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O-Block Uchiha

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Hello all, my MCAT is this Thursday and I'm feeling good. One thing still befuddles me a lil. So when we're looking at an AAs charge in solution, are we looking at the pka or pI? Take Asparatate for example, it's pI is roughly 2.7 and it's pka is 4, so if a solution had a pH of 3, would it be resorted to a plus charge due to the 2 carboxyls and the amino being protonated?? And at what pH would asparatate be a -2 charge from the carboxyl groups beig deprotonated. Your response would be greatly appreciated, thank you.

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Look at the pKa when you're talking about a protein or peptide. Amino acids in a peptide do not have free amino or carboxyl termini (with the exception of the first and last amino acids in a peptide) and so the only relevant acidic proton is the side-chain one.

The pKas of free aspartate are around 2, 4, and 9. At pHs up to 2, everything is protonated and so it's got a +1 charge. At pH 2-4, it's neutral because one carboxyl is deprotonated and the amino is still protonated. From pH 4-9, it's 1 from an additional carboxyl being deprotonated. At pH greater than 9, it's -2 due to everything being deprotonated.
 
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Look at the pKa when you're talking about a protein or peptide. Amino acids do not have free amino or carboxyl termini (with the exception of the first and last amino acids in a peptide) and so the only relevant acidic proton is the side-chain one.

The pKas of free aspartate are around 2, 4, and 9. At pHs up to 2, everything is protonated and so it's got a +1 charge. At pH 2-4, it's neutral because one carboxyl is deprotonated and the amino is still protonated. From pH 4-9, it's 1 from an additional carboxyl being deprotonated. At pH greater than 9, it's -2 due to everything being deprotonated.
helpful explantion as always, thankyou
 
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