Cold-adapted species are able to generate cryoprotective molecules to prevent the freezing damage caused by the formation and growth of internal ice crystals. These antifreeze molecules are mainly proteins and glycoproteins. Cryoprotectant plays a pivotal role in the cryopreservation of tissues and organs, rendering the development of new antifreeze molecules highly desirable.
In 2009, Walters and colleagues reported an antifreeze xylomannan, the first nonprotein thermal hysteresis producing biomacromolecule, from Alaskan beetle Upis ceramboides (PNAS, 2009, 20210). Intriguingly, this xylomannan possessed highly potent antifreeze activity and was then found in diverse cold-adapted organisms, including plants, frogs, and insects. Several prestigious carbohydrate chemists including Ito, Crich, and Serianni have launched the synthetic studies of this glycan to verify its chemical structure and to elucidate the antifreeze mechanism. However, these efforts were limited to the synthesis of short saccharide fragments and thus failed to confirm the reported structure as well as the antifreeze properties.
Recently, the Yu's group at Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, disclosed an iterative exponential glycan growth (IEGG) strategy based on the gold(I)-catalyzed "Yu glycosylation" for expeditious synthesis of long glycans. Each round of the glycan assembly entails three steps: (1) removal of the anomeric protection and subsequent esterification to prepare the glycosyl ortho-alkynylbenzoate donor; (2) deprotection at the nonreducing end to prepare the glycosyl acceptor; and (3) gold(I)-catalyzed glycosylation to access the double-sized glycan, which can be subjected to the next round of assembly (Nat. Commun. 2020, 4142).
Using this assembly strategy, Yu and co-workers accomplished the rapid synthesis of the initially proposed [→4)-β-D-Manp-(1→4)-β-D-Xylp-(1→]n xylomannans up to a 64-mer. Unfortunately, the nuclear magnetic resonance (NMR) data of the synthetic glycans differed notably from those of the reported natural isolate. To solve the structural puzzle, three other possible structures were prepared, including those with the D-mannose unit being replaced by L-mannose unit, with the (1→4) linkage being replaced by (1→3) linkage, or with the alternating xylose-mannose unit being changed to the block-wise arrangement. In fact, the NMR spectra of the block-wise glycan resembled closely to those reported for the natural sample, strongly supporting that the natural glycan should be a block-wise xylomannan or a mixture of xylan and mannan.
Disappointingly, neither the block-wise glycan and the xylan-mannan mixture whose NMR spectral data matched well with those of the natural isolate, nor the three other types of synthetic glycans, displayed antifreeze properties.
The present work leads to an end of the researches on the antifreeze xylomannan. Importantly, this work offers a paradigm in the research field relevant to polysaccharides, underscoring the need of prudence in the chemical structure as well as the function of glycans which has not been synthetically verified.
