“…Chemical standards of adenine (1), guanine (2), xanthine (3), thymine (10), inosine (13), arginine (15), histidine (16), lysine (17), tyrosine (20), valine (21), proline (22), glutamate (24), serine (27), threonine (28), alanine (29), tryptophan (31), phenylalanine (32), leucine (34), isoleucine (35), aspartic acid (37), 1-deoxynojirimycin (DNJ) (39), and fagomine (40) were purchased from the National Institute for the Control of Pharmaceutical and Biological Products, Beijing, China. Reference compounds of cytidine-5 -monophosphate (4), 2 -deoxyguanosine (5), 2 -deoxyuridine (8), 2 -deoxyinosine (9), cytidine (11), 2 -deoxyadenosine-5 monophosphate (12), thymidine (14), ornithine (18), hydroxyproline (19), citrulline (23), glutamine (25), asparagine (26), glycine (30), ␥-aminobutyric acid (36), and cysteine (38) were obtained from Sigma (St. Louis, MO). Chemical standards of 2 -deoxyadenosine (6) and 2 -deoxycytidine (7) were obtained from Aladdin Chemical, Shanghai, China.…”