Find the net charge of cysteine at pH 8.75 using the follwing pka values:
carboxyl group has pka=1.9
amino group pka= 10.7
side chain=8.4
To find the net charge of cysteine at pH 8.75, we need to consider the pKa values of the functional groups present in cysteine, which are the carboxyl group, the amino group, and the side chain.
Let's start by identifying the various ionizable groups in cysteine and their respective pKa values:
1. Carboxyl Group: The carboxyl group (-COOH) in cysteine has a pKa value of 1.9. At lower pH values, the carboxyl group will be protonated and have a positive charge (+1), while at higher pH values, it will be deprotonated and have a neutral charge (0).
2. Amino Group: The amino group (-NH2) in cysteine has a pKa value of 10.7. At lower pH values, the amino group will be protonated and have a neutral charge (0), while at higher pH values, it will be deprotonated and have a negative charge (-1).
3. Side Chain: The side chain (-CH2-SH) in cysteine contains a thiol group and has a pKa value of 8.4. At lower pH values, the side chain will be protonated and have a neutral charge (0), while at higher pH values, it will be deprotonated and have a negative charge (-1).
To determine the net charge of cysteine at pH 8.75, we need to compare the pH value with the pKa values of the respective ionizable groups:
- Carboxyl Group: At pH 8.75 (higher than the pKa of 1.9), the carboxyl group is deprotonated and has a neutral charge (0).
- Amino Group: At pH 8.75 (lower than the pKa of 10.7), the amino group is protonated and has a neutral charge (0).
- Side Chain: At pH 8.75 (lower than the pKa of 8.4), the side chain is protonated and has a neutral charge (0).
Since all the ionizable groups in cysteine have neutral charges at pH 8.75, the net charge of cysteine at pH 8.75 is 0 (neutral charge).