When a mixture of the amino acids Phe and Gly is subjected to chromatography in a pH 6 buffer on the ion-exchange resin shown in Eq. 27.9, the Phe emerges from the column much later than the Gly, even though the two amino acids have the same isoelectric point. Explain.

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-CH₂-CH-CH₂-CH-CH₂-CH-... 0=S=0 0- 0=S=O structure of sulfonated polystyrene with ionized sulfonic acid groups (27.9)