Adefovir dipivoxil

The active pharmaceutical
ingredient of adefovir dipivoxil was purchased from Sigma-Aldrich. ¹³C CP-MAS SSNMR and ³¹P MAS SSNMR experiments were
performed on a JEOL ECX 500 NMR spectrometer with a 3.2 mm JEOL double-resonance
MAS probe. The MAS speed for the ¹³C CP-MAS NMR experiment
was 10 kHz. ³¹P MAS NMR experiments were performed at the
MAS speeds of 3, 5, 6, 10, and 13 kHz. The contact time for cross-polarization
(CP) was 2 ms, with a repetition interval of 30 s, and SPINAL-64 ¹H decoupling at 3072 accumulation time. ¹³C-spin–lattice
relaxation experiment was performed using the Torchia CP method^{23 (link)} with a contact time of 2 ms and a relaxation
time of 10 s. The spin–lattice relaxation time of the phosphorous
nucleus was measured by the inversion recovery method. All solid-state
experiments were performed at room temperature.

TFV-DF, TFV, FTC, and EFV were purchased from Sigma-Aldrich, Mylan
(South Africa), and Adcock Ingram (South Africa). The internal standards,
namely, adefovir dipivoxil (ADV), zidovudine (ZDV), and 4-(4-carboxybenzyl)-2H-1,4-benzoxazin-3(4H)-one (CBB), were
all purchased from Sigma-Aldrich. LC–MS-grade methanol and
acetonitrile were purchased from Sigma-Aldrich. Analytical-grade formic
acid (FA) was purchased from Merck Millipore (Merck, South Africa).
The MALDI matrix, α-cyano-4-hydroxycinnamic acid (CHCA) was
purchased from Bruker (Bremen, Germany). A MilliQ purification system
(Bedford, MA) was used to generate ultrapure water. All other chemicals
and reagents used in this study were of analytical grade.