Organic Chem/Biochem Question...

[dentwannabe]

okay, this is in kaplan's white book..

If the following amino acid (pI=9.74) in acidic slution is completely titrated with sodium hydroxide, what will its charge be at a pH of 3, 7, 11.

H2NCH2CH2CH2CH2CHNH2COOH

the answer is D
D. positive, positive, negative

now, kaplan gives a explanation that at pH 3, the amine and carboxyl groups wil be protonated to give a net positive charge.. and continues to explain for 7, 11.


MY QUESTION IS...
Can't I just use the reasoning if pI > pH, positive charge; if pI < pH, negative charge??
Can I use this simple reasoning or do I have to go into the details of what/how deprotonation will occur.

Thanks

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[dentwannabe]

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[heshyn2000]

okay, this is in kaplan's white book..

If the following amino acid (pI=9.74) in acidic slution is completely titrated with sodium hydroxide, what will its charge be at a pH of 3, 7, 11.

H2NCH2CH2CH2CH2CHNH2COOH

the answer is D
D. positive, positive, negative

now, kaplan gives a explanation that at pH 3, the amine and carboxyl groups wil be protonated to give a net positive charge.. and continues to explain for 7, 11.


MY QUESTION IS...
Can't I just use the reasoning if pI > pH, positive charge; if pI < pH, negative charge??
Can I use this simple reasoning or do I have to go into the details of what/how deprotonation will occur.

Thanks

No, the reasoning you mentioned is wrong, because all PI means is that the PH level is at the zwitterion, meaning there is a (-) and (+) charge on the molecule at the same time, for this question you have four combinations of what the charges can be, givin the PH, and that is what the question is asking you to calculate. hope this helps.