Histidine has pka of 6. What will be the net charge in Histidine in a solution of (a) pH 8.0 (b) pH 5.0

Answer 1

This is a very generalized answer not accounting for some of the nuanced specifics of histidine as an amino acid, but I hope it helps!

So if we think of Histidine like a toy with three parts that can be "on" or "off". Each part has a switch point ( the pKa), where it flips from "on" to "off". One of these switches is right at the level of 6.0.

For pH 8.0:

Think of pH like the surrounding temperature. At 8.0, it's hotter than the switch point (6.0). So, this part of the toy is "off" and doesn't add any charge. Overall, the toy (histidine) doesn't have a special charge at this temperature. So our net charge would zero.

For pH 5.0:

At this pH, it's cooler than the switch point (6.0). So, this part of the toy is "on" and adds a charge. The toy (histidine) is like a little magnet now, with a +1 charge.

In simpler terms:

(a) At a hot level (pH 8.0), histidine isn't charged.

(b) At a cooler level (pH 5.0), histidine has a +1 charge.