Step 1: Concept Overview:
Amino acids are categorized by the chemical characteristics of their R-groups (side chains). Basic amino acids possess nitrogen-containing side chains capable of accepting a proton (H⁺) at physiological pH, acting as bases and carrying a positive charge.
Step 2: Detailed Analysis:
Let's examine the side chains of the provided amino acids:
(A) Alanine: Its side chain is a methyl group (-CH₃). It is nonpolar and neutral.
(B) Lysine: Its side chain includes a terminal amino group (-NH₂) at the end of a four-carbon chain (- (CH₂)₄-NH₂). This amino group readily accepts a proton to become -NH₃⁺, thus lysine is a basic amino acid.
(C) Threonine: Its side chain contains a hydroxyl group (-OH). It is polar but neutral (uncharged).
(D) Methionine: Its side chain contains a sulfur atom (thioether). It is nonpolar and neutral.
Step 3: Conclusion:
Of the options given, only lysine features a side chain with an amino group that gives the molecule basic properties.