Showing papers by "Bart Kahr published in 2003"

Imaging linear birefringence and dichroism in cerebral amyloid pathologies

Abstract: New advances in polarized light microscopy were used to image Congo red-stained cerebral amyloidosis in sharp relief. The rotating-polarizer method was used to separate the optical effects of transmission, linear birefringence, extinction, linear dichroism, and orientation of the electric dipole transition moments and to display them as false-color maps. These effects are typically convolved in an ordinary polarized light microscope. In this way, we show that the amyloid deposits in Alzheimer's disease plaques contain structurally disordered centers, providing clues to mechanisms of crystallization of amyloid in vivo. Comparisons are made with plaques from tissues of subjects having Down's syndrome and a prion disease. In plaques characteristic of each disease, the Congo red molecules are oriented radially. The optical orientation in amyloid deposited in blood vessels from subjects having cerebral amyloid angiopathy was 90° out of phase from that in the plaques, suggesting that the fibrils run tangentially with respect to the circumference of the blood vessels. Our result supports an early model in which Congo red molecules are aligned along the long fiber axis and is in contrast to the most recent binding models that are based on computation. This investigation illustrates that the latest methods for the optical analysis of heterogeneous substances are useful for in situ study of amyloid.

Circular Dichroism Imaging Microscopy: Application to Enantiomorphous Twinning in Biaxial Crystals of 1,8-Dihydroxyanthraquinone

Abstract: A microscope was constructed for imaging circular dichroism of heterogeneous anisotropic media. To avoid linear biases that are common with electronic circular polarization modulation, we chose a retrogressive solution: mechanical light modulation by rotating a linear polarizer with respect to a quarter wave plate continuously tuned by tilting to the operating wavelength. Our comparatively slow technique succeeds with near-perfect circular input and signal averaging using a CCD camera. We have applied the method to anomalously birefringent crystals of 1,8-dihydroxyanthraquinone that are shown to have intergrown mirror image domains, undetected by X-ray diffraction because the twinning complexity renders differences in anomalous dispersion, already small, unreliable. The origin of the anomalous birefringence and the assignment of the absolute configuration are discussed.

Optical Rotatory and Circular Dichroic Scattering

Abstract: Optical effects are observed in regularly dyed crystals that serve to mimic optical rotation and circular dichroism by rotating the azimuth and increasing the ellipticity of linearly polarized light traversing the samples. However, these effects to which we give the names optical rotatory scattering and circular dichroic scattering do not transform upon rotation of the sample like intrinsic optical rotation and intrinsic circular dichroism. We ascribe this behavior to new scattering consequences that arise here in crystals of K2SO4 containing oriented azo dyes that have been arranged and overgrown in particular growth sectors. Requisite for the apparent optical rotation and circular dichroism is a bias in the inclination of the induced dipoles with respect to the principal light propagation modes of the medium. This unique situation is a consequence of the anisotropy of growth that is unlikely to occur in other dye-doped systems such as polymers or liquid crystals.