TABLE 1.

Troubleshooting table.

Problem	Solution
p-Anisaldehyde sugar stain: The color of the p-anisaldehyde sugar stain changes from colorless to dark red	It is recommended that the stain be stored at − 20 °C in a sealed bottle. If p-anisaldehyde sugar stain is left at room temperature for a long time, it may then change its color. Please refer to reference 25 for more information on preparation and storage of this stain
Preparative-scale synthesis of GDP–fucose: Step 5 the enzyme does not work or precipitates completely	Use a new batch of enzyme to check if it has lost its activity during storage Test the pH value of the reaction solution using precise pH-indicator strips. Adjust the pH value by carefully adding 10 M NaOH solution or concentrated hydrochloric acid if necessary NaOH is a strong base and highly corrosive. Wear gloves. Concentrated hydrochloric acid is a volatile and corrosive strong acid. Wear gloves and handle with great care in a chemical fume hood.
Preparative-scale synthesis of GDP–fucose: Step 15 the GDP–fucose could not be isolated from the side-products and residual substrates	Decrease the loading amount to 1/100 of bed volume Increase the length and/or diameter of the P-2 column