Conformations of succinoglycan as observed by atomic force microscopy

Succinoglycan, a high molar mass polysaccharide, undergoes conformational t ransformations as a function of ionic strength. The nature of the transitio ns and the presence of intermolecular associations have been described prev iously using solution-based techniques. In this work, we have determined th e conformation of succinoglycan macromolecules at the solution-mica interfa ce using atomic force microscopy (AFM) and compared these data to the measu rements obtained in solution. Molecular characteristics such as chain lengt h, end-to-end distances, polymer heights (diameters), and chain rigidity we re determined as a function of ionic strength. Individual chains and dimers were found for succinoglycan deposited from pure water, whereas only indiv idual 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 f rom the AFM images indicated that succinoglycan became more rigid with incr easing ionic strength. Flexible chains corresponding to a disordered confor mation were observed in water while ordered, single helical chains were ima ged in 0.01 M KCl. In comparison to bulk solution measurements, molecular c onformations determined by AFM were shown to be affected by local concentra tion increases due to the AFM drying step and by the strength of the intera ction 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 poten tial of AFM as a polymer characterization technique that is complementary t o classical solution-based techniques and able to provide specific informat ion on the polymer conformations at the solid-water interface.