Amino acid solubility

[Yseah]

Does anyone know why L-tyrosine is less soluble than
L-phenylalanine in water ? I would have thought that the extra
hydroxyl group in tyrosine would render it more hydrophilic and
therefore more soluble in water.

Thanks in advance.

Stephen

[ghea...@netaxs.com]

Responding to msg by YS...@macollamh.ucd.ie (Yseah) on 28 Feb
1995

Predicting solubility, or even relative solubility, is
obviously not simple. You have only addressed one side of the
equation. Compounds are held in a crystal structure (solid) by
several types of interactions - ionic, Van der Waals
interactions, hydrogen bonding, etc. A compound is soluble
when the solvent (in your example water) effectively disrupts
the interactions of the compound in the solid state (separates
the molecules from one another) while providing a degree of
solvation that prevents the molecules from getting back
together and crystallizing out. There are equilibria involved
in these processes that include such things as temperature, pH,
ionic strength, etc. You could just as easily have postulated
that, since phenylalanine does not have the hydroxyl group to
provide interactions in the solid state, it would be more
soluble in water.

George A. Heavner
ghea...@netaxs.com