Absolute structure and structure-function relationships of 4R,2′R and 4S,2′S Pidotimod^®

Pidotimod^® is a dipeptide with widely recognized immunomodulatory properties and with particularly beneficial effects for the treatment of acute respiratory and urinary tract infections. Pidotimod^® presents two chiral centres which originate four stereoisomers. (4R,2′S, 4S,2′R, 4R,2′R and 4S,2′S). To date, only the 4R,2′S and 4S,2′R stereoisomers are reported in the literature. We report here the absolute crystal structure of the 4R,2′R and 4S,2′S diastereoisomers of Pidotimod^®, obtained by crystals grown by slow evaporation of a mixture of water and ethanol. The analysis of the crystal structures revealed the key role of a solvent water molecule in the crystal packing engaged in an extended hydrogen bonds network. This water-assisted H-bond network explained the recalcitrance of 4R,2′R and 4S,2′S Pidotimod^® to crystallize in pure ethanol, despite their high solubility, and the growth of well-diffracting crystals only in presence of water. Hence, Pidotimod^® 4R,2′R and 4S,2′S stereoisomers markedly differ from the 4R,2′S and 4S,2′R ones, which crystallize in absence of water. The molecular and crystal structures of the 4R,2′R and 4S,2′S Pidotimod^® stereoisomers here presented gave some hints on the differences in bioactivity with respect to the 4R,2′S stereoisomer. In fact, beyond an expected different dispositions of hydrophilic ligands, 4R,2′R and 4S,2′S showed an incremented tendency to intermolecular H-bonds with water.