Research - Mesoscale Stereochemistry
Amyloidosis
Banded Spherulites
Rhythmic Precipitates
We recognized that it was a small leap from studying dyed crystals to dyed polycrystalline tissues. Whether single crystals or polycrystalline tissue samples, the dyes and methods of analysis are the same. This has prompted a general reevaluation of classical histochemical staining procedures. Initially we studied amyloid plaques stained by Congo red. In a recent publication in PNAS we used advances in polarized light microscopy to provide the most complete optical analysis of AD plaques. Our linear birefringence and linear dichroism imaging has already revealed aspects of plaque structure that were not previously known, especially the disordered cores. Moreover, we have unambiguously established the orientation of the dipoles of Congo red with respect to the fibrils, a matter of considerable disagreement. Currently, we are trying to make movies of pathological Alzheimer's disease plaque growth using Werner Kaminsky's novel rapid birefringence imaging system. We have already established proof of concept by making movies of bovine insulin (Figure 1) spherulite growth. Bovine insulin grows just like Alzheimer's amyloid in the brain but is less costly than the Alzheimer's peptide.
Figure 1. Growth of bovine insulin captured in real time. (Top) Amplitude of the linear birefringence separated from (bottom) the extinction angle measured clockwise from the horizontal.
Hundreds of simple, off-the-shelf compounds, chiral as well as achiral, have been known for generations to produce, by mechanisms as yet uncertain, helical structures easily detected with an optical microscope. The study of such structures reached its zenith with the publication of Ferdinand Bernauer's magnificent 1929 volume, Gedrillte Kristalle. Bernauer's crystals are also known as banded spherulites. Spherulites become banded, so it is believed, as a consequence of the fact that as crystallites grow radially they twist about the radii so that the optic axis or axes periodically turn in and out of the focal plane. Bernauer lists 137 banded spherulites, many common off the shelf organic chemicals. Such twisted substances should indeed show circular extinction contrast. Fantastic, regular patterns are evident in many systems. For example, mannitol, though not studied by Bernauer, forms magnificent banded spherulties [L. Yu, J. Am. Chem. Soc. 2003, 125, 6380-6381.]. Figure 2b shows the circular extinction contrast of opposite sign in every half period. On the other hand, cholesteryl acetate show oppositely handed sectors. The contrast and its differences are likely consequences of differential circular scattering. Erica Gunn has begun an all out assault on the problem of the structural origin of mesoscale chiroptical contrast by combining our unique optical microscopies with electron microscopy, AFM, phase contrast, and confocal fluorescence microscopies.
Figure 2. False color map of the circular extinction in (a) pure cholesteryl acetate spherulites (b) 10% mannitol-PVP spherulites.
Liesegang-like ringed, rhythmic precipitates also fall into Erica's purview. Phthalic acid show a remarkable division into heterochiral fields, an amplification of the chirality of centrosymmetrically related faces growing in opposing directions from a central, needle-like nucleus. This optical texture was related to the chirality of the microtexture detected by AFM and SEM.
Figure 3. Circular extinction contrast in rhythmic precipitates of phthalic acid.