Which of the following represents a peptide chain ?

Options

A. `-NH-underset(O)underset("||")C-NH-underset(|)overset(|)C-NH-underset(O)underset("||")C-NH-`
B. `-NH-underset(O)underset("||")C-CH_(2)-CH_(2)CH_(2)-NH-CH_(2)CH_(2)-underset(O)underset("||")C-`
C. `-NH-CH_(2)-underset(O)underset("||")C-NH-CH_(2)-underset(O)underset("||")C-NH-CH_(2)-`
D. `-NH-CH_(2)CH_(2)-underset(O)underset("||")C-NH-CH_(2)-underset(O)underset("||")C-CH_(2)-`

Answer

To determine which of the following represents a peptide chain, we need to understand what a peptide chain is and how it is formed.

### Step-by-Step Solution:

1. **Definition of Peptide Chain**: A peptide chain is a sequence of amino acids linked together by peptide bonds. The peptide bond is formed between the carboxyl group (-COOH) of one amino acid and the amino group (-NH2) of another amino acid, resulting in a -C(=O)-N- linkage.

2. **Structure of Peptide Bond**: The peptide bond can be represented as -C(=O)-NH-, where:
- C represents the carbon atom of the carboxyl group.
- O represents the oxygen atom of the carbonyl group.
- N represents the nitrogen atom of the amino group.
- H represents the hydrogen atom attached to the nitrogen.

3. **Identifying Peptide Chains**: To identify a peptide chain in a list of options, look for the presence of the -C(=O)-N- linkage. This is the key characteristic of a peptide bond.

4. **Example of Peptide Chain**: Consider an example where two amino acids are linked:
- First amino acid: NH2-CHR-COOH
- Second amino acid: NH2-CHR'-COOH
- When they combine, the -OH from the carboxyl group of the first amino acid and an -H from the amino group of the second amino acid are removed, forming the peptide bond:
NH2-CHR-CO-NH-CHR'-COOH.

5. **Conclusion**: Based on the definition and structure of the peptide bond, we can conclude that the correct representation of a peptide chain will include the -C(=O)-N- linkage.

### Final Answer:
The option that represents a peptide chain is the one that includes the -C(=O)-N- structure, which is typically denoted as CONH.