Negatively Charged Submicron Heterogeneities in Aqueous Solutions of Biomolecules as Alkaline Membraneless Organelles

In this work, charge characteristics of submicron heterogeneities (SMH) spontaneously formed in aqueous solutions of various biomolecules: seven amino acids of various types (nonpolar glycine, polar serine, hydrophobic valine, aromatic phenylalanine, sulfur-containing methionine, glutamic acid and basic arginine), ATP, monosaccharide glucose and disaccharide sucrose were studied. The isoelectric points of the SMH in the amino acid solutions determined turned out to be in the pH range from 2.4 to 4, being shifted to the acidic region relative to the isoelectric points of the amino acids themselves (except for glutamic acid). The zeta potential of the SMH was measured in solutions of all the biomolecules under conditions close to the intracellular environment at pH = 7 and basic K⁺ ion content 150 mM. The zeta potential appeared to be negative in all cases. Using these values of the zeta potential, the concentration of OH-anions inside the SMH was estimated, and the pH values corresponding to this concentration turned out to be in the range of 7-10. Since the cell cytosol is an aqueous solution of various biomolecules, SMH must also form inside cells. An analogy is drawn between SMH and membraneless organelles, many of which have been discovered recently. The presence of compact regions with alkaline pH inside the cell is a fundamentally new factor in cell biology, which may have important consequences.