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Our results support the hypothesis that the thermodynamically most favorable entry / exit portal for H2O is the so - called histidine gate ( HG ) , the same as for CO .
 H2O, histidine, CO
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 H2O, histidine, CO