N-Terminal Residues of Proteins

The FDNB reacts with the free amino groups of the protein under mild (slightly alkaline) conditions where the peptide bond is quite stable. On hydrolysis of the protein the JV-terminal residues are liberated in the form of DNP amino acids. These are bright yellow compounds that can be extracted with an organic solvent, fractionated chromatographically, and estimated colorimetrically. The accuracy varies somewhat with the particular amino acids involved, due to differences in the stability of the DNP derivatives. In most cases the N-terminal residues of proteins and peptides may be estimated to within 10-15%. 

Besides reacting with the a-amino groups of proteins, FDNB also reacts with the -amino groups of the lysine residues, and an estimation of the e-DNP-lysine in the hydrolysate indicates how many of these amino groups are free in the intact protein. For all proteins studied reasonable 

Protein Assumed mol. wt. JV-Terminal residue Number per molecule 

Serum albumin (human, horse, ox) 69,000 Aspartic acid 1 

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