Optimisation of sample preparation methods for air imaging of ocular mucins by AFM

The addition of cations to the imaging buffer for AFM has been previously shown to improve the binding of biological molecules to mica. Investigations were carried out to find the concentration of NiCl2 required to immobilize mucin molecules on a freshly cleaved mica surface, for imaging using intermittent contact in air. Drop-deposition of samples prepared in HEPES buffer with 1, 2 and 5 mM NiCl2 revealed the sensitivity of the mucin molecules to salt. Dialysis of the mucin solutions dramatically reduced the amount of salt present and allowed single molecules to be imaged, revealing a variation in thickness along their length. Spray deposition of the same mucin solutions produced single molecules that, although less affected by co-adsorbed salt, showed a degree of self-folding. This shows the sensitive balance between HEPES and NiCl2 required for successful imaging of the sub-molecular features of individual mucin molecules.