one proton on nitrogen, the other must be on oxygen. Acetylglycine shows the same behavior. It can also be concluded that the bisulfate ion is a poor base for exchanging the N-H proton of secondary amides in sulfuric acid.Primary Amides.-The N-H proton magnetic resonance absorption of acetamide has been observed in saturated aqueous solutions at pH 7 and in 100 per cent sulfuric acid. Here, also, exchange of the N-H proton in sulfuric acid appears to be slow in comparison with its relaxation time. By measuring the areas under the absorption peaks it has been possible tentatively to assign O-protonation of acetamide in 100 per cent sulfuric acid. This interpretation is in accord with that of Hantzch, based upon ultraviolet spectroscopy,7 and that of Huisgen and Brade,8 based upon potentiometric pK determinations.Experimental.-All spectra were determined with the Varian 4300 B, 40-mc., 9,400-gauss n-m-r spectrometer, equipped with spinner and superstabilizer. Samples were sealed in 5-mm. o.d. Pyrex tubes. Chemical shifts were calibrated by a combination of the method of side bands9 and the use of toluene markers in capillary tubes as secondary standards. The cryoscopic measurements in 100 per cent sulfuric acid were conducted as described by O'Brien and Niemann.I The N,Ndimethylcyclopropylcarboxamide was kindly supplied by Mr. David Schuster. All other materials were obtained by the purification of samples of commercial origin.

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size of the Cotton effects, in accord with the aforementioned hypothesis. We have also brought about a destruction of the pertinent conformations by metal ions, such as Zn++.The titration from helical to unfolded forms appears to be reversible to the extent that bases, such as piperidine, can be used to bring the weakly rotating solutions formed by the addition of, say, trifluoroacetic acid back to the large amplitudes of rotation characteristic of inherently dissymmetric chromophores. However, the full initial amplitude of rotation could not be recovered (only about a third for piperidine and trifluoroacetic acid). This point and others will be discussed further in a later paper. We might just note here the very strong analogy between the helixopen chain (geometrical) transition in the urobilins and the helix-coil transition in polypeptides and proteins. Conceivably, the particular conformations of the urobilins play an important role in their in vivo biochemistry.

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