Counterion condensation in short cationic peptides: Limiting mobilities beyond the Onsager–Fuoss theory

Abstract

We investigated the effect of the background electrolyte (BGE) anions on the electrophoretic mobilities of the cationic amino acids arginine and lysine and the polycationic peptides tetraarginine, tetralysine, nonaarginine, and nonalysine. BGEs composed of sodium chloride, sodium propane-1,3-disulfonate, and sodium sulfate were used. For the amino acids, determination of the limiting mobility by extrapolation, using the Onsager-Fuoss (OF) theory expression, yielded consistent estimates. For the peptides, however, the estimates of the limiting mobilities were found to spuriously depend on the BGE salt. This paradox was resolved using molecular modeling. Simulations, on all-atom as well as coarse-grained levels, show that significant counterion condensation, an effect not accounted for in OF theory, occurs for the tetra- and nonapeptides, even for low BGE concentrations. Including this effect in the quantitative estimation of the BGE effect on mobility removed the discrepancy between the estimated limiting mobilities in different salts. The counterion condensation was found to be mainly due to electrostatic interactions, with specific ion effects playing a secondary role. Therefore, the conclusions are likely to be generalizable to other analytes with a similar density of charged groups and OF theory is expected to fail in a predictable way for such analytes.

Self-association of a highly charged arginine-rich cell-penetrating peptide

“…These are salt bridges formed between the negatively charged C-terminal COO − and the positively charged Gdm + of the ninth residues. Theoretical and experimental studies suggest that Gdm + ions form weakly stable like-charge ion pairs in aqueous medium (21)(22)(23)(24)(25)(26)(27)(28). The free energy of interaction between two Gdm + ions is minimized when they are stacked parallel to each other in staggered geometry with carbon atoms with separation between 0.35 nm and 0.46 nm (28).…”

Inversion of Membrane Surface Charge by Trivalent Cations Probed with a Cation-Selective Channel

“…However, despite the extensive experimental and theoretical work, problems regarding electrostatics at the charged membrane interfaces, counter-ion screening, and charge inversion phenomenon remain a subject of intense discussions, e.g. 11, 23 .…”

Specific ion effects on the electrophoretic mobility of small, highly charged peptides: A modeling study

“…Side bead radii are parameterized to reproduce the diffusion constants or electrophoretic mobilities of individual amino acids . In the present work, we shall parameterize the side bead radii of lysine, K, and arginine, R, using the experimental electrophoretic mobilities of the amino acids under conditions identical to those of the corresponding oligopeptides .…”

“…CE offers a simple but very effective way of separating peptides as well as other ionic species and nanoparticles on the basis of their size and charge. In recent years, numerous investigators employing a variety of modeling strategies have analyzed free solution CE mobility data of peptides in an attempt to determine their physicochemical properties and, in some cases, elucidate their structure . Our laboratory has played a part in these studies by developing a coarse‐grained bead modeling methodology (BMM) that partially accounts for the finite extent of peptides in modeling their electrophoretic mobility .…”

“…A simple and easily implemented “small ion” model that has been widely applied for many years , describes the mobility of single amino acids well, but fails when applied to the highly charged oligo species (K 4 , K 9 , R 4 , and R 9 ). The objective of the present work is to apply the BMM‐NLPB procedure to some of the mobility data of reference . Although we have examined oligolysine mobility data previously in a BGE consisting of sodium phosphate buffer , Wernesson et al examined different BGEs including NaCl and Na 2 SO 4 that will be studied here.…”