Which technique can sequence a 25-residue peptide directly?
Determining amino acid order of a 25-residue peptide requires fragmentation along peptide backbone to generate overlapping ions. Circular dichroism assesses secondary structure, SDS-PAGE estimates molecular size, and X-ray diffraction requires crystals unsuitable for short peptides. Tandem mass spectrometry, MS/MS, isolates chosen precursor ion and fragments it via collision-induced dissociation, producing b and y ion series covering sequence. Modern Orbitrap or quadrupole-time-of-flight instruments achieve sufficient resolution and mass accuracy to read full sequence directly, enabling rapid peptide sequencing without Edman degradation in proteomics workflows for biological samples.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.