amino acids df05
TRANSCRIPT
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Chem 437
Amino acids
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Bio-Polymer Chemistry• DNA
– Made up of four monomer units (adenine, guanine, thymine and cytosine)– In general, forms one structure (double helix).– Functional role: storing information
• RNA– Made up of four more abundant monomer units (adenine, guanine, uracil
and cytosine)– Many less abundant bases also exist– Simple to very complex structures– Functional role: template for protein synthesis, catalytic activity, protein
synthesis
• Proteins– Made up of 20 monomer units (α-amino-acids)– Vast array of different types of structure– Functional role: catalysts (organic synthesis), structural, regulation of all
intracellular and extracellular events, signaling, etc etc etc etc etc
• Polymers are an energetically efficient way to construct a wide range of macromolecules with the same machinery
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Basic structure of α-amino acids
Cα
R
CO2H3N+
HCα
R
CO2H3N+
H
Ball and stick model(Tetrahedral arrangement)
R R-group, varies with each amino acid
-CO2- Carboxyl group
-NH3+ Primary amino group
Zwitterion form
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Formation of the peptide bond
Peptide bond
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The α-amino acids
Cα
H
CO2H3N+
HGlycine, Gly, G
( not-chiral)
Cα
CH3
CO2H3N+
Alanine, Ala, A(non-polar/hydrophobic)
Cα
CH2
CO2H3N+
HSerine, Ser, S
(polar)
Cα
CH2
CO2H3N+
HCysteine, Cys, C
(polar)
Cα
C
CO2H3N+
HThreonine, Thr, T
(polar, two chiral centers)
Cα CO2H3N+
HValine, Val, V
(non-polar/hydrophobic)
H CH3
OH
CH CH3
CH3SH
OH
H
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The α-amino acids
Cα CO2H3N+
H
CH CH3
CH3
CH2
Leucine, Leu, L(non-polar/hydrophobic)
Cα CO2H3N+
H
CH2
CH3
C
Isoleucine, Ile, I(non-polar/hydrophobic
Two chiral centers)
H CH3
Cα CO2H3N+
H
CH2
S
CH2
Methionine, Met, M(non-polar/hydrophobic)
CH3
Proline, Pro, P(non-polar/hydrophobic,
α-imino acid)
Cα CO2H2N+
H
CH2CH2
CH2
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H
The α-amino acids
Cα CO2H3N+
H
CH2
Phenylalanine, Phe, F(non-polar/hydrophobic)
Cα CO2H3N+
H
CH2
Tryptophan, Trp, W(non-polar/hydrophobic)
Cα CO2H3N+
H
CH2
Tyrosine, Tyr, Y(polar)
OHN
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The α-amino acids
Lysine, Lys, K(polar, charged)
Cα CO2H3N+
H
CH2
CH2
CH2
NH3+
CH2
Arginine, Arg, R(polar, charged)
Cα CO2H3N+
H
CH2
CH2
CH2
CNH
NH2+NH2
Histidine, His, H(polar, charged)
Cα CO2H3N+
H
CH2
N
N
H
H
+
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CCH2
The α-amino acids
Asparagine, Asn, N(polar)
Cα CO2H3N+
H
CH2
CO NH2
Glutamine, Gln, Q(polar)
Cα CO2H3N+
H
CH2
O NH2
Cα CO2H3N+
HAspartic acid, Asp, D
(polar, charged)
CH2
CO2
Glutamic acid, Glu, E(polar, charged)
Cα CO2H3N+
H
CH2
CO2
CH2
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Labeling of the side chain atoms
Lysine, Lys, K(polar, charged)
Cα CO2H3N+
H
CH2
CH2
CH2
NH3+
CH2
βγδεζ
Cα
C
CO2H3N+
HThreonine, Thr, T
(polar, two chiral centers)
H CH3
OH
βγ
γ
• The carboxyl group and amino group attached to the Cα atom are referred to as the α−carboxy group and the α−amino group respectively
• The second amino group on lysine is referred to as the ε-amino group
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CH2
Representations of α-amino acids
Cα CO2H3N+
H
Phenylalanine, Phe, F
ball and stick
Space filling
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Chem 437
Uncommon amino acids found in proteins
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Biologically important derivatives of amino acids
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Acid-base properties
Net Charge +1 Net Charge 0 Net Charge -1
pH = 1 pH = 7 pH = 13
pKa 1.8-2.6pKa 8.8-10.4
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Acid-base properties
Cα
H
CO2H3N+
H
Cα
H
CO2HH3N+
H
Cα
H
CO2H2N
H
Glycine(A, V, I, L, P, F, M, N, Q, W)
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Acid-base properties : acidic amino acids
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Acid-base properties : basic amino acids
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Table of pKa values for amino acids
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Questions on histidine
• Draw all the ionization states of histidine in the order that they would be deprotonated
• Draw the titration curve of histidine as it is titrated with hydroxide ions (ensuring that the axis are correctly labeled and the pKa values are shown).
• Indicate on the curve the region where each of the above ionization states is the major species.
• Calculate the isoelectric point of histidine
• What percentage of histidine is in the protonated form at pH 7.0
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Answer slide
Cα CO2HH3N+
H
CH2
N
N
H
H
+
Cα CO2-H3N+
H
CH2
N
N
H
H
+
Cα CO2-H3N+
H
CH2
N
NH
Cα CO2-H2N
H
CH2
N
NH
pI = 7.6
[His][His+]
= 10
% His+ is 9.1%
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Chem 437• Substituents on the amino acids can alter their pKa values
• Anserine (N-β-alanyl-3-methyl-L-histidine) has a pKa of imidazole group is 7.04
• When amino acid residues occur within folded protein structures, the pKa values can be very different from the free amino acid (alters function)
Altering the pKa
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Chem 437
Properties of α-amino acids: Chemistry
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Properties of α-amino acids : Chemistry
α-amino acids give purple productα-imino acids give yellow product
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Reactivity of side chains
Cα CO2H3N+
HAspartic acid, Asp, D
(polar, charged)
CH2
CO2
Glutamic acid, Glu, E(polar, charged)
Cα CO2H3N+
H
CH2
CO2
CH2
Lysine, Lys, K(polar, charged)
Cα CO2H3N+
H
CH2
CH2
CH2
NH3+
CH2
Cα
CH2
CO2H3N+
HCysteine, Cys, C
(polar)
SH
Cα
CH2
CO2H3N+
HSerine, Ser, S
(polar)
OH
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Reactivity of side chains
-Phe-Ser-Tyr-Gly-Val-
Part of the amino acid sequence of Green Fluorescent Protein (GFP)
NN
O
HOOHN
Phe, etc.
Val, etcO2
Cα CH2
CO2
NH3+
H
Cysteine, Cys, C(polar)
SH2 Cα CH2
CO2
NH3+
H S CαCH2
CO2
NH3+
HS
Cystine(disulfide bond)
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Optical activity of amino acids• All amino acids are chiral except for glycine.
Two enantiomeric forms
Cα
R
2OCNH3
+H
Cα
R
CO2
H3N+ H
Mirror plane
α-alanine
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Rotation of polarized light by chiral molecules
Schematic diagram of a polarimeter
Sodium
Chiral molecules are optically active, i.e. they rotate plane polarized light
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Animation of rotation of polarized light
QuickTime™ and a Sorenson Video decompressor are needed to see this picture.
http://cwx.prenhall.com/petrucci/medialib/media_portfolio/28.html
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Rotation of polarized light by chiral molecules
• Clockwise rotation– Dextrorotatory (to the right)– Assigned a positive value
Observed rotation (º)
Optical path length (dm) × concentration (gcm-3)[α]25D
=
D D-line in the spectrum of sodium used to generate monochromatic light
25 Experiment carried out at 25ºC
• Anti-clockwise rotation– Levorotatory (to the left)– Assigned a negative value
Amino acid
Alanine +1.8Arginine +12.8Leucine -11.0Lysine +13.5Methionine -10.0
[α]25D
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Stereochemistry of α-amino acids - The D, L system
Cα
H
CH2OH
CHOHO
Cα
H
OHCH2OH
CHO
L-glyceraldehyde (-9.4) D-glyceraldehyde (+9.4)
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Stereochemistry of α-amino acids - The D, L system
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Stereochemistry of α-amino acids - CORN law
Clockwise direction
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Stereochemistry of α-amino acids
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Stereochemistry of α-amino acids - R, S system
Cα
R
CO2H3N+
HCα
H3N+ CO2
R
Hydrogen atom points to back
Cα
H3N+CO2
R
SH > OH > NH2 > CO2H > CHO > CH2OH > CH3
R(clockwise,
goes to the Right)
S(anti-clockwise,goes to the left)
• Atoms of higher atomic number bonded to a chiral center are ranked above those of lower atomic number
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Stereochemistry of α-amino acids - R, S system
• (2S,3S) and (2R,3R) are enantiomers• (2S,3R) and (2R,3S) are enantiomers• (2S,3S) and (2S,3R) are diastereoisomers• (2R,3R) and (2R,3S) are diastereoisomers
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Spectroscopic properties of α-amino acids
Beer/Lamberts Law
A = = εcl
A - absorbance (A.U.)Io = initial intensityI = emitted intensityε - molar extinction coefficient
(molar absorptivity, M-1cm-1)c - concentration (M)l - pathlength (cm)
logIIo
Detector
Io Il
Cuvette
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NMR spectroscopy of amino acids• The sample under study is placed in a strong magnetic field and a series of radio
frequency (rf) pulses are applied to it.
• Certain atoms (such as 1H) when they are exposed to these radio frequency pulses, emit an rf pulse of their own.
• The emitted rf gives information about the environment of that atom
• The specific emitted rf is represented relative to a rf from a standard (e.g. for a proton NMR, tetramethylsilane is used) in an NMR spectra
• Each peak in the spectra (referred to as the chemical shift) represents an atom (or set of atoms in identical environments) in a unique environment that give a unique rf
• Each amino acid will give a unique proton NMR as their hydrogens exist in unique environments
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1H NMR examples of amino acids
• The type of group the hydrogen is attached to will effect its position (the carboxyl hydrogen has a chemical shift > 10ppm and is not shown)
• The presence of hydrogens on a neighboring carbon will cause splitting of the chemical shift of a hydrogen
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Variation in chemical shift of 13C peaks with pH
• Atoms in groups that can be ionized show pronounced changes in chemical shift as the pH changes
• NMR can be used to study these changes in proteins
Cα CO2H3N+
H
CH2
CH2
CH2
NH3+
CH2
βγδεζ
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Analysis/separation of amino acid mixtures
• Required in the analysis of– Physiological fluids (imbalance in amino acid levels)– Hydrolyzed proteins (determine relative amounts of amino
acids)– Foodstuffs for nutritional value
• Properties of amino acids used to separate them chromatographically– Ionic properties– hydrophobicity
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Analysis/separation of amino acid mixtures
SampleSO3
-
SO3-
SO3-
O3S-SO3
-SO3
-
Cation exchangecolumn
(CH3)3+N
Anion exchangecolumn
HydrophobicInteraction column(reverse phase)
(CH3)3+N
N(CH3)3+N(CH3)3
+
N(CH3)3+
N(CH3)3+
(CH2)17CH3
(CH2)17CH3
(CH2)17CH3(CH2)17CH3
CH3(CH2)17
CH3(CH2)17
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Analysis/separation of amino acid mixtures
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
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Analysis/separation of amino acid mixtures
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
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Analysis/separation of amino acid mixtures
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Cα CO2H3N+
H
CH2
CO2H
Cα CO2H3N+
H
(CH2)4
NH3+
Cα
CH3
CO2H3N+
H
Mixture (pH ~ 3.3)
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Analysis/separation of amino acid mixtures
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
Na+
Na+
Na+
Na+
Na+
Cα CO2H3N+
H
CH2
CO2H
Cα CO2H3N+
H
(CH2)4
NH3+
Cα
CH3
CO2H3N+
H
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Analysis of amino acid mixtures
Column
A BSolution ASolution B
Time/volume
Peak100% Solution B
50% Solution B
0% Solution B
Pumps anddetector
HPLC(High Performance
Liquid Chromatography)
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Analysis/separation of amino acid mixturesSO3
-
SO3-
SO3-
O3S-SO3
-SO3
-
Cation exchangecolumn
(CH3)3+N
Anion exchangecolumn
HydrophobicInteraction (C18) column(reverse phase)
(CH3)3+N
N(CH3)3+N(CH3)3
+
N(CH3)3+
N(CH3)3+
(CH2)17CH3
(CH2)17CH3
(CH2)17CH3(CH2)17CH3
CH3(CH2)17
CH3(CH2)17
Na+Cl-
or pH
CH3CN(acetonitrile)
Na+Cl-
or pH
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Analysis/separation of amino acid mixtures
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
Na+
Na+
Na+
Na+
Na+
Peak
100% B
50% B
0% B
Solution A : 0M NaClSolution B : 1M NaCl
Solution A : pH 3.3Solution B : pH 11.0
or
or
Mixture of both variation in pHand salt concentration
Cα CO2H3N+
H
CH2
CO2H
Cα CO2H3N+
H
(CH2)4
NH3+
Cα
CH3
CO2H3N+
H
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Analysis/separation of amino acid mixtures
Peak100% B
50% B
0% B
Solution A : 0M NaClSolution B : 1M NaCl
Asp Ala Lys
A B
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
SO3-
Na+
Na+
Na+
Na+
Na+
Cα CO2H3N+
H
CH2
CO2H
Cα CO2H3N+
H
(CH2)4
NH3+
Cα
CH3
CO2H3N+
H
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Analysis of amino acid mixtures
Ion exchange column
Post-column derivatization by,– Ninyhdrin– o-pthaldialdehyde (OPA)
C
O
H
CO
H
OPA
• Ninhydrin detected byvisible light absorption
• OPA detected through fluorescence
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Phenylisothiocyanate reaction with amino acids
C18 hydrophobic Interaction column
(Detection at 254nm)
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Things to do
• Garrett and Grisham, chapter 4• Problem : 1-13