The role of sodium ions in the solubility of peptides

Abstract

The importance of water quality is often stressed for various reasons: health concerns, environmental conservation, and ecological preservation, but the needs of the research chemist often go missing from this list. With the plight of the research chemist in mind, this particular study focuses on the ionic content of water and its effects on peptide structure. WKWK, a well-studied peptide that is typically highly structured and soluble in aqueous solution, was found not to be soluble in filtered water available from a local source. Because this observation differs from all previous studies with this peptide, the water content was studied via ICP-MS. The results conclude that a high sodium cation concentration causes the peptide’s insolubility. Subsequent quantum chemical computations on smaller model systems have determined that sodium ions bind to the tryptophan residues in the peptide and cause the requisite stabilizing interaction between the tryptophan and lysine residues in WKWK to be lost. The sodium cation competes with the lysine in the peptide, resulting in the dissociation of the lysine and the unfolding of the peptide. The high sodium ion concentration effectively unfolds a typically structured peptide. While this result highlights the significance of water quality as well as ionic content and their effects on peptide structure and function, it also indicates that atomic cations may be useful for controlling similar peptides in future binding studies.