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Keywords:

  • diffusion;
  • fluorescence;
  • polyelectrolytes

Abstract

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 (Cs) 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 RHCs−1/2 over a limited range of Cs, where the changes in D were interpreted as changes in the hydrodynamic radius, RH. However, D of polylysine at first decreased but eventually passed through a maximum followed by a decrease. These large increases implied that RH 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 RH value may be related to the salting-in phenomenon. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3497–3502, 2005