2.5. Spectral analysis

For NMR analysis, the lyophilized sample was dissolved in D₂O (J&K Scientific Ltd., Shanghai, China) and frozen three times. ¹H and DEPT 135°-NMR were acquired on an Aglient DD2-600MHz spectrometer (Agilent, USA) at 25 °C. The relaxation delay was 2 and 1 respectively, receiver gain was 18 and 30 respectively. 2D ¹H— ¹H COSY and ¹H— ¹³C HSQC were collected using the pulse program process provided by the instrument.

MALDI-TOF MS spectra was recorded with an MALDI-TOF/TOF 5800 mass spectrometer (AB Sciex, USA). The matrix used was arabinoosazone, which was successfully synthesized by our laboratory according the method reported (Chen, Baker, & Novotny, 1997). For the synthesis and verification of arabinoosazone, L-arabinose and sodium acetate were purchased from Aladdin Reagent Co., LTD (Shanghai, China), phenylhydrazine hydrochloride and DMSO‑d ₆ (99.9 atom % D) were purchased from Sigma-Aldrich LLC. (Darmstadt, Germany). 1 μL mixture containing 0.1 mg/mL OP145 solution in H₂O and 0.5 M arabinoosazone matrix solution in acetone was introduced on to the sample plate and air dried. Spectrum was collected in reflector negative mode, and the instrument settings was as follows: total shots, 400; accelerating voltage, 7.5 kV; laser power, 30 μJ; laser shot rate, 200 Hz. According to GPC results and several acquisitions to adjust the molecular weight set range, the final range was 800–2000.