Molecular and crystal structures of two matrices for MALDI-TOF-MS: 2-(4-hydroxyphenylazo)benzoic acid and 3,5-dimethoxy-4-hydroxycinnamic acid

2-(4-Hydroxyphenylazo)benzoic acid (1) and 3,5-dimethoxy-4-hydroxycinnamic acid (2) (also known as sinapinic acid), two matrices for the research of matrix-assisted laser desorption/ionization time-of-flight mass spectrum (MALDI-TOF-MS), have been structurally elucidated in this paper. The former packs into the orthorhombic P212121 space group, a solvent water directed single strand helix-forming crystallization from optical inactive compound, constructing a three-dimensional framework with the help of O–H⋯O hydrogen bonding interactions and π–π stacking interactions. As for the latter, an infinite one-dimensional chain is formed via two types of dimeric eight-membered O–H⋯O hydrogen bonding rings between the neighboring carboxylic groups as well as the hydroxyl and solvent methanol oxygen atoms, respectively, but no π-stacking is observed in its layer packing.