Which of the following statements are correct about `alpha`-amino acids.

Options

A. All the amino acids which constitute proteins have D-configuration.
B. Isoelectric point of glycine is 6.1.
C. Valine is an essential amino acid
D. In `alpha`-amino acids, the basic group is `(-COO^(Theta))` and acidic groups is `(-overset(o+)(N)H_(3))`.

Explanation

To determine which statements about alpha-amino acids are correct, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement A: All the amino acids which constitute proteins have D configuration.**
- **Analysis:** This statement is incorrect. Alpha-amino acids can exist in two configurations: D (dextrorotatory) and L (levorotatory). Since the alpha carbon in amino acids is chiral, both configurations are present.
- **Conclusion:** **Incorrect**

2. **Statement B: The isoelectric point of glycine is 6.1.**
- **Analysis:** The isoelectric point (pI) is the pH at which the amino acid exists as a neutral dipolar ion and does not migrate to either electrode in an electric field. For glycine, a neutral amino acid, the isoelectric point falls within the range of 5.5 to 6.3. Therefore, stating that the pI is 6.1 is correct.
- **Conclusion:** **Correct**

3. **Statement C: Valine is an essential amino acid.**
- **Analysis:** Valine is indeed classified as an essential amino acid because it cannot be synthesized by the human body and must be obtained through diet. This aligns with the definition of essential amino acids.
- **Conclusion:** **Correct**

4. **Statement D: The general structure of amino acids includes a zwitterion form.**
- **Analysis:** The general structure of amino acids includes an amino group (NH2), a carboxyl group (COOH), a hydrogen atom (H), and a variable R group. In solution, amino acids exist predominantly as zwitterions, where the amino group accepts a proton (H+) and becomes positively charged (NH3+), while the carboxyl group donates a proton and becomes negatively charged (COO-). This confirms that the zwitterion form is indeed a characteristic of amino acids.
- **Conclusion:** **Correct**

### Final Answer:
The correct statements about alpha-amino acids are **B, C, and D**.