For aspartate, this is our side chain group, and the PKA value of Aspartate is 4.1. Now, this PKA value is the same as above. It's 3.1. So notice they both have negative charges. And that makes sense, because if both of these groups give the same type of charge, then to basically cancel out the positive charge, we have to average these two negative charges. And so we average these two PK values.