Question

What is the charge state of the following peptide at (A) pH 3? (B) pH 10?...

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.

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

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