Systematic Study of the Physicochemical Properties of a Homologous Series of Aminobisphosphonates

Aminobisphosphonates, e.g., alendronate and neridronate, are a well known class of molecules used as drugs for various bone diseases. Although these molecules have been available for decades, a detailed understanding of their most important physicochemical properties under comparable conditions is lacking. In this study, ten aminobisphosphonates, H2N(CH2)nC(OH)[P(O)(OH)2]2, in which n = 2–5, 7–11 and 15 have been synthesized. Their aqueous solubility as a function of temperature and pH, pKa-values, thermal stability, IR absorptions, and NMR spectral data for both liquid (1H, 13C, 31P-NMR) and solid state (13C, 15N and 31P-CPMAS NMR) were determined.

. The effect of length of the side chain on aqueous solubility at 21 °C. Table S1. The effect of temperature on aqueous solubility (mg/L) of aminoalkylbis(phosphonates) 2, 4 and 5 (agitation time 24 h).  The NaNO 3 and NaCl used in reference electrode were p.a. grade (Merck). The water used in the dilutions and titration solutions was purified with Milli-RO and Milli-Q water purification systems (Millipore). Potentiometric measurements: The protonation equilibria was studied in aqueous 0.1 M NaNO 3 at 25.0 ± 0.1 °C through a series of potentiometric EMF titrations carried out with a Schott-Geräte GmbH titrator TPC2000 and utilizing titration software TR600 version 5.02. The cell arrangement for the measurement of the hydrogen ion concentration [H+] was as follows: where GE denotes a glass electrode (Schott N2680), and RE is Hg, Hg 2 Cl 2  0.01 M NaCl, 0.09 M NaNO 3 . Expression (2) is valid assuming that the activity coefficients are constant.
The cell parameter E 0 and the liquid junction coefficient j H , valid in acidic solutions, were determined for each titration by adding a known amount of HNO 3 to the background electrolyte. The value of the liquid junction coefficient j OH , valid in basic solutions, was determined periodically. Only stable EMF readings were used in the calculations.
During the measurements of the protonation equilibria, aqueous 0.1 M NaOH or 0.1 M HNO 3 was added to the solution. The initial concentrations of ligands varied within the limits 0.7 mM ≤ C L ≤ 4.5 mM. Four to eight independent titrations were carried out for each ligand. The number of data points used in the calculation of the stability constants varied from 305 to 551 in the pH ranges 1.87-11.99.
Data treatment: Protonation and deprotonation of the ligands were controlled by addition of HNO 3 or NaOH. The curves of Z H versus pH were drawn to visualize the experimental data sets. Z H describes the average number of H + ions added or liberated per mole of ligand and is given by the relationship where C H denotes the total concentration of protons calculated over the zero level H 2 L 2− . In the evaluation of the equilibrium constants, the following two-component equilibria were utilized: Mathematical analysis of the systems involves a search for protonation models and equilibrium constants that best describe the experimental data. The calculations were carried out with the computer program SUPERQUAD (Gans, P.; Sabatini, A.; Vacca, A. J. Chem. Soc. Dalton Trans. 1985, 1195-1200. The sample standard deviation Σ and the χ 2 statistics used as criteria in the selection of the models were those provided by the program. As a means to improve the confidence level, the error limits for log β values determined in this study are reported as three times the standard deviation estimated by the program.    Table S3. The overall protonation of compounds 1, 2, 3, 4, and 5 in 0.1 M NaNO 3 aqueous solution at 25 °C (equations 4 and 5, zero level H 2 L 2− ).

Compound 4 logβ x ± 3σ
Compound 5   Details for Solid State NMR Measurements. The contact times for the CPMAS experiments were 2 ms for 13 C, 3 ms for 15 N and 5 ms for 31 P. The relaxation delay was 5 s and the spin rate was 10 KHz. In the high-power broad band 1 H decoupling the pulse program was spinal64 for carbon-13 and nitrogen-15 and tppm15 for phosphorus-31. The number of scans was typically hundred for carbon-13, thousands for nitrogen-15 and less than one hundred for phosphorus-31. The 13 C and 15 N chemical shifts were referenced those of glycine (176.03 ppm for carbon-13 and -345.25 ppm for nitrogen-15) measured prior to each sample. The 31 P-NMR chemical shifts are referenced that of sodium alendronate measured before. Table S5. Liquid state 13 C as well as solid state (SS) 15 N and 31 P CPMAS NMR chemical shifts.
Experimental for Supportive X-ray Powder Diffraction Measurements. The X-ray powder diffraction data were measured with PANalytical X'Pert PRO diffractometer in Bragg-Brentano geometry using step-scan technique and Johansson monochromator to produce pure Cu K 1 radiation (1.5406 Å; 45 kV, 30 mA). Lightly hand-ground powder sample was prepared on a silicon-made zero-background holder using petrolatum jelly as an adhesive. The data was collected from a spinning sample by X'Celerator detector in 2 range of 3-70° with a step size of 0.017° and counting times of 240 s per step. Programmable divergence slit (PDS) was used in automatic mode to set irradiated length on sample to 10 mm together with fixed 15 mm incident beam mask. Soller slits of 0.02° rad were used on incident and diffracted beam sides together with anti-scatter slits of 4° and 8.7 mm, respectively.