Elucidação estrutural da manana cristalina isolada do endosperma de sementes de açaí (Euterpe oleracea mart.)

Abstract

Açaí seeds (E. oleracea Mart.) are a byproduct of pulp production, with 50% of their total dry weight consisting of mannan. The depolymerization of mannan releases mannose and mannan-oligosaccharides. However, açaí seeds are recalcitrant to deconstruction by enzymatic action. Understanding the structure of this polysaccharide can help identify the limitations of its processing, with the aim of increasing the efficiency of bioconversion. For this purpose, an insoluble fraction was isolated from the seeds and evaluated by solution nuclear magnetic resonance (NMR). The 1H/13C HSQC spectra were attributed to a β-Dmannan, and the 1H/13C HMBC spectra confirmed the presence of a β(1→4) glycosidic linkage between the Manp units through cross-peaks at 4.73/78.0 ppm (H-1/C-4) and 3.88/101.8 ppm (H-4/C-1). However, due to the difficulty of solubilizing the sample, the isolated fraction was also evaluated by solid-state NMR. The results confirmed it to be a linear mannan, with the presence of crystalline polymorphs I and II, detected by signals corresponding to carbon 5 of both forms at 70.2 and 70.9 ppm, respectively. The crystallinity of this fraction was assessed by X-ray diffraction (XRD), which showed peaks at 2θ values of 16°, 18°, 20°, 24°, and 26°, corresponding to planes 110, 111, 200, 210, and 211, respectively. This profile is similar to that of a crystalline form of type I mannan, which predominated in the sample. However, the XRD profile also revealed a peak at 22.5°, characteristic of the 200 crystalline plane of cellulose Iβ, indicating possible sample contamination. The contamination was corroborated by the thermal degradation behavior of the sample and compositional analysis, which indicated a 78:19:2 ratio of Man:Glc:Gal. Consequently, the sample was reevaluated using solution NMR equipped with a cryoprobe to increase the sensitivity of the method. However, it was not possible to observe the 1H/13C signal corresponding to the anomeric position of the Gal-(1→ residue, making further optimization of the method necessary to confirm the chain branching. To overcome contamination issues, the mannan from açaí seeds was also analyzed in situ through immunolocalization. The analysis not only allowed for the identification of linear mannan in the mature endosperm cell wall but also enabled the monitoring of the transformation of galactomannan into linear mannan throughout fruit development. Thus, the structure of mannan in açaí seeds was identified for the first time. The results obtained can guide strategies to reduce the crystallinity of mannan and select more suitable enzymes for the hydrolysis of this recalcitrant biomass.