Which of the following is not an important secondary structural feature in large peptides and proteins ?

Options

A. the `alpha-` helix
B. the `beta-` tum
C. chair conformations
D. the `beta-` pleated sheet

Explanation

To determine which of the following is not an important secondary structural feature in large peptides and proteins, we first need to understand the common secondary structures found in proteins.

### Step-by-Step Solution:

1. **Identify Secondary Structures**: In proteins, the main secondary structural features are:
- **Alpha Helix**: This structure is formed by hydrogen bonds between the backbone NH group of one amino acid and the C=O group of another amino acid that is typically 3-4 residues earlier in the sequence.
- **Beta Pleated Sheet**: This structure is formed by hydrogen bonds between beta strands, which are segments of the polypeptide chain that are aligned next to each other. These strands can be parallel or antiparallel.
- **Beta Turn**: This is a type of secondary structure that allows the polypeptide chain to reverse direction, often involving 4 amino acids and stabilized by hydrogen bonds.

2. **Evaluate the Options**: The question provides several options, and we need to identify which one is not a recognized secondary structural feature:
- **Alpha Helix**: Important secondary structure.
- **Beta Pleated Sheet**: Important secondary structure.
- **Beta Turn**: Important secondary structure.
- **Chair Conformation**: This refers to a specific conformation of cyclohexane and is not a secondary structure in proteins.

3. **Conclusion**: Based on the definitions and characteristics of the secondary structures, the option that is not an important secondary structural feature in large peptides and proteins is **Chair Conformation**.

### Final Answer:
The correct answer is **Chair Conformation** (Option C).

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