Tetrahedral (T) Closed-Shell Cluster of 29 Silver Atoms & 12 Lipoate Ligands, [Ag29(R-α-LA)12](3-): Antibacterial and Antifungal Activity

Abstract

Here we report on the identification and applications of an aqueous 29-atom silver cluster stabilized with 12 lipoate ligands, i.e. Ag₂₉(R-α-LA)₁₂ or (29,12), wherein R-α-LA = R-α-lipoic acid, a natural dithiolate. Its uniformity is checked by HPLC-ESI-MS and analytical ultracentrifugation, which confirms its small dimension (~3 nm hydrodynamic diameter). For the first time, this cluster has been detected intact via electrospray ionization mass spectrometry, allowing one to confirm its composition, its [3-] charge-state, and the 8-electron shell configuration of its metallic silver core. Its electronic structure and bonding, including T-symmetry and profound chirality in the outer shell, have been analyzed by DFT quantum-chemical calculations, starting from the known structure of a nonaqueous homologue. The cluster is effective against Methicillin-Resistant Staphylococcus aureus bacteria (MRSA) at a minimum inhibitory concentration (MIC) of 0.6 mg-Ag/mL. A preformed Candida albicans fungal biofilm, impermeable to other antifungal agents, was also inhibited by aqueous solutions of this cluster, in a dose-response manner, with a half-maximal inhibitory concentration (IC₅₀) of 0.94 mg-Ag/mL. Scanning electron micrographs showed the post-treatment ultrastructural changes on both MRSA and C. albicans that are characteristic of those displayed after treatment by larger silver nanoparticles.

Keywords: Ag29; analytical ultracentrifugation; antibiotic; electrospray ionization mass spectrometry; lipoic acid; silver clusters.

Figure 1.

Electrospray ionization mass spectrometric analysis of the Ag₂₉ lipoate complex in solution. (A) Cluster can be detected intact thus confirming its composition and 3- charge state. The observed peak positions and isotopic patterns agree with those calculated (green trace) from the chemical formulas given, assuming all lipoic acid carboxyl groups are protonated (−CO₂H). (B) Cluster can be induced to fragment by loss of lipoate ligands while keeping its silver count intact at 29.

Figure 2.

Structure models optimized by DFT calculations show how (B) an isoelectronic aqueous surrogate of the (29,12)⁽³⁻⁾ cluster retains (A) the structural features of its nonaqueous homologue. Viewed along a C₃-axis. Color code: Ag (orange, gold, yellow spheres); S or Cl (lime-green); P (blue); C (gray); H (not shown). See text for details of these models.

Figure 3.

Core (left), shell (center), and core+shell (right) model structures (cf. Figure 2) used for the calculation of the HCM index of chirality : (A) Ag₂₉Cl₂₄(PH₃)₄ and (B) Ag₂₉(S₂Ph)₁₂(PPh₃)₄. Color code: Ag (gray or red spheres); S (yellow) or Cl (lime-green); P (orange); C (black); H (white).

Figure 4.

Stokes radius distribution after cluster purification.

Figure 5.

Ag₂₉LA₁₂ cluster inhibits a Candida albicans biofilm at 0.94 mg Ag/mL half maximal inhibitory concentration IC50. Error bars represent + standard deviation from the mean.

Figure 6.

(a, b) SEM images of a preformed biofilm of Candida albicans: (a) without treatment, displaying true hyphae and yeast cells displaying a smooth surface on the cell wall (CW) and a dense mesh-like network of yeasts cells and hyphal elements; and (b) after 24 h treatment of clusters at 0.94 mg-Ag/mL, showing the preformed biofilm of Candida albicans with scarce hyphae, and disrupted outer CW. Clusters becomes aggregated 24 h after incubation on the preformed biofilm, (c, d) SEM images of MRSA: c) MRSA cells without treatment; and (d) post-treatment, showings cell size and morphology changes of MRSA along with clusters interacting and agglomerated on the outer cell surface. Cluster concentration was 0.6 mg-Ag/mL for 24 h treatment.

Scheme 1.. Process of R-α-Lipoic Acid Reduction to R-Dihydrolipoic Acid, Then Added to Silver Nitrite and Excess Sodium Borohydride to Produce the Cluster-Complex Ag₂₉LA₁₂^a

^aAll reagents are in the aqueous phase.