IMAGING THE MOLECULAR DIMENSIONS AND OLIGOMERIZATION OF PROTEINS AT LIQUID SOLID INTERFACES/

Individual Concanavalin A (ConA) molecules have been imaged at the liq uid/solid interface with an atomic force microscope (AFM). Three-dimen sional sizing with very high resolution (<5 Angstrom) has been obtaine d by a novel approach based on height distributions, which avoids the tip convolution effects which normally affect scanning probe microscop y techniques. Each height measurement correlates to a particular molec ular orientation on the surface. A large number of such measurements p rovide a statistical ensemble of orientations. The complete height dis tribution reflects the three-dimensional size of the protein sample an d hence its tertiary and quaternary structure. A surface adsorption an d orientation model, based on a minimization of surface adsorption ene rgy, is proposed. This model is in good agreement with the observed he ight distribution of Con A molecules at the liquid/solid interface. An alysis of Con A and succinylated Con A molecules on mica demonstrates that Con A dimers are the prevalent species at the liquid/solid interf ace. This is in contrast to the tetrameric organization of Con A norma lly observed in solution. The new possibilities opened by height distr ibution analysis on the physical characterization of biomolecules at i nterfaces are also discussed.