AMINO ACIDS Amino acids Amino acids are the compounds which contain both an amino (-NH2) and carboxylic (COOH) groups in the same molecule. The carbon atom next to the carboxylic group is regarded as α carbon atom. In the amino acids obtained from proteins the amino group is attached to this carbon atom. Hence they are called α –amino acids. The general structure of α –amino acids is shown below
Alpha - carbon atom
H2N
H
O
C
C
R
carboxylic group OH
side chain
amino group About 20 amino acids are obtained by the hydrolysis of proteins. They differ only in the nature of the side chain (R) its formula is H2N – CH2 – COOH R represents the side chain. Classification of amino acids Amino acids are broadly classified as a). Aliphatic amino acids Ex. H2NCH2COOH i) Glycine
NH2 ii) Alanine
CH3
iii) Serine
H2N
CH
COOH
CH
COOH
b). Aromatic amino acids i) Tyrosine
NH2
OH
CH2
HO
CH2
CHCOOH
NH2 ii) Phenylalanine
CH2
CHCOOH
c). Heterocyclic amino acids
i)
Proline
H2C
CH2
H2C
CHCOOH NH
ii)
CH2
Tryptophan NH
CHCOOH NH2
Aliphatic amino acids may be further classified into following classes depending upon their nature. i). Neutral amino acids : They have one amino group and one carboxyl group in their structure Ex.
1) Glycine
H2NCH2COOH
2) Alanine
H2NCHCOOH CH3
ii). Acidic amino acids : They possess one amino group and two carboxylic acids groups in their structure. Thus they are acidic in nature. Ex. Aspartic acid
Aspartic acid CHCOOH
H2N
CH2COOH iii). Basic amino acids : They contain two amino groups and one carboxylic group. Thy are basic in nature Ex. Lysine
Lysine H2N
CH (CH2)4
COOH NH2
iv). Hydroxy amino acids : They contain one hydroxyl group also Ex. Serine Serine H2N
CHCOOH CH2
OH
v). Sulphur containing amino acids: They contain a sulphur atom Ex. Cysteine
Cysteine H2N
CHCOOH CH2SH
Methods of synthesis of amino acids Proteins on hydrolysis give mixture of amino acids. Various methods are used to isolate each amino acid from the mixture. However several laboratory methods are also developed to synthesis specific amino acids. 1. From α – halogenated acids α – amino acids may be prepared by the action of ammonia on α – chloro or acids
α – bromo
0
ClCHCOOH
+
R alpha - chloro acid
2 NH3
50 c
H2NCHCOOH R amino acid
+
NH4Cl
2. Streckar synthesis An aldehyde is treated with HCN to from corresponding cyanohydrin. This is treated with conc. Ammonia and the resulting α – amino nitrile is hydrolysed with acid to give corresponding amino acids.
H
H CH3
C
+
O
NH3 CH3
HCN
OH
C
-HO 2
acetaldehyde
CN
H
H hydrolysis
CH3
C
NH2
CH3
C
NH2
CN
COOH Amino nitrile can also be prepared in one step by the action of aqueous solution of KCN and amino nitrite alanine NH4Cl mixture upon aldehydes and ketones. 3. Gabriel synthesis An ester of α – halogenated acid is treated with potassium phthalimide and the resulting product is then hydrolysed to give amino acids.
O C NK
+
- KCl
ClCH2COOC2H5
C O O +
C H NHCH2COOC2H5
H2O H2NCH2COOH glycine
C O
+
COOH COOH phtalic acid
+
C2H5OH ethly alcohol
Properties of amino acids Amino acids are generally colourless crystalline solids, soluble in water but insoluble in ether and other organic solvents. They melt at very high temperature but may decompose in melting. All α – amino acids possess asymmetric carbon atom (ex. cept glycine) and thus they are optically active. Zwitter or dipolar nature of amino acids Zwitter ion or a dipolar ion had both positive and negative charges in the same molecule. Since amino acids contain both acidic group (-COOH) and a basic group (-NH2), in aqueous solution, -COOH group donates a proton and the –NH2 group of the same molecule accepts it. This results in the formation of dipolar ion. In neutral solution, the following equilibrium exists.
+ NH2
NH3 -
H
C
COOH
H
R
C
COO
R
Zwitterion The dipolar ion is charged but electrically neutral. Since amino acids contain amino and carboxyl groups they possess the properties of both a base and an acid. Therefore they are amphoteric in nature. Isoelectric point When an ionized amino acid is placed in electric field. It will actually migrate towards cathode or anode depending upon the pH of the medium. In acidic solutions amino acids exist as cation and migrates towards cathode. +
H3NCHCOO
+
+
+
H
H3N
R
CHCOOH R
Zwitter ion cation In basic medium, the amino acid exists as a negative anion and thus migrates towards anode. +
+
+
H3NCHCOO + OH H3N CHCOO + H2O Each amino acid has definite pH at which it does not migrate towards either cathode or anode but exist as a neutral molecule. This pH is called the isoelectric point of the amino acid R the amino acid has its lowest solubility. It is also observed R that all amino acids do and at this pH, Zwitter ion cation
not have the same isoelectric point. Neutral amino acids have isoelectric point ranging from pH 5.5 to 6.3. isoelectric points for acidic amino acids are at lower pH and range about 2.7 to 3.2. For basic amino acids the isoelectric point lies between 7.6 and 10.8 pH. Peptide bond formation The acidic carboxyl group of an amino acid can react with the basic amino group of another amino acid molecule with the elimination of a water molecule. O
This results in the formation of
bond . This known as a peptide linkage C
N
or peptide bond or amide linkage. H
O
H H2N
H
C
C
OH
+
H HN
R
H2N
- H2O C
COOH
R
H
O
C
C
H
R
N
C
H
R
COOH
peptide linkage When two molecules of amino acids are linked by peptide linkage, a dipeptide is formed. This dipeptide can react with next amino acid molecule giving rise to a tripeptide. When several amino acid molecules are linked by peptide linkages a polypeptide is formed. Polypeptide may be partially represented as follows.
H
O
N
R C
C N
C H R
H
H
This peptide bond is the backbone of the polypeptide chain. The amide of peptide group is flat and the carbonyl and amino group lie in one plane having H (of NH) and O (of CO) trans with respect to each other. X – rays studies indicate that the C – N bond length of peptide – CONH is 1.32 A which is shorter than the usual 1.47 A showing slight double bond character. This double bond nature of C – N bond may be due to resonance and because of this there is restricted rotation about C – N bond giving rise to cis – trans isomerism.
H 00
C
C
C
C
NH
N
C
C +
O
O -
Synthesis of polypeptides Bergmann method (Carbobenzoxy method) Dipeptide can be synthesized by the condensation of amino group of noe amino acid with the carboxylic group of the other through the formation of peptide bond. It can be synthesised as shown below.
H H2N
H
O
C
C
OH
+
H HN
H
COOH
CH3
glycine
H2N
C
-HO 2
alanine
H
O
C
C
H
H N
C
H
CH3
COOH
This is only one possibility of such combination. There is a possibility of getting the mixture of dipeptides (such as AlaGly, AlaAla, Gly – Gly) rather than a single desired peptide. In order to condense the two amino acids in the desired manner, the one first reacting amino group of the amino acid should be protected and then other one should have the carboxyl group protected so that when they react there is only one possibility of such combination. Then proper reagent should be used to remove the protective groups after formation of the dipeptide bond.
A number of protecting agents have been recommended. A good protecting agent is characterized by the fact that it can react with amino or carboxylic group easily and can be removed after the synthesis of the dipeptide. Without affecting the peptide bond. Bergmann and zervas in 1932 used a carbobenzoxy derivatives as protecting agent for amino groups. The following sequences illustrates the synthesis of dipeptide using carbobenzoxy chloride for protecting amino group. 1. The amino group of one amino acid (say Glycine) which is not involved in the formation of peptide bond is protected by first reacting the compound with carbobenzoxy chloride to get carbobenzoxy amino acid. Carbobenzoxy is also known as benzylchloro formate and it is prepared by the action of carbonyl chloride on benzyl alcohol.
C6H5CH2OH
+
COCl2
C6H5CH2OCOCl
+
HCl
carbobenzoxy chloride 2. The reagent is condensed with Amino acid to get carbobenzoxy amino acid. It is then treated with PCL5 or SOCl2 to convert into its corresponding acid chloride.
C6H5CH2OCOCl
+
C6H5CH2OCONHCH2COOH
H2NCH2COOH
carbobenzoxy amino acid
glycine PCl5
C6H5CH2OCONHCH2COCl
C6H5CH2OCONHCH2COOH
3. The product formed is treated with another amino acid (say alamine) to form carbobenzoxy peptide.
C6H5CH2OCONHCH2C OCl
+
- HCl H NHCHCOOH CH3 alanine
C6H5CH2OCONHCH2 CONH CHCOOH CH3
4. The carbobenzoxy group is removed by reduction
H2
Pd
C6H5CH2OCONHCH2CONHCHCOOH CH3
H2NCH2CONHCHCOOH
+
C6H5CH3
+
CO2
CH3 The above steps are repeated to synthesis higher peptides. glycylalanine (dipeptide) If the amino acid contains sulphur, then palladium can not be used for catalytic reduction, since sulphur poisons the catalyst. In such cases, the blocking group carbobenzoxy group, can be removed by means of phosponium iodide or sodium in liquid ammonia.
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