13090
They are present only when the metal center is attached to the surface via a flexible linker ( a propyl group ) , which allows the active site to either react with the substrate or relax , reversibly , to the surface , thus providing stability .
 Chemical: propyl
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: propyl