Proteins are present in all living cells and fulfil numerous functions in the chemistry of life. The — Analytical Chemistry Chemistry Question
Structure of Peptides
Proteins are present in all living cells and fulfil numerous functions in the chemistry of life. They are composed of α-aminocarboxylic acids. Peptides are ‘small’ proteins with a relative small number of amino acids. The peptide bond is a amide bond formed by interaction of the amino group of an amino acid with the carboxylic acid group of its neighbour.
[VISUAL]
Which dipeptides can be derived from phenylalanine F and alanine A? Give the structures.
Model Answer
[VISUAL]
In the structure analysis of peptides the N-terminal and C-terminal residues play an important role. Sanger’s method for the determination of the N-terminal residue (this is an amino acid unit in the peptide with the free NH2 group) involves treatment with 2,4.dinitrofluorobenzene under mid alkaline conditions, followed by a total acid hydrolysis of all peptide linkages. The N-terminal amino acid then has a yellow tag which can readily be spotted in the paper chromatographic analysis. Sanger received the Nobel prize in 1958 and 1980.
Which reaction takes place with Sanger’s reagent (for reason of simplicity write the N-terminal side of the peptide as H2NR). Give the equation.
Model Answer
This is a nucleophilic aromatic substitution.
[VISUAL]
The C-terminal residue which contains the free COOH group in the peptide, is performed by a selective enzymatic hydrolysis of the C-terminal amino acid unit by carboxypeptidase (from the pancreas). From the tetrapeptide composed of the amino acids F, A, glycine G and Leucine L, the C-terminal residue was identified as F. Sanger’s method indicated that the N-terminal unit is G.
Deduce the possible structure(s) of this tetrapeptide. Give the structure(s).
Model Answer
[VISUAL]
Only the positions of G and F are determined, the other two are in the middle, but no information is provided if this is AL or LA.