I had a couple of questions concerning cysteine, N-acetylcysteine and carbocysteine.

I have been able to find pKa values for cysteine and N-acetylcysteine but none for carbocysteine.

The values I have for each are as follows (please confirm)
Cysteine pka = 1.9, 8.1, 10.3
N-acetylcysteine pka = 4 and 9.5
Carbocysteine pka = ????

I figure that carbocysteine has two pka values

Any assistance would be helpful with my LC methods. Thank you