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 (similar to 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 similar to 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. (c) 2005 Wiley Periodicals, Inc.