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A high-pressure liquid chromatographic (HPLC) procedure has been developed for the determination of vitamin D3 in resins, oils, dry concentrates, and dry concentrates containing vitamin A. Specificity of the method for vitamin D3 in the presence of isomers and other related constituents was shown by ultimate recovery of the vitamin D3 and the individual constituents and their characterization by other methods such as ultraviolet spectrophotometry. Precision and accuracy are within 2%, and as many as 20 determinations may be completed in a working day. Excellent agreement with the official AOAC biological assay was found. A comparison of the response of isomers of vitamin D3 by the HPLC method vs. other instrumental and chemical procedures shows HPLC to be the most specific method for determining the bioactive isomers.

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This investigation of the kinetics and thermodynamics of the thermal isomerization reaction of la,25-dihydroxycholecalciferol, the physiologically active metabolite of vitamin D3, is based on the simultaneous determinations of la,25-(OH)2D~ and its previtamin analog by nuclear magnetic resonance spectroscopy which distinguishes these compounds from possible impurities. The kinetics at different temperatures are used to obtain the activation parameters for the sigmatropic [ 1,7] thermal interconversion process which is shown to be compatible with a reaction that is unimolecular and concerted. The nature of the transition state of the activated complex, the reaction energetics, and the relative stabilities of la,25-(OH)zD3 and vitamin D3 are discussed.

This investigation of the kinetics and thermodynamics of the thermal isomerization reaction of 1α,25‐dihydroxycholecalciferol, the physiologically active metabolite of vitamin D3, is based on the simultaneous determinations of 1α,25‐(OH)2D3 and its previtamin analog by nuclear magnetic resonance spectroscopy which distinguishes these compounds from possible impurities. The kinetics at different temperatures are used to obtain the activation parameters for the sigmatropic [1,7] thermal interconversion process which is shown to be compatible with a reaction that is unimolecular and concerted. The nature of the transition state of the activated complex, the reaction energetics, and the relative stabilities of 1α,25‐(OH)2D3 and vitamin D3 are discussed.