Which of the following statements are correct?

A. All amino acids contains amino \(-NH_2\) and \(-COOH\) functional groups.

B. All amino acids are optically active.

C. All amino acids are solid at room temperature.

D. In aqueous solution amino acids exists as Zwitter ions.

Choose the correct answer from the options given below:

A, C, D only

The correct answer is option 4. A, C, D only.

Let us break down each statement and explore the reasoning behind why it is correct or incorrect.

Statement A: "All amino acids contain amino \(-NH_2\) and \(-COOH\) functional groups."

Amino acids are organic molecules that serve as the building blocks of proteins. They all have a central carbon atom, known as the \(\alpha \)-carbon \((C_\alpha)\), to which four different groups are attached:

Amino group \((-NH_2)\): This is the basic group.

Carboxyl group \((-COOH)\): This is the acidic group.

Side chain (R group): This varies among different amino acids and determines their specific properties.

The presence of both an amino group and a carboxyl group is a defining feature of all **standard alpha amino acids**.

Conclusion: The statement is true. This statement is correct because every standard amino acid contains both the amino \((-NH_2)\) and carboxyl \((-COOH)\) functional groups.

Statement B: "All amino acids are optically active."

Optical activity refers to the ability of a compound to rotate the plane of polarized light, which is a characteristic of chiral molecules. A molecule is chiral if it has a carbon atom attached to four different groups, resulting in non-superimposable mirror images (enantiomers). Most amino acids have a chiral center at the alpha carbon \((C_\alpha)\), which makes them optically active. However, glycine is an exception because its side chain (R group) is a hydrogen atom \((-H)\). This means that glycine has two identical hydrogen atoms attached to the alpha carbon, making it achiral and, therefore, not optically active.

Conclusion: The statement is false. This statement is incorrect because glycine, an amino acid, is not optically active.

Statement C: "All amino acids are solid at room temperature."

Amino acids are generally crystalline solids at room temperature. They have relatively high melting points because of the strong ionic interactions between the zwitterionic forms in the solid state.  In the solid state, amino acids exist as zwitterions, which are dipolar ions where the amino group is protonated \((\text{-NH}_3^+)\) and the carboxyl group is deprotonated \((\text{-COO}^-)\). This ionic nature contributes to their solid, crystalline form at room temperature.

Conclusion: The statement is true. This statement is correct as all standard amino acids are indeed solids at room temperature.

Statement D: "In aqueous solution, amino acids exist as zwitterions."

Zwitter ions are molecules that contain both positive and negative charges, but the overall molecule is neutral. In aqueous solutions, amino acids typically exist as zwitterions. At physiological pH (around 7.4), the amino group \((-NH_2)\) of an amino acid tends to accept a proton and becomes \((\text{-NH}_3^+)\), and the carboxyl group \((-COOH)\) tends to lose a proton, becoming \((\text{-COO}^-)\). This results in a molecule that has both a positive and a negative charge, which is the zwitterionic form. The zwitterionic form of amino acids is significant because it influences their solubility, reactivity, and interactions with other molecules.

Conclusion: The statement is true. This statement is correct because amino acids do exist as zwitterions in aqueous solutions.

Thus the correct answer is option 4. A, C, D only.