《有机化学》课程PPT教学课件（Organic Chemistry with Biological Applications, 3th Edition, John McMurry, 2016）Chapter 19 Biomolecules - Amino Acids, Peptides, and Proteins

19-2 Amino Acids and the Henderson- Hasselbalch Equation:Isoelectric Points Using the Henderson-Hasselbalch equation the ratio of [A-]to [HA]in the solution can be calculated PH=pK,+l0g HA [A】or Iog HA

Using the Henderson-Hasselbalch equation the ratio of [A- ] to [HA] in the solution can be calculated - - a a A A pH = p + log or log = pH - p HA HA         K K 19-2 Amino Acids and the Henderson￾Hasselbalch Equation: Isoelectric Points

Amino Acids and the Henderson-Hasselbalch Equation:Isoelectric Points Species present in a 1.00 M solution of alanine at pH =9.00 From Table 19.1 [+H3NCH(CH3)CO2H]has pKa(a-CO2H)=2.34 [+H3NCH(CH3)CO2]has pKa(a-NH3)=9.69 0 0 H3NCHCOH H2O H3NCHCO H30+ pKa1=2.34 CH3 CH3 0 9 H3NCHCO H2O → H2NCHCO-H30+ pKa2=9.69 CH3 CH3

Species present in a 1.00 M solution of alanine at pH = 9.00 ▪ From Table 19.1 ▪ [ +H3NCH(CH3 )CO2H] has pKa (a-CO2H) = 2.34 ▪ [ +H3NCH(CH3 )CO2 - ] has pKa (a-NH3 + ) = 9.69 Amino Acids and the Henderson-Hasselbalch Equation: Isoelectric Points

Amino Acids and the Henderson-Hasselbalch Equation:Isoelectric Points [A- pH-pKa=9.00-9.69=0.69 so A- antilog(-0.69)=0.20 HA and[A]=0.20[HA] also A]+[HA]=1.00M

Amino Acids and the Henderson-Hasselbalch Equation: Isoelectric Points         antilog ( 0.69) 0.20 and A  0.20 HA HA A pH pK 9.00 9.69 0.69 HA A log a = - = = = - = - = - - - also so A - +HA=1.00 M

Amino Acids and the Henderson-Hasselbalch Equation:Isoelectric Points Solving the two simultaneous equations gives [HA]=0.83and[A-]=0.17 Therefore,in a 1.00 M solution at pH 9.00 83%of alanine molecules are neutral zwitterionic [H3NCH(CH3)CO2] 17%of alanine molecules are deprotonated [H2NCH(CH3)CO2]

Solving the two simultaneous equations gives [HA] = 0.83 and [A-] = 0.17 Therefore, in a 1.00 M solution at pH = 9.00 83% of alanine molecules are neutral zwitterionic [ +H3NCH(CH3 )CO2 - ] 17% of alanine molecules are deprotonated [H2NCH(CH3 )CO2 - ] Amino Acids and the Henderson-Hasselbalch Equation: Isoelectric Points

Amino Acids and the Henderson-Hasselbalch Equation:Isoelectric Points Similar calculations at other pH values leads to titration curve 12 H2NCHCO- pKa2=9.69 CH3 10 0 H3NCHCO- +H2NCHCO- 8 Isoelectric CH3 CH3 point =6.01 pH 6 H3NCHCO- CH3 0 0 4 pKa1=2.34 H3NCHCOH H3NCHCO CH3 CH3 0 + H3NCHCOH 0.0 0.5 1.0 1.5 2.0 CH3 Equivalents of HO- Cengage Leaming.All Rights Raserved

Similar calculations at other pH values leads to titration curve Amino Acids and the Henderson-Hasselbalch Equation: Isoelectric Points