Stereochemical identification of glucans by a donor acceptor donor conjugated pentamer enables multicarbohydrate anatomical mapping in plant tissues

Abstract

Optotracing is a novel method for analytical imaging of carbohydrates in plant and microbial tissues. This optical method applies structure-responsive oligothiophenes as molecular fluorophores emitting unique optical signatures when bound to polysaccharides. Herein, we apply CarbotraceTM680, a short length anionic oligothiophene with a central heterocyclic benzodithiazole (BTD) motif, to probe for different glucans. The donor-acceptor-donor type electronic structure of CarbotraceTM680 provides improved spectral properties compared to oligothiophenes due to the possibility of intramolecular chargetransfer transition to the BTD motif. This enables differentiation of glucans based on the glycosidic linkage stereochemistry. Thus a-configured starch is readily differentiated from b-configured cellulose. The versatility of optotracing is demonstrated by dynamic monitoring of thermo-induced starch remodelling, shown in parallel by spectrophotometry and microscopy of starch granules. Imaging of Carbotrace TM680 bound to multiple glucans in plant tissues provided direct identification of their physical locations, revealing the spatial relationship between structural (cellulose) and storage (starch) glucans at sub-cellular scale. Our work forms the basis for the development of superior optotracers for sensitive detection of polysaccharides. Our non-destructive method for anatomical mapping of glucans in biomass will serve as an enabling technology for developments towards efficient use of plant-derived materials and biomass.