Question

At $pH = 4$, glycine exists as:

• A. $H_{3}N^{+}-CH_{2}-COO^{-}$
• B. $H_{3}N^{+}-CH_{2}-COOH$
• C. $H_{2}N-CH_{2}-COOH$
• D. $H_{2}N-CH_{2}-COO^{-}$

Hint:

Low pH (Acidic) $\rightarrow$ Cation form; High pH (Basic) $\rightarrow$ Anion form.

Answer 1

The Correct Option is B

Step 1: Concept
Amino acids exist as Zwitterions at their isoelectric point, but change form in acidic or basic media.
Step 2: Analysis
At $pH = 4$ (acidic medium), the carboxylate group ($-COO^{-}$) of the Zwitterion accepts a proton.
Step 3: Conclusion
The structure becomes a cation: $H_{3}N^{+}-CH_{2}-COOH$.
Final Answer: (B)