Influence of D/H Ratio in Water on Fluorescence and Biological Activity of Drug Solutions

The ratio of hydrogen isotopes—protium and deuterium—in water significantly influences the physicochemical and biological properties of dissolved pharmaceutical substances. This study investigated the impact of water isotopic composition on both the kinetic parameters and biological activity of amoxicillin (AMX) and Zn2+ solutions. The kinetics of complexation were examined fluorometrically in waters with varying D/H ratios: H2O (D/H = 145 ppm), heavy water (D2O, 99.9%), and deuterium-depleted water (DDW, D/H = 10 ppm). The fluorescence intensity of the solutions increased significantly with higher deuterium content in the solvent: ID2O/IDDW = 1.5. The addition of Zn2+ induced the changes in fluorescence intensity. The isotope effects kH2O/kD2O = 1.9 and kH2O/kDDW = 2.2 indicate the involvement of water molecules in the rate-limiting step of the process. The decrease in biological activity of AMX due to complexation was confirmed using the biosensor Spirostomum ambiguum. Depletion of D/H and 18O/16O ratios in water increases the biosensor’s lifetime. The longest biosensor lifetime was observed in heavy-isotopes-depleted water (HIDW, D/H = 10 ppm, 18O/16O = 400) solutions. Therefore, the variations in the isotopic composition of water from different regions should be considered in the preparation of liquid dosage forms and in the quality control of drugs.