23613
Uridine ( U ) imino proton chemical shifts , measured in the slow hydrogen - bond exchange regime at low temperatures in a freonic solution , show that electron - withdrawing 5 - bromo and 5 - fluoro substituents on the uracil base strengthen NHN hydrogen bonds in ( X ) U . A base pairs formed by the free nucleosides .
 Uridine, hydrogen, 5 - bromo, 5 - fluoro, uracil, NHN, hydrogen, nucleosides
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 Uridine, hydrogen, 5 - bromo, 5 - fluoro, uracil, NHN, hydrogen, nucleosides