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A single salt-like link between protein and exchanger is not enough to explain the strength of this union. The bond between protein and exchanger is not hydrolyzed by water as one would expect from the salt of a weak acid and weak base.Multiple bonds (2) or a constellation of charges in each bond would overcome this weakness. How-ever, one cannot overlook the fact that there are many groups on both the exchanger and the protein which could participate in hydrogen bonding. As the derivative is dried, these bonds may become even more important to the strength of this union and may increase the stability of the protein.

The bathochromic shifts resulting from the interaction of mercuric acetate with indole derivatives, such as 2-and 3-substituted aryl and alkyl indoles, tetrahydrocarbazoles, indole propionic acid, and derivatives of tryptophan, including tryptophan-containing proteins, have been recorded.The requirements for the shift are the presence of the nitrogen hetero atom, and the availability of the a-electrons of the 2,3-double bond of the indole nucleus. Electronegative substituents on both the 2 and 3 positions prevent this characteristic reaction. The 1,2-di-and 1,2,3-triacetoxymercury derivatives of indole and the di-(1,2-diacetoxymercuryindole propionic acid) mercury salt have been isolated and characterized. The indole nucleus of tryptophan in proteins provides a binding site for mercury, when ionizing mercury salts are used. The presence of excess mercuric acetate during N-bromosuccinirnide oxidation of polypeptides and proteins, containing both tyrosine and tryptophan, makes possible the selective oxidation of tryptophan and cleavage of the adjacent C-tryptophyl peptide bonds.The average red shift is 5-10 mp and the extinction is nearly doubled.

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