IONIZATION OF AMINO ACIDS

Given in the table below are pKa values for ionizable groups of the 20 protein-derived amino acids.

Amino Acid -CO2H Group -NH3+ Group Side Chain
alanine 2.35 9.87   
arginine 2.01 9.04 12.48
asparagine 2.02 8.80  
aspartic acid 2.10 9.82 3.86
cysteine 1.86 10.25 8.00
glutamic acid 2.10 9.47 4.07
glutamine 2.17 9.13  
glycine 2.35 9.78  
histidine 1.77 9.10 6.10
isoleucine 2.32 9.76  
leucine 2.33 9.74  
lysine 2.18 8.95 10.53
methionine 2.28 9.20  
phenylalanine 2.58 9.24  
proline 2.00 10.60  
serine 2.21 9.15  
threonine 2.09 9.10  
tryptophan 2.38 9.39  
tyrosine 2.20 9.11 10.07
valine 2.29 9.72  

 

Table from Introduction to Organic and Biochemistry, 4th ed. William H. Brown, Brooks/Cole Publishing Company, 1987, p. 356. Used with permission from publisher.