What does FDNB primarily react with in proteins?

FDNB, or 1-fluoro-2,4-dinitrobenzene, primarily reacts with the N-terminus of proteins. This compound is widely used in biochemistry for the identification and analysis of amino acid sequences. The reactivity of FDNB is due to its electrophilic nature, which allows it to react specifically with the amino group present at the N-terminus of proteins. When FDNB interacts with the N-terminal amino group, it forms a stable dinitrophenyl derivative, which can be cleaved under specific conditions to analyze the amino acid composition further.

This property makes FDNB a valuable tool for amino acid sequencing and determining the N-terminal residues of proteins. In contrast, while other regions of a protein, such as side chains of amino acids, may also undergo various types of reactions, FDNB's specificity for the N-terminus is what makes it particularly useful for these applications. The other options pertain to regions that do not specifically engage with FDNB in the same manner, highlighting the unique role of the N-terminus in these reactions.