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Model 2 The pl of a peptide is determined by examining the ionizable groups. The protonated and unprotonated forms of each ionizable group are in equilibrium. Consider the peptide Lys-Glu-Ser shown below at pH 7.2. The complete structure is on on the left and a stylized structure with just the ionizable groups is on the right. These two structures below represent the same peptide, but with differing levels of detail. The molecule on the right allows one to focus on how ionizable groups could change at different pH values. While the N-terminal is depicted as protonated, a sample of Lys-Glu-Ser is composed of a population of molecules and within that population some molecules may contain a non-protonated N-terminal. pH 7.2. group at H3N CH-CN CH CH CO- H3N+ COO- H pKa=8.5 pKa= 3.5 OH COO- CH2 CH₂ CH₂ NH3+ pKa=10.0 pKa=4.2 CH₂ CH₂ OH CH2 CO CH₂ NH₂+ In the stylized structures below, only one molecule is drawn. However each diagram represents a collection of many molecules. Therefore, "half protonated" implies that half the molecules present are protonated and half are not. H3N COOH H3N OH COOH NH3+ -COOH/ COO- H3N+ OH COOH NH3 COO OH COOH/ NH3 COO- NET charge +2 = At pH=1 all groups are protonated. At pH=3.5 the C terminal is half proton- ated and half deprotonated. At pH=4.2 the R group carboxyl is half protonated, half deprotonated. NET charge = 0.5 NET charge +1.5 H3N+ /H2N COO- H₂N- COO- H₂N- OH OH COO- COO- NH3 NHINH2 COO- OH COO- NH2 At pH=8.5 the N terminal is half protonated and half deprotonated. NET charge = -0.5 At pH=12 all groups are deprotonated. NET charge = -2.0 NOTE: In peptides and proteins, the N-terminal and C-terminal groups have different pKs from the parent amino acid. The pK, of the N-terminal is about 8.5 whereas C-terminal pK is about 3.5 At pH=10.0 the R group amino of lysine is half protonated, half deprotonated. NET charge-1.5

Model 2 The pl of a peptide is determined by examining the ionizable groups. The protonated and unprotonated forms of each ionizable group are in equilibrium. Consider the peptide Lys-Glu-Ser shown below at pH 7.2. The complete structure is on on the left and a stylized structure with just the ionizable groups is on the right. These two structures below represent the same peptide, but with differing levels of detail. The molecule on the right allows one to focus on how ionizable groups could change at different pH values. While the N-terminal is depicted as protonated, a sample of Lys-Glu-Ser is composed of a population of molecules and within that population some molecules may contain a non-protonated N-terminal. pH 7.2. group at H3N CH-CN CH CH CO- H3N+ COO- H pKa=8.5 pKa= 3.5 OH COO- CH2 CH₂ CH₂ NH3+ pKa=10.0 pKa=4.2 CH₂ CH₂ OH CH2 CO CH₂ NH₂+ In the stylized structures below, only one molecule is drawn. However each diagram represents a collection of many molecules. Therefore, "half protonated" implies that half the molecules present are protonated and half are not. H3N COOH H3N OH COOH NH3+ -COOH/ COO- H3N+ OH COOH NH3 COO OH COOH/ NH3 COO- NET charge +2 = At pH=1 all groups are protonated. At pH=3.5 the C terminal is half proton- ated and half deprotonated. At pH=4.2 the R group carboxyl is half protonated, half deprotonated. NET charge = 0.5 NET charge +1.5 H3N+ /H2N COO- H₂N- COO- H₂N- OH OH COO- COO- NH3 NHINH2 COO- OH COO- NH2 At pH=8.5 the N terminal is half protonated and half deprotonated. NET charge = -0.5 At pH=12 all groups are deprotonated. NET charge = -2.0 NOTE: In peptides and proteins, the N-terminal and C-terminal groups have different pKs from the parent amino acid. The pK, of the N-terminal is about 8.5 whereas C-terminal pK is about 3.5 At pH=10.0 the R group amino of lysine is half protonated, half deprotonated. NET charge-1.5

Biochemistry
Biochemistry
9th Edition
ISBN:9781319114671
Author:Lubert Stryer, Jeremy M. Berg, John L. Tymoczko, Gregory J. Gatto Jr.
Publisher:Lubert Stryer, Jeremy M. Berg, John L. Tymoczko, Gregory J. Gatto Jr.
Chapter1: Biochemistry: An Evolving Science
Section: Chapter Questions
Problem 1P
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1. Refer to Model 2 for information relevant to this question

a) A sample of the peptide Lys-Glu-Ser has a net charge of zero between what two pH values?

b) What is the pI of Lys-Glu-Ser?

 

2. Would Lys-Glu-Ser have the same pI as Ser-Glu-Lys? Explain

 

3. Draw a dipeptide (use R1 and R2 for the side chain R groups) and the resonance tructures of the peptide bond.

Model 2 The pl of a peptide is determined by examining the ionizable groups. The protonated and unprotonated forms of each ionizable group are in equilibrium. Consider the peptide Lys-Glu-Ser shown below at pH 7.2. The complete structure is on on the left and a stylized structure with just the ionizable groups is on the right. These two structures below represent the same peptide, but with differing levels of detail. The molecule on the right allows one to focus on how ionizable groups could change at different pH values. While the N-terminal is depicted as protonated, a sample of Lys-Glu-Ser is composed of a population of molecules and within that population some molecules may contain a non-protonated N-terminal. pH 7.2. group at H3N CH-CN CH CH CO- H3N+ COO- H pKa=8.5 pKa= 3.5 OH COO- CH2 CH₂ CH₂ NH3+ pKa=10.0 pKa=4.2 CH₂ CH₂ OH CH2 CO CH₂ NH₂+ In the stylized structures below, only one molecule is drawn. However each diagram represents a collection of many molecules. Therefore, "half protonated" implies that half the molecules present are protonated and half are not. H3N COOH H3N OH COOH NH3+ -COOH/ COO- H3N+ OH COOH NH3 COO OH COOH/ NH3 COO- NET charge +2 = At pH=1 all groups are protonated. At pH=3.5 the C terminal is half proton- ated and half deprotonated. At pH=4.2 the R group carboxyl is half protonated, half deprotonated. NET charge = 0.5 NET charge +1.5 H3N+ /H2N COO- H₂N- COO- H₂N- OH OH COO- COO- NH3 NHINH2 COO- OH COO- NH2 At pH=8.5 the N terminal is half protonated and half deprotonated. NET charge = -0.5 At pH=12 all groups are deprotonated. NET charge = -2.0 NOTE: In peptides and proteins, the N-terminal and C-terminal groups have different pKs from the parent amino acid. The pK, of the N-terminal is about 8.5 whereas C-terminal pK is about 3.5 At pH=10.0 the R group amino of lysine is half protonated, half deprotonated. NET charge-1.5
Transcribed Image Text:Model 2 The pl of a peptide is determined by examining the ionizable groups. The protonated and unprotonated forms of each ionizable group are in equilibrium. Consider the peptide Lys-Glu-Ser shown below at pH 7.2. The complete structure is on on the left and a stylized structure with just the ionizable groups is on the right. These two structures below represent the same peptide, but with differing levels of detail. The molecule on the right allows one to focus on how ionizable groups could change at different pH values. While the N-terminal is depicted as protonated, a sample of Lys-Glu-Ser is composed of a population of molecules and within that population some molecules may contain a non-protonated N-terminal. pH 7.2. group at H3N CH-CN CH CH CO- H3N+ COO- H pKa=8.5 pKa= 3.5 OH COO- CH2 CH₂ CH₂ NH3+ pKa=10.0 pKa=4.2 CH₂ CH₂ OH CH2 CO CH₂ NH₂+ In the stylized structures below, only one molecule is drawn. However each diagram represents a collection of many molecules. Therefore, "half protonated" implies that half the molecules present are protonated and half are not. H3N COOH H3N OH COOH NH3+ -COOH/ COO- H3N+ OH COOH NH3 COO OH COOH/ NH3 COO- NET charge +2 = At pH=1 all groups are protonated. At pH=3.5 the C terminal is half proton- ated and half deprotonated. At pH=4.2 the R group carboxyl is half protonated, half deprotonated. NET charge = 0.5 NET charge +1.5 H3N+ /H2N COO- H₂N- COO- H₂N- OH OH COO- COO- NH3 NHINH2 COO- OH COO- NH2 At pH=8.5 the N terminal is half protonated and half deprotonated. NET charge = -0.5 At pH=12 all groups are deprotonated. NET charge = -2.0 NOTE: In peptides and proteins, the N-terminal and C-terminal groups have different pKs from the parent amino acid. The pK, of the N-terminal is about 8.5 whereas C-terminal pK is about 3.5 At pH=10.0 the R group amino of lysine is half protonated, half deprotonated. NET charge-1.5
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