In the original study, a search for alternative ligands to account for the unexplained density was unsuccessful. The mass spectrum of the crystallization mixture revealed a molecular ion consistent with BU72, but no others of similar mass . However, the intact adduct would not be detectable in solution, and one decomposition product per binding site would yield negligible concentrations relative to saturating BU72. An alternative test would be for modification of His54: proteolysis of the receptor and mass spectrometry of the fragments should reveal either the adduct or decomposition products. A simpler alternative would be to substitute a short Gly-Xaa-His-containing peptide for the receptor, although this might also result in side-reactions. The initial nickel complex itself should be detectable spectroscopically, and may indeed give a noticeable yellow color to the solution .