Product in indole detection by Ehrlich's reagent

Ehrlich's reagent (p-dimethylaminobenzaldehyde [DMAB, 1] in 95% EtOH with HCl as catalyst) was employed in spot tests of indoles, providing a diagnosis of, for example, liver diseases, hemolytic processes, occlusion of the common bile duct, and carcinoid syndrome. Although the reagent has been widely used for more than a century, it is not clear how many indole molecules react with a DMAB molecule and whether the reaction takes place at the α- or β-position of the indole molecule. Research here shows that the reaction of DMAB (1) with indole (2) in a 1:2 ratio gives β-bis(indolyl)methane (3). The reaction occurs at the β-position of indole under the conditions of the Ehrlich test, as confirmed by the crystal structure of 3.