**4. Conclusions**

Pure DPA and its 1:1 DPA\_PA salt crystals have been obtained from the slow solvent evaporation method and both belong to the centrosymmetric monoclinic crystal system having a *P*21/*n* space group. The asymmetric unit of pure DPA contains two molecules while 1:1 DPA\_ PA salt contains one protonated DPA and one PA¯ anion.

Crystal structure analysis of pure DPA showed two closely associated molecules formed amide–amide dimer through an N-H···O hydrogen bond and resulting in the R<sup>2</sup> <sup>2</sup>(8)

ring motifs. Dimeric units were assembled in the *ab*-plane through C-H···O and C-H···π interaction, whereas it packed loosely along the *c*-axis via weak non-covalent interaction. Crystal structure analysis of 1: 1 DPA\_ PA salt showed a strong association between the drug and the salt former leading to compact molecular packing, with an increase in crystal density compared to DPA alone. In salt, two inversion symmetry related protonated DPA molecules formed amide homodimer through an N-H···O hydrogen bond in R<sup>2</sup> <sup>2</sup>(8) ring motifs and such dimer is hydrogen bonded to two PA¯ anions through N-H···O and C-H···O hydrogen bonds to form the basic dimeric unit comprising two protonated DPA and two PA¯ anion. Furthermore, dimeric units linked to four *n*-glide related neighboring dimeric units through a strong N+–H···O¯ hydrogen bond resulted in a 2-Dimentional packing network. Such a 2-D network assembled in centrosymmetric fashion along the *a*-axis through weak C-H···O interaction resulting 3-D packing in the *ac*-plane.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/cryst11040379/s1, Figure S1. PXRD patterns of (**a**) commercial DPA; (**b**) DPA crystal, (**c**) simulated DPA, (**d**) PA, (**e**) DPA\_PA salt and (**f**) simulated DPA\_PA salt, Figure S2. FT-IR spectra of (**a**) commercial DPA; (**b**) DPA crystal, (**c**) PA, (**d**) DPA\_PA salt, Figure S3. DSC profiles of (**a**) commercial DPA, (**b**) DPA crystal, (**c**) PA, (**d**) DPA\_PA, Figure S4. TG curves of commercial DPA (yellow), DPA crystal (gray), PA (orange), DPA\_PA salt (blue).

**Author Contributions:** Conceptualization, Y.O., D.U., and O.D.P.; formal analysis, M.I.T., Y.O., D.U., O.D.P. and H.U.; investigation, Y.O., Y.U. and D.U.; writing of original draft preparation, M.I.T., T.F. and S.W.; writing of review and editing; T.F., O.D.P. and E.Y.; visualization, M.I.T., T.F., Y.U. and S.W.; supervision, T.F., O.D.P., K.F., H.U. and E.Y.; project administration, T.F., K.F. and E.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** Part of this work was supported by the Mochida Memorial Foundation for Medical and Pharmaceutical Research 2019–2020 (to T.F.) and JSPS KAKENHI Grant Number JP18H04504 and 20H04661 (to H.U.).

**Acknowledgments:** The authors are thankful to Takashi Kikuchi (Rigaku Corporation) for refining and solving the disorder in DPA\_PA salt.

**Conflicts of Interest:** The authors declare no conflict of interest.
