Synthesis of Water Soluble Undecagold Clusters for Specific Labelling of Proteins

W ater soluble undecagold clusters have been used successfully for the labelling of proteins and the sub­ sequent visualization of the label by electron micro­ scopy [1, 2, 3], In addition, undecagold clusters have also been used as heavy atom labels in X-ray crystal­ lography of very large proteins [4], The monofunc­ tional clusters needed for these experiments can be prepared by blocking all but one of the 21 amino groups of the water soluble undecagold cluster first described by Bartlett et al. [5, 6] or by use of a mix­ ture of appropriately substituted phosphines for the synthesis of the gold complex [7], However, in addi­ tion to the monofunctional clusters formed by these methods, a relatively large amount of the precious starting material is converted into clusters which do not have a functional group or have more than one and are thus unsuitable for specific labelling. For ex­ periments in electron microscopy usually only trace amounts of the cluster samples are sufficient, where­ as in crystallographic work relatively high amounts of monofunctional clusters are required. In this paper a more effective method of synthesis of monofunction­ al undecagold clusters is described. In the experiments described below, a gold cluster having only identical, hydrophilicly substituted ligands is used as the starting material. Heating a solution of this cluster and a phosphine having one free amino group leads to an exchange of ligands. The desired mono-amino derivative can be easily separated from the reaction mixture and further sub­ stituted by a maleimido group, giving a cluster which