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Amino acids are organic compounds containing carboxylic acid group and amino group. They have specific pKa values depending upon the number of carboxylic acid and amino groups. Similarly their pKa value is also determined by other groups like imidazole group in the side chain. They are classified as alpha amino acids if they contain the both carboxylic acid amino group in the same carbon atom. Similarly they are classified as essential and non essential amino acid.

Essential amino acids cannot be synthesized in our body and has to be supplied through our diet regularly. Phenylalanine is an example for essential amino acid. Non essential amino acids are synthesized within our body. Glycine is an example for non essential amino acid.
In the same way they are classified as hydrophobic and hydrophillic amino acid. Hydrophilic amino acids will dissolve in water. They are building blocks of proteins and the study of metabolism of synthesis and decomposition of protein is important to study.


What is Phenylalanine?

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Phenylalanine is an essential amino acid with formula C6H5CH2CH(NH2)COOH. It is the derivative of another amino acid, alanine, with one benzene ring substituted in γ position. It is highly insoluble in water and the benzene ring contains free Π electrons they are stacked one over another and in most of the cases the amino acid is buried within the protein. The phenylalanine structure is given as

Phenylalanine Structure

The structure contains amino group and carboxylic group attached with the same carbon atom. Hence it is alpha amino acid. It contains one asymmetric carbon atom and two optical isomers d-phenylalanine and l-phenylalanine. 

L-phenylalanine is biologically important optically active isomer and is naturally available. D-phenylalanine and dl-phenylalanine are synthesized in the laboratory.  Due to the presence of long benzene ring it is classified as  hydrophobic amino acid.


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L-phenylalanine is optically active isomer and is naturally found in mammals breast milk. It is having analgesic and antidepressant properties and is widely used in medicines. It is the precursor in the synthesis of important enzymes like dopamine, adrenaline, noradrenaline and skin pigment melamine. The structure of l-phenylalanine


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d-phenylalanine is enatiomer of l-phenylalanine and the structure of d-phenylalanine is given here.
D-Phenylalanine Structure

The role of d-phenylalanine is not significant and is prepared in the laboratory either as simple enatiomer or as a racemic form (a mixture of d and l isomers). D-phenylalanine is used as analgesic as it inhibit the enzymes that breakdown the endorphin.

Another form called as dl-phenylalanine is equal molar mixture of d and l phenylalanine. It is extensively used for its analgesic and antidepressant activities.  The analgesic action of  dl-phenylalanine is due to the presence of d-phenylalanine and the antidepressant activity is explained by the ability of l-phenylalanine to produce norepinephrine and dopamine. Elevated levels of norepinephrine and dopamine will have antidepressant effects in the body. 

Structure of Phenylalanine

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The structure of phenylalanine contains a carboxylic acid group and amino group. Similarly it contains a hydrophobic benzene ring at the γ position. Naturally occurring phenylalanine is l isomer and the another form d isomer is not significant. As the structure of phenylalanine contains hydrophobic benzene ring it is less solvated and hence the amino group is less hydrated.

Synthesis of Phenylalanine

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Phenylalanine is synthesized by the plants and cannot be synthesized in the animals body. Hence it should be given through diet regularly. The natural sources containing the phenylalanine are meat, fish and milk. But commercially it is prepared by action of bacteria E.Coli which naturally produces phenylalanine.

The stereoselectivity of producing L-phenylalanine alone can be achieved by genetically modifying the E.Coli to get the desired product.

Effects of Phenylalanine

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  1. Phenylalanine is used to cure depression, attention deficit hyperactivity disorder, Parkinson's decease and the skin decease called as vitiligo.
  2. It can be applied over the skin to get ride of skin disorders. In our body phenylalanine is converted to tyrosin which in turn is converted to L-DOPA.
  3. L-DOPA is further converted to dopamine, noradrenaline and finally to adrenalilne.
  4. It is also interfering the production of seretonin. There is a genetic disorder associated with phenylalanine called as phenylketonurea(PKU).
  5. It is the inability of the body to mobilize phenylalanine. The persons suffering from PKU cannot catabolise phenylalanine and hence they have to monitor the intake of phenylalanine.

Phenylalanine and Aspartame

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  1. A popularly known artificial sweetener, aspartame which is sold in trade names like Equal etc contains phenylalanine. It is the methyl ester of phenylalanine dipeptide and aspartic acid.
  2. On strong acidic and basic conditions it decomposes to give methanol which is strongly poisonous.
  3. Further hydrolysis and degradation of aspartame gives phenylalanine which is harmful for persons with PKU gene and may lead to death.
  4. Although the uses of aspartame is still in dispute it is safer to use it in cool drinks with pH around 4.5.


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  • Phenylketonuria is a metabolic disorder in which there is a lack of an enzyme for the metabolism of phenylalanine.
  • Out food contains many protein sources like red meat, milk which contains phenylalanine.
  • In addition to this the artificial sweetener like aspartame gives phenylalanine due to degradation. Hence it accumulates in the blood in the form of an acid called as phenyl pyruvic acid.
  • High level of phenyl pyruvic acid will affect the central nervous system leading to mental retardation and some time lead to severe brain damage.
  • Phenylketonuria can be found by some of the special characteristics like loss of pigmentation in hair, skin and mousy or musty odor and seizure of muscles in some cases.
  • The only solution for this disorder to have a control of diet with proper low protein levels. They should avoid the food items marked as Phenylketonurics contains phenylalanine.
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