Arginine “Magic”: Guanidinium Like-Charge Ion Pairing from Aqueous Salts to Cell Penetrating Peptides

Abstract

It is a textbook knowledge that charges of the same polarity repel each other. For two monovalent ions in the gas phase at a close contact this repulsive interaction amounts to hundreds of kilojoules per mole. In aqueous solutions, however, this Coulomb repulsion is strongly attenuated by a factor equal to the dielectric constant of the medium. The residual repulsion, which now amounts only to units of kilojoules per mole, may be in principle offset by attractive interactions. Probably the smallest cationic pair, where a combination of dispersion and cavitation forces overwhelms the Coulomb repulsion, consists of two guanidinium ions in water. Indeed, by a combination of molecular dynamics with electronic structure calculations and electrophoretic, as well as spectroscopic, experiments, we have demonstrated that aqueous guanidinium cations form (weakly) thermodynamically stable like-charge ion pairs. The importance of pairing of guanidinium cations in aqueous solutions goes beyond a mere physical curiosity, since it has significant biochemical implications. Guanidinium chloride is known to be an efficient and flexible protein denaturant. This is due to the ability of the orientationally amphiphilic guanidinium cations to disrupt various secondary structural motifs of proteins by pairing promiscuously with both hydrophobic and hydrophilic groups, including guanidinium-containing side chains of arginines. The fact that the cationic guanidinium moiety forms the dominant part of the arginine side chain implies that the like-charge ion pairing may also play a role for interactions between peptides and proteins. Indeed, arginine-arginine pairing has been frequently found in structural protein databases. In particular, when strengthened by a presence of negatively charged glutamate, aspartate, or C-terminal carboxylic groups, this binding motif helps to stabilize peptide or protein dimers and is also found in or near active sites of several enzymes. The like-charge pairing of the guanidinium side-chain groups may also hold the key to the understanding of the arginine "magic", that is, the extraordinary ability of arginine-rich polypeptides to passively penetrate across cellular membranes. Unlike polylysines, which are also highly cationic but lack the ease in crossing membranes, polyarginines do not exhibit mutual repulsion. Instead, they accumulate at the membrane, weaken it, and might eventually cross in a concerted, "train-like" manner. This behavior of arginine-rich cell penetrating peptides can be exploited when devising smart strategies how to deliver in a targeted way molecular cargos into the cell.

Cartilage penetrating cationic peptide carriers for applications in drug delivery to avascular negatively charged tissues

“…Despite the similar net charge in arginine rich CPC +8 and lysine rich CPC +N, the higher uptake and retention of CPC +8 is attributed to the chemistry of the guanidinium head-group on arginine residues that allow formation of stable bidentate hydrogen bonds with polarizable oxo-anions such as the sulfates in aggrecan glycosaminoglycans [44, 52], which further stabilize the long-range electrostatic interactions. Additionally, guanidinium cations can form thermodynamically stable (weakly) like-charge pair in water, where a combination of dispersion and cavitation forces from the medium can overwhelm the coulombic repulsion [53]. Because of this, arginine moieties are capable of binding more strongly (due to synergistic effects of charge, H-bond and hydrophobic interactions) and cooperatively (due to formation of like-charge pair between arginine) with negatively charged GAGs compared to lysine moieties, which instead exhibit mutual coulombic repulsion.…”

Arginine‐enveloped virus inactivation and potential mechanisms

“…Additionally, the opposite charge distribution on arginine and butyroyl‐arginine can cause the formation of clusters due to self‐aggregation . Clusters can also form through like‐charge pairing of guanidinium moieties …”

“…In addition to protein binding, arginine molecules in solution tend to self‐aggregate into clusters . Arginine–arginine clusters are thought to occur due to like‐charge pairing of guanidinium moieties or hydrogen bonding from head to tail . These clusters are known to crowd out protein–protein or protein–surface interactions .…”

“…93 Clusters can also form through like-charge pairing of guanidinium moieties. 94,95 Clusters magnify localized concentrations, strengthening protein 33 and lipid interactions. [96][97][98][99][100] Specifically, arginine clusters crowd out hydrophobic protein-protein interactions to prevent aggregation.…”

Membrane Active Peptides and Their Biophysical Characterization

“…The uptake of the charged lysine or arginine residues across the lipid bilayer should be energetically unfavorable, but the prevalence of arginine rich peptides as membrane active peptides has suggested that arginine delivery across the membrane is a facile process [294]. The molecular basis and energetics of the translocation of arginine-rich CPPs through membranes are still not well established [295,296].…”