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Unformatted text preview: CHEM 4511 PS 3 Fall 10 1. 2. (A) and (B) • Place in Fisher convention – Amino group is on left side so “L” (C) The pI is generally greater than the pKa for the α‐CO2H and less than that pKa for the α‐NH3+, so both groups will be ionized. It would also be impossible to identify a pH in which both the α‐NH2 and α‐CO2H groups would be unionized. (D) Calculate pI for Ala: pI = pI = 6.11 pH = pKa + log A AH = . . H/H equation: (see table 4.1 for pKa’s) • for the α‐NH2, we can calculate that: 1 out of 5755 is unionized • for the α‐CO2H group, we can calculate that: 1 out of 5755 is unionized Product of this is: 1 out of 3.3 × 107 is unionized 3. N +H 3 (C) * CO2 (A) 2 chiral centers (B) 4 possible optical isomers (R,R ; R,S ; S,R ; S,S) * 4. (A) O H3+N N H pKa2 H N O O- O pKa1 (B) The –CO2H group is father away from NH3+ • When deprotonates to CO2‐, no longer getting as much electrostatic stabilization from –NH3+ Decreased stabilization with physical separation, more difficult to ionize CO2H in longer peptides. 5. 6. (A) Ala (B) = pI . . = 6.11 (C) pI = = . . = 7.69 7. pI = = . . = 3.03 8. 9. (A) fully protonated
CO2H NH H3 N
+ 3 + ~3 pKa1 H3 N
+ CO2H NH3+
+ CO2- NH + 3 H3 N A T L D A K CO2
+ CO2- NH + 3 H2 N A T L D A K CO2
+ CO2- NH 2 H2 N A T L D A K CO2- A T L D A K CO2H A T L D A K CO2 pKa2 pKa3 pKa4 Net change: +2 +1 0 -1 -2 (B) Net change pH 7.0 = 0 pI = = . = 5.95 ...
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