Low solubility in water may be the reason for the low bioavailability of biologically active substances . Quinine in its free base form is a poorly water-soluble substance (0.05 g dm -3 ) , and the solubility of QASs 1Br-4Br is strongly correlated with the length of the alkyl substituent: 1Br with a short (butyl) substituent (9.3 g dm -3 , Table ) is dissolved more than 70 times better than its analog 3Br with a long (dodecyl) substituent (0.1 g dm -3 ). In contrast, the presence of a hydroxyl group in the short substituent of 4Br increases the solubility in water to more than 250 g dm -3 . It should be noted here that the chemical structure of the anion has an even more significant effect on the solubility of quinine-based QASs in water, and the replacement of the bromide anion with either an Lasparaginate or L-alaninate anion results in ready solubility (more than 1000 g dm -3 ), with the exception of 3Ala, which still showed very good solubility (more than 500 g dm -3 ). Given the relationship between the solubility of active substances in water and their bioavailability , combining quaternary cations with low affinity for water with amino acid anions may result in increased bioavailability of new QASs, although more thorough studies are required to confirm the validity of utilizing such a strategy.