Tapping mode atomic force microscopy of the hyaluronan derivative, hylan A

Hylan A, a water-soluble hyaluronan derivative with enhanced molecular weight attributed to protein-mediated crosslinks, has been examined by tapping mode atomic force microscopy (TMAFM). Hylan A was deposited on mica from dilute aqueous solution and imaged in air through a thin layer of adsorbed water. Isolated chains, drawn into extended forms to varying degrees by the movement of a droplet interface across the surface ("molecular combing"), were observed. The majority of the extended chains could be characterized as long linear single-stranded molecules, as is observed for hyaluronan. In a smaller number of observations on extended hylan A, it was possible to observe apparent connections between two chains, presumably mediated by covalent linkage via protein. Hylan A was also commonly observed in less extended forms. Isolated chains of this type often exhibited intramolecular association, the simplest form of which was antiparallel double-stranded (probably double-helical) junction zones. More extensive intramolecular association, via intermittent double- or multi-stranded connections, led to a fenestrated appearance. Intermolecular association was also observed, leading to the formation of network-like matrices. The variety and types of structures observed for hylan A by TMAFM were in excellent agreement with predictions based on physicochemical studies of both hylan A and hyaluronan.