Lysine zwitterion at pH 7. Would the NH2 residue be protonated or not?TKS

[PB]

I know that the carboxy and amino fn grp would be charged but what about the side group of say aspartic acid or lysine?

[lzxnl]

I've been told to consider the acid/base side groups as protonated/deprotonated according to the acid/base parts in the amino acid. So that implies deprotonated aspartic acid at neutral pH. To be honest, this system is weird and I don't get why we're not given pKas to use, aside from the generic excuse "it's VCE".

[PB]

true

[BubbleWrapMan]

Lysine isn't a zwitterion at pH 7, so if you were asked to draw the zwitterion at pH 7 it wouldn't be a well-written question. Both amino groups will be protonated, and the carboxyl group will be deprotonated, giving it a net charge of +1. Similarly, aspartate (aspartic acid) would have a charge of -1.

They probably used the term 'zwitterion' loosely, i.e. it has different charges on it.

[thushan]

I've been told to consider the acid/base side groups as protonated/deprotonated according to the acid/base parts in the amino acid. So that implies deprotonated aspartic acid at neutral pH. To be honest, this system is weird and I don't get why we're not given pKas to use, aside from the generic excuse "it's VCE".

VCE kids will learn about Ka values (and pKa is just the negative log of that) in Unit 4 nliu.

[lzxnl]

Yes, but VCE students are not given Ka values of each amino acid's acid and base group.

[thushan]

Eh, can make a question on it giving the values in the question.

[BubbleWrapMan]

I think all you have to understand if you're not given pK_a values is that the acidic side chains (D/E) will be negatively charged and the basic side chains (H/K/R) will be positively charged at pH = 7.

[lzxnl]

But we don't know that; some acid side chains may have extraordinarily high pKa values so even at pH=7 the side group isn't deprotonated. Likewise for the bases.