Analytical ultracentrifugation as a tool for exploring COSAN assemblies.

Area of Science:

• Supramolecular chemistry
• Colloid and interface science
• Biophysical chemistry

Background:

• Cobaltabis(dicarbollide) (COSAN) anionic boron clusters act as "sticky nano-ions" and self-assemble into micelles.
• Previous studies using scattering techniques provided average structural parameters but struggled with discrete species resolution.
• The micelle formation of COSAN and its protein aggregation behavior remain subjects of debate.

Purpose of the Study:

• To resolve discrete species in COSAN micelle formation and protein aggregation using sedimentation velocity analytical ultracentrifugation (SV-AUC).
• To clarify the aggregation behavior of COSAN into micelles and the nature of COSAN-induced protein assemblies.
• To determine the size/shape distribution and aggregation numbers of protein assemblies.

Main Methods:

• Sedimentation velocity analytical ultracentrifugation (SV-AUC) was employed to resolve discrete species in colloidal systems.
• SV-AUC was used to confirm the critical micelle concentration (cmc) of COSAN and analyze COSAN micelle aggregation numbers.
• The method was applied to study myoglobin aggregation induced by COSAN at various COSAN-to-protein ratios.

Main Results:

• SV-AUC confirmed the COSAN cmc at 16 mM and revealed low aggregation numbers for COSAN micelles (8 in water, 14 in salt).
• COSAN was shown to induce myoglobin aggregation into discrete oligomeric species, including dimers, tetramers, and higher-order assemblies.
• COSAN binding to myoglobin was quantified, with specific binding numbers determined for monomeric and dimeric forms at low ratios.

Conclusions:

• SV-AUC provides clarity on the discrete nature of COSAN micelle aggregation and COSAN-mediated protein assembly.
• COSAN micelles exhibit low aggregation numbers, supporting recent hypotheses about their structure.
• This study highlights the complementary role of SV-AUC in understanding supramolecular assemblies and nano-ion interactions with biomacromolecules.