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Optical calculi for anistropic media are reviewed and extended for use in a phase-modulation spectrophotometer. It is demonstrated under which conditions signals proportional to linear and circular dichroisms may be collected, and how artifacts may be introduced from either the optical systems or the medium itself. Finally, conversions among the optical calculi in use are tabulated and discussed.

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The theory of electric dichroism (ED), with the permanent dipoles of helical macromolecules being oriented by a static external electric field, is discussed. The effects on the ED of orientation due to an anisotropic polarizability, flexibility of the backbone and side chain, aggregation, and polydispersity in the degree of polymerization (DP) are also considered. This theory, and a new method for measuring ED, are applied here in a study of poly(zz-butyl isocyanate) in nonpolar solvents over a range of DP from 200 to 2400. The ED results show that the polymer behaves as a rigid rod up to DP ~600, with an onset of flexibility at higher DP. The ED spectrum down to 192 µ shows only positive dichroism which arises from a weakly dichroic band at <203 m/z and a stronger one at 249 µ. The directions of polarization with respect to the helix axis, determined for these bands, are 53 and 44°for the <203-and the 249-µ bands, respectively. Consistent with the nonplanarity of the >N-C-O group, the ED data indicate that there is very little electron delocalization over the whole macromolecule. Also, there appears to be no strong or intermediate strength exciton coupling observable in the spectral region studied. The directions of polarizations could be primarily along the C=0 bond (for the <203-µ transition) and primarily in the plane of the N-C=0 group and perpendicular to the 0=0 bond (for the 249-µ transition); however, it is not possible to make a definite assignment of these polarization directions at this time.

The use of Mueller matrix algebra to analyze the optical properties of anisotropic media is discussed. It is shown that the treatment of collinear transmission in a general anisotropic medium can be simplified, since the contribution from the average absorption can be factored out of the Mueller matrix. The various anisotropic optical properties contribute to the rotation of the plane of the incident linearly polarized light, and these properties can be measured. The major contributions to the net rotation (for weak interaction of the anisotropic medium with the light beam) are the linear dichroism, the linear birefringence, and the change in optical rotatory dispersion upon application of an external field. Since these all depend differently on the angle, , between the major axis of the output elliptically polarized light and the direction of the external (electric) field, they can be separated and measured; for light propagating perpendicular to the axis of a uniaxial medium, the linear dichroism is much larger than the other two effects in the region of an absorption band. A sum rule is applicable to the total linear dichroism spectrum. On the basis of these considerations, a technique is developed for measuring electric dichroism, in which the Cary Model 60 spectropolarimeter is used (together with a parallel-plate cell) to obtain the dependence of the rotation on , on the strength of a static external electric field, and on the wavelength of the incident light. This method is much more sensitive than a conventional direct measurement of the separate absorbances of two mutually perpendicularly polarized light beams. Representative data on several -helical poly(amino acids) indicate that the rotation shows the expected dependence on 9 and on field strength.

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This report summarizes the current state of scientific knowledge regarding food allergy and celiac disease, including information on dose-response relationships for major food allergens and for gluten, respectively. The report presents the biological concepts and data needed to evaluate various approaches to establish thresholds that would be scientifically sound and efficacious in relation to protection of public health. Each approach has strengths and weaknesses, and the application of each is limited by the availability of appropriate data. It is likely that there will be significant scientific advances in the near future that will address a number of the limitations identified in this report. The Threshold Working Group expects that any decisions on approaches for establishing thresholds for food allergens or for gluten would require consideration of additional factors not covered in this report. Furthermore, one option that is implicit in the report's discussion of potential approaches is a decision not to establish thresholds at this time.