Charged polypeptide diffusion at a very high ionic strength

Two charged polypeptides of opposite charge, poly(glutamic acid) (negative charge) and polylysine (positive charge), were end‐labeled with Alexa fluorescent dyes, and their translational diffusion coefficient ( D ) values in dilute solutions (∼10 −4 mg mL −1 ) were studied at the biological pH with fluorescence correlation spectroscopy as a function of the ionic strength ( C s ) mediated by the addition of NaCl. At a moderate ionic strength, D increased consistently with expected chain contraction because of electrostatic screening. At a very high ionic strength, D of poly(glutamic acid) increased more rapidly, following the empirical power law R H ∼ C s −1/2 over a limited range of C s , where the changes in D were interpreted as changes in the hydrodynamic radius, R H . However, D of polylysine at first decreased but eventually passed through a maximum followed by a decrease. These large increases implied that R H decreased considerably, in turn implying a strong contraction of the chain conformations even though the polymer remained soluble and showed no evidence of aggregation. For polylysine, the unexpected minimum R H value may be related to the salting‐in phenomenon. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3497–3502, 2005