Dyeing polymorphs: The MALDI host 2,5-dihydroxybenzoic acid

The dye methyl red was used to distinguish concomitant polymorphs of the matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) host, 2,5-dihydroxybenzoic acid. Red and purple crystals were readily separated by inspection. They corresponded to the known Form I (P2₁/c, a = 4.9110(3) Å, b = 11.8280(7) Å, c = 11.0580(6) Å, β = 91.059(3)°, V = 642.22 ų) and Form II (P2₁/c, a = 5.5610(2) Å, b = 4.8690(1) Å, c = 23.6880 (8) Å, β = 100.191(1)°, V = 631.27 ų), respectively. X-ray structures of cocrystals of dihydroxybenzoic acids and methyl red, measurements of the linear dichroism of mixed crystals, and force-field calculations were used to investigate the mechanism of differential coloring. The relevance of the observations to investigations of the MALDI mechanism are discussed.