Because usually on the outside, we have the polar water molecules, and this will observe the hydrophobic effect and will tend to interact with the non polar side chains found inside that protein. So these are the seven ionizable amino acids. It's ionizable because they have these side chain groups that can lose or gain h atoms at specific PH values. Now, all 20 varia amino acids contain an alphacroboxyl group and an alpha amino group. And so, technically, at specific PH values, all of our amino acids can actually gain and lose h atoms. But because normally our body is at a specific PH value at around seven is is usually negatively charged.