Ch_322b_24.02

# At low ph it has three acid functions that

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has one amino and two carboxyl functions. At low pH, it has three acid functions that deprotonate as HO - is added in order of their acid strengths. HOOCCH 2 CHCOOH NH 3 + cationic form of aspartic acid (pK a1 = 2.1) HO - H 3 O + HOOCCH 2 CHCO 2 - NH 3 + zwitterionic form (pK a2 = 3.9) HO - H 3 O + O 2 CCH 2 CHCO 2 - NH 3 + - anionic form (pK a3 = 9.8) HO - H 3 O + O 2 CCH 2 CHCO 2 - NH 2 - dianionic form The isoelectric point of aspartic acid is the average of pK a1 and pK a2 : pI = 2.1 + 3.9 2 = 3.0 At pH = 3.0, the dominant form of aspartic acid is the neutral zwitterion. Summary of Observations on pI Values CHCO 2 - NH 3 + pI 6-7 ~3 ~9 HOOC CHCO 2 - NH 3 + H 2 N CHCO 2 - NH 3 +

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Quiz 24.02 The structure of valine and its pK a1 and pK a2 values are shown below. (CH 3 ) 2 CHCHCO 2 - NH 3 + valine pK a1 = 2.3 pK a2 = 9.7 (i) What is the isoelectric point of valine? (ii) What are the dominant structures of valine in solution at pH = 2.0? pH = 6? pH = 12? pI = (pK a1 + pK a2 )/2 = 6.0 (CH 3 ) 2 CHCHCO 2 H NH 3 + cation (CH 3 ) 2 CHCHCO 2 - NH 3 + zwitterion but neutral (CH 3 ) 2 CHCHCO 2 - NH 2 anion
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