Neutral amino acids lack ionizable side chains

Steps:

• Amino acids in solution at physiological pH form zwitterions with net zero charge from alpha groups; overall charge depends on side chain (R group).
• Examine R groups: alanine (CH3) and glycine (H) are nonpolar and non-ionizable.
• Aspartate's R group (CH2COO-) deprotonates to negative charge.
• Lysine's R group (CH2(CH2)3NH3+) protonates to positive charge.
• Thus, only alanine and glycine have no net charge.

Why B is correct:

• Alanine and glycine have neutral, non-ionizable side chains, so their zwitterionic form remains uncharged per standard amino acid ionization definitions.

Why the others are wrong:

• A: Aspartate's acidic side chain carries a negative charge.
• C: Both aspartate (negative) and lysine (positive) have charged side chains.
• D: Lysine's basic side chain carries a positive charge.

Final answer: B