Question

what charge do basic peptides have in order to stick to the negativily charged beads for cation exchange i had assumed basic peptides had negative charges

Cation and Anion Exchange Cation exchange media (solid supports) are negatively charged and bind best to basic peptides peptides and proteins. Anion exchange media (solid supports) are positively charged and bind best to acidic peptides and proteins.

Answer 1

Peptides are short chains of amino acids. Based on the number of amino acid groups present they are classified in to dipeptides, tripeptides, tetrapeptides etc. Basic peptides contains positively charged amino acids like histidine, lysine, arginine etc. These positively charged amino acid building blocks in basic peptide chains may interact with the negatively charged cation exchange media.

As an example we can consider the Histidine amino acid.

Histidine is an essential amino acid and contains a positively charged imidazole functional group.The unprotonated imidazole is nucleophilic (positively charged) and can serve as a general base, while the protonated form can serve as a general acid. When peptides possess such positively charged amino acids as their building blocks, they also become essentially positively charged.

- 6.0 PK HN- NH+ CH2 +H3N-CH-c= 0 pk = 9.2 Opk, = 1.8

An example of cationic peptide (basic) is protamine, which is highly positively charged due to the presence of 21 arginine residues.