Regarding secondary structure of a protein, correct statement(s) is/are
(A) peptide bonds possess regional planarity
(B) C = O and `-NH-` of different peptide chains are held by Van der Waal attarctions
(C ) closely packed arrangement so as to minimise repulsion between ''R'' groups.

Options

A. only C
B. only B
C. A and B only
D. A and C only

Answer

To determine the correct statements regarding the secondary structure of a protein, we will evaluate each statement one by one.

### Step-by-Step Solution:

1. **Evaluate Statement A: "Peptide bonds possess regional planarity."**
- Peptide bonds are indeed planar due to the partial double bond character between the carbonyl carbon and the nitrogen atom. This results in a rigid structure that restricts rotation, leading to a planar configuration around the peptide bond.
- **Conclusion:** Statement A is correct.

2. **Evaluate Statement B: "C=O and -NH of different peptide chains are held by Van der Waals attractions."**
- In the secondary structure of proteins, the C=O (carbonyl) group of one amino acid and the -NH (amide) group of another amino acid form hydrogen bonds, not Van der Waals attractions. Hydrogen bonds are crucial for stabilizing structures like alpha helices and beta sheets.
- **Conclusion:** Statement B is incorrect.

3. **Evaluate Statement C: "Closely packed arrangement so as to minimize repulsion between 'R' groups."**
- The secondary structure does involve a closely packed arrangement of amino acids, which helps to minimize the repulsion between the side chains (R groups) of the amino acids. This arrangement is also stabilized by hydrogen bonding.
- **Conclusion:** Statement C is correct.

### Final Conclusion:
- The correct statements regarding the secondary structure of a protein are **A and C**.