A red zwitterionic co-crystal of acetaminophen and 2,4-pyridinedicarboxylic acid.

We report on a co-crystal of acetaminophen (APAP) and 2,4-pyridinedicarboxylic acid (PDA). The co-crystal was discovered by screening using the soln.-mediated phase transformation (SMPT) technique. Despite the bulk solids of each component being white in color, the new co-crystal phase exhibited a red color. The new phase was analyzed using single-crystal X-ray diffraction and identified as (APAP)·(PDA)·(1). Structural anal. revealed PDA to exist in a hitherto unreported zwitterionic form in the co-crystal. A structural anal. of pure PDA revealed the presence of the zwitterion form in (PDA)·(H2O) (2), as well. The components of 1 self-assemble as a three-dimensional (3D) hydrogen-bonded network with a pronounced 2D structure. The origin of the red color was investigated using d. functional theory calcns., which demonstrate a decreasing π-π* sepn. involving the components of the solid. © 2010 Wiley-Liss, Inc. and the American Pharmacists Assocn. J Pharm Sci 99:3676-3683, 2010. [on SciFinder(R)]