At what pH would you find Arg to be 90 deprotonated pka valu

At what pH would you find Arg to be 90% deprotonated? (pka values are 2.2, 9.0, and 12.5)

Solution

As Argenine is a Basic amino acid, it contain an extra Amino group in its Guanidino Side Chain. (Normal, simple amino acids possess 1 Amino group and 1 Carboxylic group). And Its PI (Iso electric point = 9+12.5 / 2

= 21.5 / 2

= 10.75.

The above value is thr PI value for Arginine, and at this pH value arginine is 50% deprotonated. Upon increasing the pH, from 10.75 onwards, till 12.5 we will get 90% deprotonation. The value is around 11.5 to 12 pH