Why are 2-amino acids soluble in water and have high melting points?

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Answer 1 · 2017-11-27T00:58:13

Because they are Zwitter-ions.

Put in very simple terms, they are charged.

The neutral form for any given (#alpha#-)amino acid is:

#"H"_2"N"-"HRC"_alpha-"COOH"#

However, the zwitterionic state tends to exist near pH #7#:

#"H"_color(red)3"N"^color(red)"+"-"HRC"_alpha-"COO"^(color(red)(-)#

The amino group is protonated, and the carboxyl group is deprotonated.

This results in strong ionic interactions within the amino acids, resulting in surprisingly high melting points (#200^@ "C" - 300^@ "C"#).

These are difficult to measure though, as most AA's decompose first before they melt...

By the way, it also explains why many of them readily dissolve in polar solvents like water, depending on what #R# is.