Conformations of succinoglycan as observed by atomic force microscopy

Succinoglycan, a high molar mass polysaccharide, undergoes conformational transformations as a function of ionic strength. The nature of the transitions and the presence of intermolecular associations have been described previously using solution-based techniques. In this work, we have determined the conformation of succinoglycan macromolecules at the solution-mica interface using atomic force microscopy (AFM) and compared these data to the measurements obtained in solution. Molecular characteristics such as chain length, end-to-end distances, polymer heights (diameters), and chain rigidity were determined as a function of ionic strength. Individual chains and dimers were found for succinoglycan deposited from pure water, whereas only individual chains were found for 0.01 M KCI. In 0.5 M KCI, succinoglycan formed a gel-like structure at the mica surface. Analysis of persistence lengths from the AFM images indicated that succinoglycan became more rigid with increasing ionic strength. Flexible chains corresponding to a disordered conformation were observed in water while ordered, single helical chains were imaged in 0.01 M KCl. In comparison to bulk solution measurements, molecular conformations determined by AFM were shown to be affected by local concentration increases due to the AFM drying step and by the strength of the interaction between the macromolecules and the mica substrate. In water and 0.01 M KCl, comparison of the measured end-to-end distances, with calculated 2D or projected end-to-end distances, revealed that the polysaccharide was not at equilibrium with the mica surface. These findings demonstrate the potential of AFM as a polymer characterization technique that is complementary to classical solution-based techniques and able to provide specific information on the polymer conformations at the solid-water interface.