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E -50--60 --70--80-I I 1 1 h (w). 200 2 50 300 350 Figure 7. in EPA at +25 and -192".Circular dichroism curves of 5a-pregnan-20-one (XI) can be expected for XII. The relevant C.D. curves are shown in Figure 7 and, on the basis of the strong negative rotational strength, conformer XI does indeed seem to be the preferred one. The rotational strength of XI ( R O Z 5 = -6.9 X c.g.s., Ro-lg2 = -7.4 X c.g.s.) is only slightly affected by lowering the temperature. Since the rotational strengths of the two preferred conformations are predicted by the octant rule4 to be appreciably different, this result implies a large free energy for the interconversion of XI and XI1 probably approaching 2 kcal. (see Figure 3), if entropy differences in the two conformers are small. ExperimentalAll measurements were performed in Spectrogride solvents. The EPA solvent consisted of ether-isopentane-ethanol in a ratio of 5 : 5 : 2 by volume; C.D. curves were obtained, using a Baird-Atomic Jouan dichrograph operating with a photomultiplier voltage of 1.2 kv. and following the procedures and molecular ellipticity calculations outlined earlier. 39 We wish to thank Drs. P. Crabbe and A. D. Cross (Syntex, S . A., Mexico City) and Dr. G. Snatzke (University of Bonn) for furnishing pertinent manuscripts. Also, Dr. J. S. Mills (National Gallery, London) provided a sample of 3P-acetoxyhexanordammar-20-0ne,~O and Dr. CrabbC supplied samples of 30-acetoxy-l6P-cyano-A5-pregnen-20-one4l and 3P-acetoxy-16P-methyl-A5-pregnen-20-one. The technical assistance of Ruth Records and W. s. Briggs is gratefully acknowledged.Acknowledgment.

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Abstract.-The species existing in aqueous solution at pH 8.3 when copper(JJ) interacts with two equivalents of L-histidine has been studied by a variety of techniques. The evidence is consistent with a fluctuating type of coordination for the histidiiiato-copper(II) complex, which varies according to conditions and other coordinating ligands involved.The complex derived from the interaction of cupric ion and a histidinlate ion in solution has been variously described as coordination involving (a) the amino and imidazolyl groups in the square-plane (histamine-like),1 (b) the amino and carboxylate groups in the square-plane (amino acid-like),2 (c) all three possible coordinating sites occupying two planar positions and one apical site (tridentate).3 Despite this array of claims concerning the precise structure of the copper-histidinate complexes, this system continues to be used in a variety of model chemical and biological studies.We now present preliminary results which indicate an equilibrium in solution between 1 and 2; the presence of significant amounts of 3 is not required.

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