Amino acids week 7(mine new)

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Transcript Amino acids week 7(mine new)

Amino Acids
• Organic compounds containing both the amine
-NH2 and carboxyl -COOH functional groups.
• Amine e.g. CH3CH2NH2 ethylamine
• Carboxylic acid e.g. CH3COOH ethanoic acid.
• Simplest amino acid: aminoethanoic acidglycine if you are a biologist.
Week 7
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State the general formula for an α-amino acid as RCH(NH2)COOH.
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State that an amino acid exists as a zwitterion at a pH value called the
isoelectric point.
•
State that different R– groups in α-amino acids may result in different
isoelectric points.
•
Describe the acid–base properties of α-amino acids at different pH values.
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Amino Acids
• Naturally occurring amino acids are all α amino
acids.
• This means that both the carboxyl and the
amino functional groups are on the SAME
carbon atom.
• This leads to the general formula on the next
slide.
• These compounds are BIFUNCTIONAL since
both functional groups act independently of
one another.
Week 7
General formula of an α-amino acid
R means any organic side chain.
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Week 7
Structures of some amino acids
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Physical Properties
• White solids
• With relatively high melting points glycine
(the simplest) has a melting point of 235°C.
• Normally readily soluble in water
• Almost totally insoluble in non-polar solvents
• Soluble in both acids and bases (alkalis).
Reactions
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The carboxylic acid group is a proton donor:
CO2H D COO- + H+
The amine group is a proton acceptor (a base):
-NH2 + H+ D –NH3+
The amino acids tend to exist as ZWITTERIONS.
These are formed when the carboxyl group and the
amine group have undergone an internal acid-base
reaction.
• A proton is transferred FROM the acid TO the amine
group so that both ends are charged.
• The overall charge is zero because the positive and
negative charges cancel each other out.
Week 7
Formation of a zwitterion from glycine
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Isoelectric Point
• pH value at which the amino acid exists as a
zwitterion.
• Varies from amino acid to amino acid since the
inductive effects of the side chains affects
the acid and base strengths of different
amino acids differently.
• The isoelectric point has an impact on the
acid-base behaviour of the amino acids.
Amphoteric Behaviour
• Amphoteric means can react with both acid and base .
• pH below isoelectric point:
• The amino acid is a base and accepts a proton from
the the acid.
• The amino acid is a positively charged ion.
• pH above isoelectric point:
• The amino acid is an acid and donates a proton to the
base.
• The amino acid forms a negatively charged ion.
Leucine
Isoelectric point pH 5.98
1. Draw the displayed formula
of leucine.
2. Show the structure of
leucine at pH 5.98, 2.0 and
7.
Answers
-
Nearly full displayed
formula.
Skeletal zwitterionic
form at pH 5.98
H+
At pH7 behaves as an acid
At pH2 behaves as a base
Week 7
Acid–base reactions of an amino acid
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Week 7
•
Explain the formation of a peptide (amide) linkage between α-amino acids to
form polypeptides and proteins.
•
Describe the acid and alkaline hydrolysis of proteins and peptides to form αamino acids or carboxylates.
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Peptides and Polypeptides
• A peptide linkage is the –CONH- link
• This is formed in a CONDENSATION reaction
between two amino acids with the loss of a water
molecule.
• A peptide is a compound containing amino acids linked
by peptide bonds.
• Dipeptide = 2 amino acids
• Tripeptide = 3 amino acids etc.
Week 7
Formation of a dipeptide between glycine and alanine
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Week 7
Alternative reaction of alanine and glycine
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Peptides and Polypeptides
• A polypeptide is a long chain of amino acids joined by
peptide links.
• A protein is a long polypeptide chain with more than
60 amino acid units.
• A polypeptide with 4 different amino acids.
Hydrolysis
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The breaking of a bond by reaction with water.
Can be acid or base catalysed.
Acid hydrolysis:
Generally refluxed with aqueous (6mol dm-3) HCl for
24 hours.
The product is the protonated form of the
constituent amino acids.
Alkaline hydrolysis
Reflux with aqueous sodium hydroxide.
Sodium salt formed.
Week 7
Acid hydrolysis of a dipeptide
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Week 7
Alkaline hydrolysis of a polypeptide chain
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Isomerism
Isomerism
Structural
Isomers
Stereo
Isomers
E/Z (cistrans)
Isomers
Optical
Isomers
Stereo isomerism –same structural formula but
different arrangements of groups in space.
Week 7
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Describe optical isomers as non-superimposable mirror images about an
organic chiral centre.
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Identify chiral centres in a molecule of given structural formula.
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Explain that optical isomerism and E/Z isomerism are types of
stereoisomerism.
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• All molecules have a mirror image –
but for most molecules it is the
same molecule.
H
H
H
C
C
H
F
H
F
H
fluoromethane
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• For some molecules the mirror image is a different
molecule (the mirror image is non-superimposable).
When this happens the 2 forms of the molecule are
described as OPTICAL ISOMERS. Optical isomerism
is a form of stereoisomerism.
H
OH
OH
C
C
COOH
CH3
(-) lactic acid
in sour milk
HOOC
H3C
H
(+) lactic acid
in muscles
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• Molecules that are optical isomers are called
enantiomers.
• Enantiomers have identical chemical and
physical properties, except:
• Their effect on plane polarised light;
• Their reaction with other chiral molecules.
Many natural molecules are chiral and most
natural reactions are affected by optical
isomerism.
• In nature, only one optical isomer occurs (e.g.
all natural amino acids are rotate polarised
light to the left).
• Light is a form of electromagnetic
radiation.
• The wave vibrations are perpendicular
to the direction of travel of the wave.
normal light
(w aves vibrate in all directions)
plane-polarised light
(vibrates in only one direction)
plane-polarised light after
clockw ise rotation
• Optical isomers rotate the plane of
plane polarised light.
(-)-enantiomer
(anticlockw ise rotation)
(+)-enantiomer
(clockw ise rotation)
(±)-racemate
(no overall effect)
• http://www.youtube.com/watch?v=3WZZX
POsPNI&feature=related
Optical Isomerism
• Left and right hands are an example of nonsuperimposable mirror images.
OPTICAL ISOMERISM
Occurrence
another form of stereoisomerism
occurs when compounds have non-superimposable mirror images
Isomers
the two different forms are known as optical isomers or enantiomers
they occur when molecules have a chiral centre
a chiral centre contains an asymmetric carbon atom
an asymmetric carbon has four different atoms (or groups)
arranged tetrahedrally around it.
OPTICAL ISOMERISM
Occurrence
another form of stereoisomerism
occurs when compounds have non-superimposable mirror images
Isomers
the two different forms are known as optical isomers or enantiomers
they occur when molecules have a chiral centre
a chiral centre contains an asymmetric carbon atom
an asymmetric carbon has four different atoms (or groups)
arranged tetrahedrally around it.
CHIRAL CENTRES
There are four different colours
arranged tetrahedrally about
the carbon atom
2-chlorobutane exhibits optical isomerism
because the second carbon atom has four
different atoms/groups attached
Most α amino acids are optically active – having non
superimposable mirror image isomers. Exception?
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Week 7
The butan-2-ol molecule has a chiral carbon
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Week 7
Optical isomers of CH3CH2CH(NH2)CH3
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Week 7
Simplified three-dimensional representation of two optical isomers
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TASK
1)
2)
3)
4)
Which of the following molecules are
optically active?
propan-2-ol
2-chlorobutane
1-chlorobutane
3-methylhexane
5)
6)
7)
8)
butanone
2-methylbutanoic acid
butan-2-ol
1-chloro-3-methylpentane
propan-2-ol
CH3
CH
CH3
OH
NOT OPTICALLY ACTIVE
2-chlorobutane
CH3
CH
CH2
CH3
Cl
H
CH2CH3
CH2CH3
C
C
CH3
Cl
H3C
Cl
OPTICALLY ACTIVE
H
1-chlorobutane
CH2
CH2
CH2
Cl
NOT OPTICALLY ACTIVE
CH3
3-methylhexane
CH3 CH2 CH CH2 CH2 CH3
CH3
H
CH2CH2CH3
CH2CH2CH3
C
C
CH3
CH2CH3
CH3
CH3CH2
OPTICALLY ACTIVE
H
O
butanone
CH3
C
CH2
CH3
NOT OPTICALLY ACTIVE
propan-2-ol
CH3
CH
CH3
OH
NOT OPTICALLY ACTIVE
CH3 O
2-methylbutanoic acid
CH3
CH3
CH2
CH
C
CH2CH3
CH2CH3
C
C
H
COOH
H
HOOC
OPTICALLY ACTIVE
OH
CH3
OH
butan-2-ol
CH3
CH3
CH2
CH
CH3
CH2CH3
CH2CH3
C
C
H
OH
H
HO
OPTICALLY ACTIVE
CH3
1-chloro-3-methylpentane
CH3
CH3
CH3
CH2
CH
Cl
CH2
CH2
CH2CH3
CH2CH3
C
C
H
CH2CH2Cl
H
CH2ClCH2
OPTICALLY ACTIVE
CH3
CH3
CH3
O
O
H
C
CH2
H3C
S carvone (caraway seed)
Caraway Seed has a warm, pungent,
slightly bitter flavour with aniseed
overtones.
H2C
C
H
CH3
R carvone (spearmint)
CH3
CH3
CH2
C
H
H
C
CH2
CH3
H3C
S limonene (lemons)
R limonene (oranges)
Week 7
E and Z isomers of but-2-ene
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