We present persistence length measurements on neurofilaments (NFs), an intermediate filament with protruding side arms, of the neuronal cytoskeleton. Tapping mode atomic force microscopy enabled us to visualize and trace at subpixel resolution photoimmobilized NFs, assembled at various subunit protein ratios, thereby modifying the side-arm length and chain density charge distribution. We show that specific polyampholyte sequences of the side arms can form salt-switchable intrafilament attractions that compete with the net electrostatic and steric repulsion and can reduce the total persistence length by half. The results are in agreement with present X-ray and microscopy data yet present a theoretical challenge for polyampholyte interchain interactions.

We demonstrate a novel technique for molecular imprinting and immobilization on a surface of a polymer containing azo dyes (azopolymer). The azopolymer was found to be capable of immobilizing micrometer- and nanometer-scale macromolecules (e.g., lambda-DNA, immunoglobulin G (IgG), bacterial protease, and 1-mum polystyrene particles) through photoirradiation with blue-wavelength light. Fluorescence and atomic force microscopy studies revealed that the azopolymer surface deformed along with the shape of the macromolecules, holding them in place after photoirradiation. The desorption of the immobilized macromolecules from the azopolymer surface in an aqueous medium was observed to be very slow, on the time scale of 10 min to weeks, depending on the photoirradiation time. Immunological and enzymatic studies showed that IgG and bacterial protease immobilized on the azopolymer surface retained their original functionality. These results suggest that the azopolymer physically, not chemically, binds the macromolecules because of the increase in contact area between the macromolecules and the azopolymer surface after photoirradiation.

A photo-immobilization based process is presented for direct imaging of hierarchical assemblies of biopolymers using atomic force microscopy (AFM). The technique was used to investigate the phase behavior of F-actin aggregates as a function of concentration of the divalent cation Mg2+. The data provided direct experimental evidence of a coil-on-coil (braided) structure of F-actin bundles formed at high Mg2+ concentrations. At intermediate Mg2+ concentrations, the data showed the first images of the two-dimensional nematic rafts discovered by recent x-ray studies and theoretical treatments.

Phase imaging during tapping mode atomic force microscopy (TMAFM) has revealed that an optical near field caused a change in the viscoelastic property on the surface of an urethane-urea copolymer film containing donor-acceptor substituted azobenzenes. Monolayers of polystyrene microspheres with 100 nm diameter and 19 nm diameter were fabricated on the surface of the copolymer film and exposed to a 488 nm wavelength laser beam coincident with the absorption band of the azobenzene derivatives. After removal of the monolayer, the phase image of the film's surface was obtained by TMAFM. The phase shift of a cantilever oscillation (the shift was induced by a tip-sample interaction) indicated that the area affected by the optical near field of the microsphere became relatively softer (the phase shift was smaller) and the vicinal area became harder (the phase shift was larger). These results suggested that the optical near field produced a change in the density on the surface of the copolymer in nanometric dimensions. The copolymer was capable of transcribing the optical near field within the resolution of 20 nm on the basis of the viscoelastic feature.