Question

Indicate the net charge for the peptide, C-H-A-V-E-C-A-R-R-I-S-T-H-E-G-R-E-A-T-E-S-T, at the given pH values:

(a) pH 1, net charge:

(b) pH 5, net charge:

(c) pH 8, net charge:

(d) pH 14, net charge:

Answer 1

Solution:

NH2- Cys - His - Ala - Val - Glu - Cys - Ala - Arg - Arg - Ile - Ser - Thr - His - Glu - Gly - Arg - Glu - Ala - Thr - Glu - Ser - Thr -COOH

Step 1: We will write down the structure with the charges that we expect at required pH.

Peptide N terminus, C terminus and only side chains of Cys, Tyr, Asp, Glu, Lys, Arg, His are ionizable and contribute in charge.

Below the pKa value it has negative charge and if pH is higher it has positive charge.

At pH 1

Cys – NH2 + Side chain = -1 + -1

His – side chain only = -1 (x2)

Glu -side chain only = -1 (x4)

Cys – side chain only = -1

Arg – side chain only = -1(x3)

Thr – COOH = -1

Net charge = -13

At pH 5

Cys – NH2 + Side chain = -1 + -1

His – side chain only = -1 (x2)

Glu -side chain only = +1 (x4)

Cys – side chain only = -1

Arg – side chain only = -1(x3)

Thr – COOH = +1

Net charge = -2

At pH 8

Cys – NH2 + Side chain = -1 + -1

His – side chain only = +1 (x2)

Glu -side chain only = +1 (x4)

Cys – side chain only = -1

Arg – side chain only = -1(x3)

Thr – COOH = +1

Net charge = +3

At pH 14

Cys – NH2 + Side chain = +1 + +1

His – side chain only = +1 (x2)

Glu -side chain only = +1 (x4)

Cys – side chain only = +1

Arg – side chain only = +1(x3)

Thr – COOH = +1

Net charge = +13