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The source behind the cooperativity could be traced back to the primary amino acid structure and , more specifically , the presence of positively charged amino acids in positions that enable electrostatic interaction with phosphoryl groups on neighboring peptides .
 amino acid, amino acids, phosphoryl
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 amino acid, amino acids, phosphoryl