**4. Conclusions**

method.

1:1 DPA:TA salt crystals were obtained from the slow evaporation method, and its crystal structure belongs to the triclinic *P*-1 space group. The asymmetric unit of 1:1 DPA:TA salt contains one protonated DPA and one TA anion.

In the crystal structure of DPA:TA salt, two inversion symmetry related protonated DPA molecules formed an amide homodimer through a N-H···O hydrogen bond in the R 2 <sup>2</sup>(8) ring motifs, and such a dimer is hydrogen bonded to two TA anions through charge assisted N+–H···O¯ and C–H···O¯ hydrogen bonds to form a basic centrosymmetric dimeric unit, comprising two protonated DPA and two TA anions. Furthermore, such a dimeric unit linked to a unit translated neighboring dimeric unit along the *b*-axis through an O3¯···H4–O4 hydrogen bond between carboxylate O3-oxygen and the carboxyl (O-H) of the next TA anion along the *b*-axis results in a one-dimensional chain, which is further supported by C20–H20B···O4 interaction between DPA and TA. Such a 1D chain assembled along the *a*-axis through C16–H16···O3¯ and C21–H21C···O5 interaction generates a twodimensional structure. Such a 2D layer structure, assembled centrosymmetrically along the *c*-axis through relatively weaker C–H···O interactions, results in a 3D layer in the *ac*-plane.

**Author Contributions:** Conceptualization, M.I.T.; formal analysis, M.I.T. and Y.U.; investigation, M.I.T. and Y.U.; writing–original draft preparation, M.I.T. and T.F.; writing–review and editing, T.F. and E.Y.; visualization, M.I.T., Y.U., and T.F.; funding acquisition, T.F.; supervision, K.F. 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:** This study was supported by the Mochida Memorial Foundation for Medical and Pharmaceutical Research 2019-2020 (to T.F.).

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