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 ). This confirms the validity of combining cationic active substances with amino acid anions to obtain biologically active compounds with high water solubility.