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Ionic Properties of Amino Acids

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Within this range there exists a ratio between the charged and uncharged groups ... zwitterion. Different pKa. Effects of Ph on Amino Acid R-group Charge. pK=pI ... – PowerPoint PPT presentation

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Title: Ionic Properties of Amino Acids


1
Ionic Properties of Amino Acids
  • Lab B.1, pages 48 to 64
  • In lab manual

2
Bronsted Acids and Bases
  • Have groups which can accept or donate hydronium
    ions (H)
  • Have acid dissociation constants Ka
  • At pKa ratio of charged to uncharged is 1
  • Two pH units away from pKa functional groups are
    completely (almost) charged or discharged.
  • Within this range there exists a ratio between
    the charged and uncharged groups
  • Amino Acids have at least 2 of these

3
Acid-Base Pairs
  • pH greater than pKa functional group is in its
    base form (Deprotonated)
  • pH is less than pKa functional group is in its
    acid form (Protonated)

pKa
4
Alanine An Ampholyte
zwitterion
Different pKa
5
Effects of Ph on Amino Acid R-group Charge
pKpI
6
Henderson-Hasselbach Equation
  • pHpKa log base/acid
  • Description on page 49

7
Buffering Capacity
  • Molecules which resist changes in pH
  • Ability to resist change in pH is referred to as
    strength of buffer or buffering capacity

8
Glycine As a Buffer Page 5051
  • Ampholyte
  • Has both acidic and basic groups
  • As base is added the acidic group group looses
    protons first
  • Glycine can buffer in two ph ranges
  • 2.3 9.6

B
A
Exists in 3 forms
9
3 forms of Glycine at pH
red
black
blue
10
H-H equation and acid-base equilibrium
  • _at_ pKa, half of the group is protonated, half
    deprotonated
  • _at_ pKa 1 (ph Units), 90 deprotonated (ionized)
  • _at_ pKa 2, 99 deprotonated
  • _at_ pKa 1, 10 deprotonated
  • _at_ pKa 2, 1 deprotonated

11
Glutamic Acid
4 forms
12
Histidine
13
Technique can be used to characterize proteins
  • Determines Isoelectric point
  • Point at which 50 protonated 50 deprotinated
  • Protein has neutral charge and comes out of
    solution
  • A mechanism for separating proteins according to
    their amino acid composition

14
Gameplan
  • Will show you how to use Henderson-Hasselbach
    equation
  • To determine the buffering capacity of an unknown
    amino acid
  • Determine the molecular weight of the unknown
  • Know mg unknown
  • Know moles of acid or base to completely
    protinate or deprotonate
  • Expect lab exercise 2 hours

15
Important
  • This lab works when you measure carefully
  • Read pH only after allowing 30 seconds for mixing
  • Do not mix at speeds that make bubbles CO2
  • Be sure that magnetic stir bar does not hit
    electrode
  • End point of titrating a group is when pH changes
    rapidly with addition of acid/base

16
Use titration procedure described on page 52 53
  • Add a volume of acid or base to your unknown or
    to water and then determine resulting pH
  • Plot as Figure B.1-4 page 54(see also page 64)
  • Spread sheet on page 62 63
  • In the NaOH titration you will need about 7 ml
    of base to complete titration

17
Data collection
  • See manual page 55-56
  • Enter data in table format for water and water
    unknown
  • Do for acid titration and base titration
  • Examples of titration plots on page 64
  • Calculate molecular weight as on page 56 for both
    acid and base.
  • B. The base calculation is more accurate
  • WHY??

18
Tips
  • Calibrate pH meter for Acid OR Base
  • Page 51
  • Titrate water
  • Weigh out unknown
  • Titrate unknown water (do not recalibrate
  • Repeat for other titration

19
Acid titration
Protonates COO COOH
20
Base titration
Deprotonates NH3 NH2
21
Protocol
  • Follow dilution protocols on page 62 63
  • Calculate molecular weight after subtracting
    water titration
  • Moles NaOH to titrate unknown minus moles to
    titrate water to same pH equals moles to titrate
    amino acid
  • Moles NaOH equals Moles amine

22
Report
  • See page 58 report requirements

23
Look at thought questions
  • Page 59
  • Some of these may show up on exams

24
SAFETY FIRST!
  • Acid burns sting or burn.
  • Base burns feel slippery after time, skin will
    be raw.
  • Splashes on skin Wash with lots of water.

25
Lab Exit Requirements
  • Page 57
  • Show TA s 1 2 ( Data table and titration
    curves 62-63)
  • Calculate molecular weight
  • TA will provide name of the unknown

26
Clean Up
  • All materials down sink with running water

27
Value of this lab
  • Number of labs 1
  • Lab Report 10 Points
  • Pre lab questions 3 Points

28
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