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If an amino acid has two amino groups and only one carboxyl group (like arginine and lysine), then they are positively charged (++ vs. -) at neutral pH. On the other hand, if amino acids have two carboxyl groups and only one amino group (like glutamic acid and aspartic acid), they are negatively charged at pH7 (-- vs. +). You are not correct that the groups ...


You are right, most of the amino acids have pI around 6, so they are slightly negative at pH 7. Calculating approximate isoelectric points for amino acids and peptides The isoelectric point depends on the amino and carboxylic-acid groups and the groups of the side chain. E.g. lysine has an amino group on the side chain, so it has a pI of 9.74.


At pH 7.4 a carboxyl group is mostly present as COO-, while an amino group is present as NH3+. So when a molecule has both groups, such as an amino acid, the molecule as a whole is neutral. As it is a Zwitterion (equal + and - charges) the hydrogen can also flip flop between amino and carboxyl group. Making the molecule 'truly' neutral as both groups are not ...

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