Charge on a weak polyelectrolyte.

Fluorescence measurements with single-molecule sensitivity are used to measure the hydrodynamic size and local pH of a weak polyelectrolyte, poly-2-vinyl pyridine end labeled with pH-sensitive dye, the polyelectrolyte having concentration so low (nanomolars) that molecular properties are resolvable only from fluorescence experiments and cannot be accessed by light scattering. We find that the local pH near the dye, inferred from its brightness, is consistently three orders of magnitude higher than the bulk pH. Upon varying the bulk pH, we measure the collapse point at which hydrophobic attraction overwhelms electrostatic repulsion between charged elements along the chain, and conclude that adding monovalent salt shifts this coil-to-globule collapse to higher pH than in the absence of salt. The influence of salt appears to shift the ionization equilibrium of this weak polyelectrolyte in the direction of the chain possessing enhanced electric charge at a given pH. Phenomenologically, this is opposite to the case for strong polyelectrolytes, although the mechanism differs.