Synthesis of undecagold cluster molecules as biochemical labeling reagents. 3. Dimeric cluster with a single reactive...

The synthesis and characterization of a dimeric derivative of the undecagold cluster complex Au11(CN)3[P-(C6H4CH2NH2)3]7, 1, are described. The dimer, 2, consists of cross-linked molecules of 1 in which 41 of the 42 amino groups are acylated, leaving a single free amino group per dimer. This amino group is reactive and can be used to prepare derivatives of 2 for use in labeling biological macromolecules in preparation for electron microscopic analysis. Limited acylation of 1 by 1.3 equiv of citraconic anhydride in aqueous solution, followed by extensive acylation with acetic anhydride at pH 7-7.5, leads to a mixture of monomeric and dimeric products. Hydrolytic removal of N-citraconyl groups at pH 3.2 unmasks a few amino groups. Cation-exchange chromatography at pH 7 separates the products into three main groups, species containing one, two, or three free amino groups, in overall yields of 33%, 28%, and 15%, respectively. The monoamino species consist mainly of 2 and a monomeric product, which are separated by gel exclusion chromatography. 2 migrates as a dimer upon polyacrylamide gel electrophoresis in sodium dodecyl sulfate, and it contains a single free amino group, detectable by its ion-exchange behavior as a monocation at pH 7 and by its reactivity with [14C]phthalic anhydride.