Amino Acids Amino Acids N N H H 3 + + C C O O 2 – – an an -amino acid that is an -amino acid that is an intermediate in the biosynthes intermediate in the biosynthes of ethylene of ethylene + + H H 3 N N CH CH 2 CH CH 2 C C O O 2 – – a a -amino acid that is one of -amino acid that is one of the structural units present i the structural units present i coenzyme A coenzyme A + + H H 3 N N CH CH 2 CH CH 2 CH CH 2 C C O O 2 – – a a -amino acid involved in -amino acid involved in the transmission of nerve the transmission of nerve impulses impulses They are classified as They are classified as , , , , , , etc etc . . amino acids according the carbon amino acids according the carbon that bears the nitrogen. that bears the nitrogen.
Transcript
Amino AcidsAmino Acids NNHH33
++
CCOO22––
an an -amino acid that is an-amino acid that is anintermediate in the biosynthesisintermediate in the biosynthesisof ethyleneof ethylene
++HH33NNCHCH22CHCH22CCOO22
––a a -amino acid that is one of-amino acid that is one ofthe structural units present inthe structural units present incoenzyme Acoenzyme A
++HH33NNCHCH22CHCH22CHCH22CCOO22
–– a a -amino acid involved in-amino acid involved inthe transmission of nervethe transmission of nerveimpulsesimpulses
They are classified as They are classified as , , , , , , etcetc. amino acids . amino acids according the carbon that bears the nitrogen.according the carbon that bears the nitrogen.
Table 27.1Table 27.1
CC CC
OO
OO––
RRRR
HH
HH33NN++
The major differences among the side chains:The major differences among the side chains:
Size and shapeSize and shapeElectronic characteristicsElectronic characteristics
Glycine is achiral. All of the other amino acids in Glycine is achiral. All of the other amino acids in proteins have the proteins have the LL-configuration at their -configuration at their carbon.carbon.
Table 27.1Table 27.1
General categories of General categories of -amino acids-amino acids
nonpolar side chainsnonpolar side chainspolar but nonionized side chainspolar but nonionized side chainsacidic side chainsacidic side chainsbasic side chainsbasic side chains
Properties of GlycineProperties of Glycine
The properties of glycine:The properties of glycine:
high melting point:high melting point: (when heated to 233°C (when heated to 233°C it decomposes before it melts)it decomposes before it melts)solubility:solubility: soluble in water; not soluble in soluble in water; not soluble in nonpolar solventnonpolar solvent
OO
OHOHHH22NCHNCH22CC••••
••••
••••
•••• ••••
––••••
OO
OOHH33NCHNCH22CC ••••
••••
•••• ••••++
more consistent with thismore consistent with this than thisthan this
called a called a zwitterionzwitterion or or dipolar iondipolar ion
Acid-Base Properties of GlycineAcid-Base Properties of Glycine
The zwitterionic structure of glycine also follows The zwitterionic structure of glycine also follows from considering its acid-base properties.from considering its acid-base properties.
A good way to think about this is to start with the A good way to think about this is to start with the structure of glycine in strongly acidic solution, structure of glycine in strongly acidic solution, say pH = 1.say pH = 1.
At pH = 1, glycine exists in its protonated form At pH = 1, glycine exists in its protonated form (a monocation).(a monocation).
OO
OHOHHH33NCHNCH22CC++
••••
••••
•••• ••••
Acid-Base Properties of GlycineAcid-Base Properties of Glycine
Therefore, the more stable neutral form of Therefore, the more stable neutral form of glycine is the zwitterion.glycine is the zwitterion.
The measured pThe measured pKKaa of glycine is 2.34. of glycine is 2.34.
Glycine is stronger than a typical carboxylic acid Glycine is stronger than a typical carboxylic acid because the positively charged N acts as an because the positively charged N acts as an electron-withdrawing, acid-strengthening electron-withdrawing, acid-strengthening substituent on the substituent on the carbon. carbon.
Acid-Base Properties of GlycineAcid-Base Properties of Glycine
Acid-Base Properties of GlycineAcid-Base Properties of Glycine
––••••
OO
OOHH33NCHNCH22CC ••••
••••
•••• ••••++
The pThe pKKaa for removal of this proton is 9.60. for removal of this proton is 9.60.
This value is about the same as that for NHThis value is about the same as that for NH44++
(9.3).(9.3).
HOHO––––••••
OO
OOHH22NCHNCH22CC ••••
••••
•••• ••••
••••
A proton attached to N in the zwitterionic form of A proton attached to N in the zwitterionic form of nitrogen can be removed as the pH is increased nitrogen can be removed as the pH is increased further. further.
Isoelectric Point pIsoelectric Point pII
––••••
OO
OOHH33NCHNCH22CC ••••
••••
•••• ••••++
––••••
OO
OOHH22NCHNCH22CC ••••
••••
•••• ••••
••••
OO
OHOHHH33NCHNCH22CC++
••••
••••
•••• ••••
ppKKaa = 2.34 = 2.34
ppKKaa = 9.60 = 9.60
The pH at which the The pH at which the concentration of the concentration of the zwitterion is a maximum is zwitterion is a maximum is called the called the isoelectric pointisoelectric point. . Its numerical value is the Its numerical value is the average of the two paverage of the two pKKaas.s.
The pThe pII of glycine is 5.97. of glycine is 5.97.
Can be used for Can be used for identification or purificationidentification or purification
Titration Curve AlanineTitration Curve Alanine
Table 27.3Table 27.3 Amino Acids with Acidic Side ChainsAmino Acids with Acidic Side Chains
Aspartic acidAspartic acidppKKa1a1 = = 1.881.88
ppKKa2a2 == 3.653.65
ppKKa3a3 == 9.60 9.60
ppI I == 2.772.77
HH33NN CC CC
OO
OO––
HH++
OCCHOCCH22
OO
––
For amino acids with acidic side chains, pI is the For amino acids with acidic side chains, pI is the average of paverage of pKKa1a1 and p and pKKa2a2..
Table 27.3Table 27.3 Amino Acids with Basic Side ChainsAmino Acids with Basic Side Chains
LysineLysine
ppKKa1a1 = = 2.182.18
ppKKa2a2 == 8.958.95
ppKKa3a3 == 10.5310.53
ppI I == 9.749.74
HH33NN CC CC
OO
OO––
HH++
CHCH22CHCH22CHCH22CHCH22NHNH33
++
For amino acids with basic side chains, pI is the For amino acids with basic side chains, pI is the average of paverage of pKKa2a2 and p and pKKa3a3..
Synthesis of Amino Acids Synthesis of Amino Acids From From -Halo Carboxylic Acids-Halo Carboxylic Acids
CHCH33CHCOHCHCOH
BrBr
OO
22NNHH33++HH22OO
CHCH33CHCOCHCO
NHNH33
OO
++
––
(65-70%)(65-70%)
++ NHNH44BrBr
Strecker SynthesisStrecker Synthesis
NNHH44ClCl
NaNaCNCNCHCH33CHCH
OO
CHCH33CHCHCC
NNHH22
NN
CHCH33CHCHCCOO
NHNH33
OO
++
–– (52-60%)(52-60%)
1. H1. H22O, HCl, heatO, HCl, heat
2. HO2. HO––
Using Diethyl AcetamidomalonateUsing Diethyl Acetamidomalonate
CCCC
CCOCHOCH22CHCH33
HH
OO OO
CHCH33CHCH22OO
CHCH33CCNNHH
OO
Can be used like as diethyl malonate.Can be used like as diethyl malonate.
ExampleExample
1. NaOCH1. NaOCH22CHCH33
2. 2. CC66HH55CHCH22ClCl
OO OO
CHCH33CHCH22OCCCOCHOCCCOCH22CHCH33
HHCHCH33CCNNHH
OO
OO OO
CHCH33CHCH22OCCCOCHOCCCOCH22CHCH33
CHCH22CC66HH55CHCH33CCNNHH
OO
(90%)(90%)
ExampleExample
OO OO
CHCH33CHCH22OCCCOCHOCCCOCH22CHCH33
CHCH22CC66HH55CHCH33CCNNHH
OO
HBr, HHBr, H22O, heatO, heat
OO OO
HOCCCOHHOCCCOH
CHCH22CC66HH55HH33NN++
OO
HCCOHHCCOH
CHCH22CC66HH55HH33NN++(65%)(65%)
––COCO22
Acylation of Amino GroupAcylation of Amino Group
The amino nitrogen of an amino acid can be The amino nitrogen of an amino acid can be converted to an amide with the customary converted to an amide with the customary acylating agents.acylating agents.
OO
HH33NNCHCH22COCO––++++ CHCH33COCCHCOCCH33
OO OO
CHCH33CCNNHCHHCH22COHCOH
OO OO
(89-92%)(89-92%)
Esterification of Carboxyl GroupEsterification of Carboxyl Group
The carboxyl group of an amino acid can be The carboxyl group of an amino acid can be converted to an ester. The following illustrates converted to an ester. The following illustrates Fischer esterification of alanine. Fischer esterification of alanine.
++ CHCH33CHCH22OHOH
HClHCl
OO
HH33NNCHCOCHCO––++
CHCH33
(90-95%)(90-95%)
OO
HH33NNCHCOCHCHCOCH22CHCH33
++
CHCH33
––ClCl
Ninhydrin TestNinhydrin Test
Amino acids are detected by the formation of a purple Amino acids are detected by the formation of a purple color on treatment with color on treatment with ninhydrinninhydrin..
OHOH
OO
OO
OHOH++
OO
HH33NNCHCOCHCO––++
RR
OOOO
OO
NN
OO
––
OO
RCHRCH ++ COCO22 ++ HH22OO ++
DecarboxylationDecarboxylation
Decarboxylation is a common reaction of Decarboxylation is a common reaction of --amino acids. An example is the conversion of amino acids. An example is the conversion of LL-histidine to histamine. Antihistamines act by -histidine to histamine. Antihistamines act by blocking the action of histamine.blocking the action of histamine.
LL-DOPA for Parkinsons Disease-DOPA for Parkinsons Disease
OOHH
COCO22––
HHHH
HHHH33NN
++
LL-Tyrosine-Tyrosine
OOHH
HHHH
HHHH22NN
HHOO
DopamineDopamine
NeurotransmittersNeurotransmitters
OOHH
HH
HHHH
OHOHHH22NN
HHOO
NorepinephrineNorepinephrine
OOHH
HHHH
OHOH
HHOO
EpinephrineEpinephrine
CHCH33NHNH
HH
PeptidesPeptides
Peptides are compounds in which an amide Peptides are compounds in which an amide bond links the amino group of one bond links the amino group of one -amino acid -amino acid and the carboxyl group of another.and the carboxyl group of another.
An amide bond of this type is often referred to An amide bond of this type is often referred to as a peptide bond.as a peptide bond.
