Draw an α amino acid with an ethyl −−CH2CH3 side chain. Draw the molecule as it would appear at physiological pH.

To draw an α amino acid with an ethyl (−CH2CH3) side chain at physiological pH, we need to understand the structure of amino acids and how they behave in solution.

Structure of α Amino Acids

1. Amino acids consist of a central carbon (the α carbon) bonded to four different groups:
   • An amino group (-NH2)
   • A carboxyl group (-COOH)
   • A hydrogen atom (-H)
   • A side chain (R group), in this case, the ethyl group (−CH2CH3)

Drawing the Molecule

At physiological pH (around 7.4), amino acids exist predominantly in their zwitterionic form, where the carboxyl group (-COOH) loses a proton to become -COO⁻, and the amino group (-NH2) gains a proton to become -NH3⁺.

2. The structure of the α-amino acid with an ethyl side chain at physiological pH can be illustrated as follows:

        H
        |
    H2N - C - COO⁻
        |
      CH2
        |
      CH3

Summary of the Structure

• The central carbon (α carbon) is attached to an amino group (-NH3⁺), a carboxylate group (-COO⁻), a hydrogen atom, and an ethyl side chain (−CH2CH3).
• The ethyl group is present at the R position, while the zwitterionic form indicates the protonated amine and deprotonated carboxylate, which is typical for amino acids at physiological pH.

This representation captures the structure of the α amino acid with an ethyl side chain at physiological pH.