21059
Here we compare three commonly used force fields , CHARMM , OPLSAA , and AMBER in study of the competitive binding of a single wall carbon nanotube ( SWCNT ) with a native proline - rich motif ( PRM ) ligand on its target protein SH3 domain , a ubiquitous protein - protein interaction mediator involved in signaling and regulatory pathways .
 proline
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 proline