Protein are nitrogeneous organic compound having very high molecular mass. They are polyamide formed from `alpha`-amino acid. The bond formed between two amino acid is called peptide bond `(-underset(O)underset(||)C-NH-)`. The product obtained by this peptide bond formation are called peptide and they may be divided as di, tri, tetra, penta peptide.
Consider following statements concerning protein.
1. All amino acids which are constituents of proteins are `alpha`-amino acid.
2. `alpha`-amino acids are all optically active and have L-configuration.
3. An especially favourable conformation for the peptide linkage in protein is the `alpha`-helix arrangement .
4. `alpha`-amino acids are connected by ester linkage.
Which of the following statement are correct?

Options

A. 1 and 3
B. 1 and 2
C. 2 and 3
D. 2, 3 and 4

Explanation

To solve the question regarding the statements concerning proteins, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement 1: All amino acids which are constituents of proteins are alpha-amino acids.**
- **Analysis**: This statement is correct. Proteins are indeed composed of alpha-amino acids, which are characterized by having the amino group (-NH2) attached to the carbon atom adjacent to the carboxyl group (-COOH).
- **Conclusion**: **Correct**

2. **Statement 2: Alpha-amino acids are all optically active and have L-configuration.**
- **Analysis**: While it is true that alpha-amino acids are optically active due to the presence of a chiral center (the alpha carbon), they are not all exclusively in the L-configuration. There are D- and L-forms of amino acids, but most naturally occurring amino acids are in the L-configuration. Therefore, this statement is misleading.
- **Conclusion**: **Incorrect**

3. **Statement 3: An especially favorable conformation for the peptide linkage in protein is the alpha-helix arrangement.**
- **Analysis**: This statement is correct. The alpha-helix is a common structural motif in proteins that is stabilized by hydrogen bonds between the backbone amide groups. This conformation is indeed favorable for peptide linkages.
- **Conclusion**: **Correct**

4. **Statement 4: Alpha-amino acids are connected by ester linkage.**
- **Analysis**: This statement is incorrect. Alpha-amino acids are connected by peptide bonds, not ester linkages. Peptide bonds are formed between the carboxyl group of one amino acid and the amino group of another, resulting in the release of a water molecule.
- **Conclusion**: **Incorrect**

### Final Conclusion:
The correct statements are:
- Statement 1: Correct
- Statement 2: Incorrect
- Statement 3: Correct
- Statement 4: Incorrect

Thus, the correct statements concerning proteins are **1 and 3**.