The Base Side of Noble Metal Clusters: Efficient Route to Captamino-Gold, Au(-S(CH)N(CH)), n = 25 - 144.

Monolayer-protected clusters (MPCs), typified by the (Au, Ag)-thiolates, share dimensions and masses with aqueous globular proteins (enzymes), yet efficient bioanalytical methods have not proved applicable to MPC analytics. Here we demonstrate that direct facile ESI(+)MS analysis of MPCs succeeds, at the few-picomol level, for aqueous basic amino-terminated thiolates. Specifically, captamino-gold clusters, Au-n(SR)(p), wherein -R = -(CH2)(2)N(CH3)(2), are prepared quantitatively via a direct one-phase (aq/EtOH) method and are sprayed under weakly acidic conditions to yield intact 6.8 kDa complexes, (n,p) = (25, 18), with up to 5 H+ adducts, or 34.6 kDa MPCs (144, 60) at charge state z = 8+. These exceed all prior reports of positive charging of MPCs except for those bearing per-cationized (quat) ligands. pH-mediated reversible phase transfer (aqueous to/from DCM-rich phases) are consistent with peripheral exposure of all tertiary amino groups to solutions. This surprising development opens the way to all manner of modifications or extensions, as well as to advanced analyses inspired by those applied to intact biomolecules.