Simple interpretation of ionization and helix-coil stability shift when a polyelectrolyte adsorbs

Abstract

Direct measurements of the ionization of poly-L-glutamic acid (PGlu) as it adsorbs onto a polymer brush of opposite electrical charge indicate a transition from expected enhanced ionization when the surface coverage is sparse to considerable suppression below the value in bulk solution when the average charge-charge spacing exceeds a critical level. As additional PGlu molecules adsorbed, the additional charge that they brought to the surface was balanced by their suppressed ionization, so that the average spacing between charged segments remained nearly constant. Charge-charge repulsion therefore limits the extent of ionization because the equilibrium between the ionized and protonated states responds to the magnitude of repulsion. Already present as a physical process in bulk solution, this becomes particularly prominent near a surface because the adsorbed state tolerates a higher charge concentration than the bulk solution tolerates without becoming insoluble. The a-helix conformation of this polypeptide in the adsorbed state is predictably destabilized (at low surface coverage) or stabilized (at high surface coverage). This study shows that generic features of charge regulation reduce surface ionization below the bulk value. This occurs when the charge-charge spacing reaches a critical level, about 7 A in a polar aqueous environment, and shows that charge regulation occurs without the need to postulate changes of the local dielectric constant.