The polarization effect of one or several tilted, parallel air-spaced fused silica plates is calculated. It is generally necessary to include higher order reflections. A device consisting of 1–4 plates tilted 10–45° from normal incidence is proposed for the calibration of differential linear dichroism (LD) in the measurement ranges 10−3 to 10−1 decadic absorbance units and the wavelength region 200–700 nm. The experimental LD measured in these ranges on commercial circular dichrometers, converted to LD mode according to Norden and Davidsson, was in excellent agreement with the calculated, nonempirical results. Since the response function of the instrument in this conversion mode is the same for LD and CD, the method also provides a means for absolute calibration of CD. The results can also be applied to polarized scattering and fluorescence. (For previous papers on this theme, see Refs. 6, 9 and 12.)

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SynopsisWe have studied the denaturation of DNA by linear dichroism (LD) techniques in the following systems: (1) in aqueous solution a t low pH, (2) in 90% (w/w) ethylene glycol and in 90% glycerol solutions, and (3) in a matrix of polyvinyl alcohol (PVA). We report that denatured DNA a t low ionic strength can be oriented by shear forces in all these systems and that it exhibits positive LD with markedly varying LDIA over the absorption band centered a t 260 nm (A being the ordinary absorbance) as compared to the negative LD with approximately constant LD/A of ordinary native DNA. These results are interpreted in terms of a large tilt of the planes of the DNA bases. DNA adopts denatured forms in (1) aqueous solutions a t pH < 3.5, (2) ethylene glycol and glycerol a t low Na+ concentrations, and (3) PVA after heating. If the denaturation is done in dry PVA, a complete re-formation of the double helix is obtained after humidifying (60% w/w) the matrix. This shows that the matrix can keep the two strands in a position allowing a perfect fitting on reassociation. Previous attempts to determine any intrinsic LD of denatured DNA have failed, probably due to the fact that, unless a very low ionic strength is used, the flexibility of the single strands prevents the orientation required to give a detectable LD signal even with recently developed high-sensitivity measurement techniques.

Soyflakes and soybrokens having 8% to 16% wet basis (w.b.) moisture contents were extracted for 8 h (about 50% extraction) using the azeotrope (91%) of isopropyl alcohol (IPA) at 7.75 ml/min flow rate. The moisture contents of soyflakes and soybrokens significantly affected oil recovery with IPA. Regression analysis was performed to optimize moisture contents of soyflakes and soybrokens during IPA extraction. The optimum moisture content was found to be 13.4% and 12.6% (w.b.) for soyflakes and soybrokens, respectively. Qualities of IPA-extracted oil [color and free fatty acid (FFA) content] and of de-oiled soy meal (whiteness value and crude protein content) were determined and compared with those of absolute n-hexane-extracted oil and meal at 4.15, 6.35, and 7.15 ml/min flow rates. Prior to the IPA and n-hexane extractions, soyflakes and soybrokens were hydrated to the optimum moisture content. The colors of IPA-extracted oil and soymeal were somewhat darker than those extracted by n-hexane. IPA-extracted oil had significantly lower FFA content than the n-hexane-extracted oil. De-oiled soy meals obtained from IPA extraction had lower whiteness values indicating darker color compared to nhexane. Crude protein contents were similar in both oil and meal obtained from both solvent extraction processes.