Question

Question #ce9be

Answer 1

The overall charge on the tetrapeptide is at `"pH 7.6"` is `"-1"`.

Explanation:

Two principles to remember:

• If the `"pH"` of the environment is less than the `"p"K_a` of the weak acid (`"HA"` or `"BH"^+`), the acid will be predominantly in the protonated form (`"HA"` or `"BH"^+`).
• If the `"pH"` of the environment is greater than the `"p"K_a` of the weak acid, the acid will be predominantly in the deprotonated form (`"A"^-` or `"B"`).

Let's list the charges at various pH values.

`" "" "" ""H"_3"N"^+"-gly-glu-cys-ala-COOH"`
`color(white)(1)"p"K_a" "color(white)(1)9.5" "" "color(white)(1)4.25" "" "" "" "2.1`
`stackrel(——————————————————————)("pH 0"" "" "+" "" "" "0" "" "" "" "color(white)(1)0)`
`" ""pH 7.6"" "+" "" "color(white)(1)-" "" "" "" "-`
`" ""pH 10"" "color(white)(1)0" "" "" "-" "" "" "color(white)(1)-`

At `"pH 7.6"`, you deprotonate both carboxyl groups but not the `N`-terminal `"H"_3"N"^+` group.

You have two negative charges and a positive charge, for a net charge of `-1`.

At `"pH > 9.5"`, you deprotonate all groups, so the charge on the tetrapeptide is `-2`.