Question

What is the charge state of the following peptide at (A) pH 3? (B) pH 10? (pKa values: N-terminus 8.0, C-terminus 3.0, Asp 4.0, Arg 12.5). Show your work.

Answer 1

At pH 3:

pKa values:

N-terminus 8.0,

C-terminus 3.0,

Asp 4.0,

Arg 12.5

N-terminus (pKa = 8.0)
So for charges compare pKa's to pH:
pKa 8.0 > pH 3.0, so the amino terminus gets a proton = +1 charge

C terminus - (pKa= 3.0)
pKa 3.0 = pH 3.0, equilibrium attained. No sharing of proton.

Asp (pKa = 4.0)
pKa 4.0 > pH 3.0, so the aspartine gets a proton = +1 charge

Arg (pKa= 12.5)
pKa 12.5 > pH 3.0, so the arginine gets a proton = +1 charge

Net charge is +3.0

At pH 10:

N-terminus (pKa = 8.0)
So for charges compare pKa's to pH:
pKa 8.0 < pH 10.0, so the amino terminus loses a proton = -1 charge

C terminus - (pKa= 3.0)
pKa 3.0 < pH 10.0, so the C terminal loses a proton = -1 charge.

Asp (pKa = 4.0)
pKa 4.0 < pH 10.0, so the aspartine loses a proton = -1 charge

Arg (pKa= 12.5)
pKa 12.5 > pH 10.0, so the arginine gets a proton = +1 charge

Net charge is -2.0