structural hierarchy, i.e., at the mineral-collagen composite. Any change in the nanostructure affects the affinity or bonding effectiveness between and within the phases at this level, and hence determines the overall strength and quality of bone. In this study, we propose a novel concept to assess change in the nanostructure and thereby change in the bonding status at this level by revealing change in the orientation distribution characteristics of mineral crystals. Using X-ray diffraction method, a parameter called Degree of Orientation (DO) has been quantified. The DO accounts for the azimuthal distribution of mineral crystals and represents their effective amount along any direction. Changes in the DOs in cortical bone samples from bovine femur with different preferential orientations of mineral crystals were estimated under external loads. Depending on the applied loads, change in the azimuthal distribution of the DOs and the degree of reversibility of the crystals was observed to vary. The characteristics of nanostructural change and thereby possible affect on the strength of bone was then predicted from the reversible or irreversible characteristics of distributed mineral crystals. Significant changes in the organization of mineral crystals were observed; however, variations in the applied stresses and elastic moduli were not evinced at the macroscale level. A novel concept to assess the alteration in nanostructure on the basis of mineral crystals orientation distribution has been proposed. The importance of nanoscale level information obtained noninvasively has been emphasized, which acts as a precise tool to estimate the strength and predict the possible fracture risks in bone.

Information on Bond Orientation Distributions in Lipids and Liquid Crystals from Segmental Order Parameters

The information content of segmental order parameters in liquid-crystalline lipids is analyzed in terms of orientation distributions. First, we discuss the use of segmental order tensors, determined from motionally averaged NMR anisotropies, for obtaining approximations to the orientation distribution of the director in the molecular segment. In addition to the distribution consisting of second-order spherical harmonics, two physically reasonable five-parameter distributions are examined, and the relation of the two shape parameters of these distributions to the principal order parameters is presented. The usefulness of these distributions for the determination of torsional angles is also discussed. Furthermore, we introduce a novel approach for obtaining model-independent limits to bond orientation probabilities from bond order parameters. The analysis shows, for instance, that the C−H bonds of the glycerol sn-3 CH2 group in phosphatidylcholine, with bond order parameters of −0.22, are oriented preferentially at angles between 52° and 128° from the director. The probability that these bonds are oriented at angles smaller than 30° or larger than 150° is less than 25%. Assuming an upper limit to the ruggedness of the distribution, even more stringent probability limits can be given. For instance, using a minimum standard deviation of 10° for any conformation, a C−H order parameter of −0.22 corresponds to a probability of less than 25% for C−H orientations below 51° and above 129°. The approaches described here are generally applicable to uniaxial liquid-crystalline systems.

Determination of the orientation distribution of adsorbed fluorophores using TIRF. II. Measurements on porphyrin and cytochrome c

The theory for determination of the orientation of adsorbed fluorescent molecules using total internal reflection fluorescence, as explained in part I of this series, is illustrated by measurements on adsorbed tetramethylpyridinium porphyrin (H2TMPyP) and porphyrin cytochrome c molecules. The results are encouraging, although for porphyrin cytochrome c the scatter in the obtained order parameters is substantial. For H2TMPyP molecules adsorbed on glass the orientation distribution depends on the solution concentration. At low concentration, the H2TMPyP molecules are more or less randomly oriented, whereas at high concentrations a broad distribution around an angle of 46 degrees between the porphyrin plane and surface was found. For cytochrome c adsorbed on glass and indium tin oxide it was impossible to interpret the data in terms of orientation distributions because of the scatter in the results. The total fluorescence as a function of the polarization angle psi of the incident light beam corresponds to an average angle between the porphyrin group and the surface of 30 degrees-40 degrees. Despite the strong electric dipole moment of the protein, the orientation distribution seems to be independent on the (imposed) electrical potential of the interface.