Amino acids contain amino (–NH₂) and carboxyl (–COOH) functional groups. Depending upon the relative position of amino group with respect to carboxyl group, the amino acids can be classified as α, β, γ, δ and so on. Only α-amino acids are obtained on hydrolysis of proteins. They may contain other functional groups also. All α-amino acids have trivial names, which usually reflect the property of that compound or its source. Glycine is so named since it has sweet taste (in Greek glykos means sweet) and tyrosine was first obtained from cheese (in Greek, tyros means cheese.) Amino acids are generally represented by a three letter symbol, sometimes one letter symbol is also used. Amino acids are classified as acidic, basic or neutral depending upon the relative number of amino and carboxyl groups in their molecule. Equal number of amino and carboxyl groups makes it neutral; more number of amino than carboxyl groups makes it basic and more carboxyl groups as compared to amino groups makes it acidic. The amino acids, which can be synthesized in the body, are known as nonessential amino acids. On the other hand, those which cannot be synthesized in the body and must be obtained through diet, are known as essential amino acids.

Which of the following reagents is used to determine the C-terminal end in a polypeptide?

Hydrazine 

The correct answer is option 1. Hydrazine.

Out of the options you provided, the reagent used to determine the C-terminal end in a polypeptide is: \1. Hydrazine \((N_2H_4)\)

Hydrazine:

Reacts with all peptide bonds except the C-terminal one, cleaving the polypeptide chain and generating aminoacyl hydrazides for all internal residues. The C-terminal amino acid remains unmodified due to the absence of a peptide bond after it. The aminoacyl hydrazides can be analyzed using chromatography or mass spectrometry to identify the C-terminal amino acid.

2,4-Dinitrophenylhydrazine (DNPH):

This is a reagent used for N-terminal sequencing, not C-terminal analysis. It reacts specifically with the N-terminal amino acid, forming a stable dinitrophenylhydrazone derivative.

2,4-Dinitrofluorobenzene (Sanger's reagent):

Similar to DNPH, this reagent also targets the N-terminal amino acid for Edman degradation sequencing, not C-terminal identification.

3,5-Difluoronitrobenzene:

This reagent is not commonly used for protein sequencing, and its specific application in C-terminal analysis is not well-established.

Therefore, while alternative methods like carboxypeptidases (enzymes) exist for C-terminal analysis, hydrazine remains the most common and straightforward chemical reagent for identifying the C-terminal amino acid in a polypeptide chain.