[amand79]

It is said that amino acids are soluble in polar solvents. I am confused as to how non polar amino acids are soluble in polar solvents. Is it not the case that like dissolves like? Thank you.

[AWK]

Don't treat the rules too dogmatically. The solubility of amino acids is not a simple dependence of the solvent polarity. There are no non-polar amino acids - they are only more or less polar. Amino acid racemates often have very different solubilities from pure enantiomers, although they are a physical mixture of these enantiomers. The highly polar Asp and Glu have lower solubilities than the less polar Phe.

https://www.semanticscholar.org/paper/Solubilities-of-the-common-L-%CE%B1-amino-acids-as-a-of-Amend-Helgeson/94df8ddf7af41b33ee734810e94e39b7a0cfb5ff
see Table 1

[Babcock_Hall]

I would also think about the acid-base properties of even the simpler amino acids, such as alanine.