One of the first APols to be designed, called A8-35 (7), has become by far the most extensively used APol. Its solution behavior has been studied in details by a broad range of physical techniques (8-11), and modeled by molecular dynamics (12). A8-35 is comprised of a relatively short polyacrylate chain (~35 acrylate residues), some of the carboxylates of which have been randomly grafted with octylamine (~9 of them) or isopropylamine (~14 units), for an average MW of ~4.3 kDa (Fig. 2). N.B. In earlier papers, the average length of A8 35 molecules had been estimated to ~70 residues, based on size exclusion chromatography (SEC) data using reference polymers as MW standards. Re-evaluation of this issue using direct methods (unpublished data by F. Giusti and J. Rieger) has led to the current value.
The ~12 acid groups that have remained free are charged in aqueous solutions (8), which makes the polymer highly water-soluble (> 200 g.L-1), while the octylamide moieties render it amphipathic. Isopropylamide residues lower the charge density.
In aqueous solutions, A8-35 forms well-defined, small, globular particles (9) above a critical association concentration of ~0.002 g.L-1 (11). Each A8-35 particle has a mass of ~40 kDa and, therefore, comprises an average of 9-10 A8 35 molecules (9). MD simulations show the particles to be micelle-like, with a hydrophobic core and hydrophilic surface, and less fluid than detergent micelles (12).
Because the solubility of A8-35 depends on the presence of charges on its carboxylate groups, it is affected by every factor that will affect the latter, such as lowering the pH below neutrality or adding multivalent cations (9, 13-15). This can create difficulties (due to aggregation) for some applications (see Table 1 in Applications), for which better-suited APols would be desirable (see Amphipols with different chemical structures). These limitations however seldom turn out to be a serious hindrance.
It has proven quite difficult and lengthy to develop, validate and characterize alternative chemical structures presenting as satisfying properties as those of A8-35. For this reason, most of the detailed studies of APols and MP/APol complexes to date have been carried out using this particular APol. This provides the putative user with a considerable body of information about what can and what cannot be attempted with it.