Structural characterization of a 3-hydroxychromone dye trehalose conjugate for fluorescent labelling of mycobacteria

Abstract

3-Hydroxychromone (3HC) dye trehalose conjugates have previously been exploited as environment-sensitive fluorescent labels to detect mycobacteria by fluorescence microscopy. We have studied the 3HC dye 2-(6-(diethylamino)benzofuran-2-yl)-3-hydroxy-4H-chromen-4-one (3HC-2) trehalose conjugate 3HC-2-Tre by X-ray crystallography, spectroscopic methods augmented by DFT calculations and conducted preliminary labelling experiments with Mycobacterium abscessus, a non-tuberculous mycobacterium and opportunistic pathogen. In the crystal structure of 3HC-2-Tre ⋅ 3.14 H2O, the 3HC-2-Tre molecules exhibit an intramolecular Osingle bondH⋅⋅⋅O hydrogen bond between the 5-hydroxy group of trehalose and the chromone carbonyl oxygen atom, and form hydrogen-bonded dimers via the trehalose units with approximate local C2 symmetry. The chromophore adopts an s-cis conformation of the chromone and benzofuran systems about its central Csingle bondC bond, as previously encountered in the crystal structure of the free dye 3HC-2, and the trehalose moieties approximately retain the shape as found in the crystal structures of anhydrous trehalose and the dihydrate. Absorption spectra of 3HC-2-Tre acquired in four different solvents revealed small positive solvatochromy. Fluorescence microscopy detected M. abscessus cells treated with 3HC-2-Tre in the GFP channel.