19500
Using nonresonant Raman spectroscopy , we demonstrate using model compounds that the frequency of this C2 - D vibration shifts by 35 cm ( - 1 ) upon changes in the His protonation state .
 C2 - D, His
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 C2 - D, His