Amino acids at different pH

Question

How do I know how amino acids look like using only the Ip and pH-value (without pKa)

For example: Draw lysine at pH 9.5 The Ip of lysine is 9.7

I feel like I can't do it without the pKas :(

Answer 1

The isoelectric point is the pH at which you have a zwitterion, so it is neutral. Below that pH, the carboxyl group(s) will be protonated (no charge) and amino groups will begin to be protonated giving the amino acid an overall positive charge. Vice versa as the pH is raised above the pI.

So, at pH 9.5 (lower than pI) you might expect the COOH group to be protonated and the NH2 groups to also be protonated (maybe not both of them). So while I agree that to get an accurate picture of the structure, you should know the Ka values for all functional groups, I think you can predict an overall positive or negative charge based on knowing only the pH and the pI.