Elutes first in cation exchange at pH 7?
Cation exchange matrix bears negative functional groups attracting positively charged species. At pH 7, acidic amino acids like aspartate with pI around 3 are deprotonated and negatively charged, thus repelled and eluted first without binding. Basic residues like lysine remain positively charged and bind strongly. Neutral aliphatic residues such as valine and leucine carry little net charge at neutral pH, resulting in minimal interaction. Hence in a mixture, aspartate exits earliest, allowing charge-based fractionation. Adjusting pH relative to amino acid pI values facilitates predictable elut
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.