Tautomeric phenomenonwas observed in the HPLCchromatogramobtained from a novel potent immunosuppressant, FK506, in ethanol solution. Two tautomeric compounds derived from FK506 were isolated and purified by HPLC.Their structures were elucidated by spectral analyses. The mechanismsfor this tautomerization were also established based on the results of structure analysis.Cis (major component1*) and trans conformational isomers of FK506 resulting from the restricted rotation of the amide bond in the pipecolic acid moiety ( Fig. 1) have been known to exist in chloroform as the equilibrium mixture1'2) which could be detected by HPLCunder low temperature column condition3*. However, this equilibrium is not detectable on the HPLCchromatogram obtained by increasing the column temperature in order to improve a peak shape, while FK506 in ethanol solution provided three peaks which suggest the existence of another kind of equilibrium phenomenon (Fig. 2). This means that the origin of the equilibrium in ethanol is different from the equilibrium associated with the rotation around the amide bond. Wereport the conclusion that a new equilibrium is explained by tautomerism of FK506; the resulting mixture consists of three species, FK506, Tautomeric compoundsI and II in ethanol solution4*. Herein wereport structures of two tautomeric compounds of FK506in comparisonwith spectral data of FK5064) and postulate a mechanism of interconversion between these three compounds.
Materials and Methods Isolation of Tautomeric Compounds
An improved first Rayleigh-Sommerfeld method (IRSM1) is intensively applied to analyzing the focal properties of metallic cylindrical focusing micro mirrors. A variety of metallic cylindrical focusing mirrors with different f-numbers, different polarization of incidence, or different types of profiles are investigated. The focal properties include the focal spot size, the diffraction efficiency, the real focal length, the total reflected power, and the normalized sidelobe power. Numerical results calculated by the IRSM1, the original first Rayleigh-Sommerfeld method (ORSM1), and the rigorous boundary element method (BEM) are presented for quantitative comparison. It is found that the IRSM1 is much more accurate than the ORSM1 in performance analysis of metallic cylindrical focusing mirrors, especially for cylindrical refractive focusing mirrors with small f-numbers. Moreover, the IRSM1 saves great amounts of computational time and computer memory in calculations, in comparison with the vectorial BEM.
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