In cation exchange chromatography, the stationary matrix is negatively charged and binds positively charged molecules; species with negative or least positive net charge elute first, while strongly positive species bind tightly and elute last.
At pH 6.0:

• Glutamic acid ($pK_R = 4.25$): Since $pH \gt pK_R$, the side-chain carboxyl group is deprotonated ($-COO^-$). Net charge $\approx -1$. It is repelled by the negative resin and elutes FIRST.
• Histidine ($pK_R = 6.0$): Since $pH = pK_R$, the imidazole side chain is 50% protonated. Net charge $\approx +0.5$. It binds weakly and elutes SECOND.
• Lysine ($pK_R = 10.5$): Since $pH \lt pK_R$, the epsilon-amino group is fully protonated ($-NH_3^+$). Net charge $\approx +1$. It binds strongly to the resin and elutes LAST.