Why is solubility phe > tyr ?

Davy Kalonia kalonia at uconnvm.uconn.edu
Thu Jun 13 17:36:22 EST 1996

Achim Recktenwald wrote:
> I have a question concerning the solubilities of the amino acids phenylalanine and
> tyrosine.
> I had posted this message already a few weeks ago, but received no The solubility is an equilibrium quantity.  The free energy of 
solubilization (del G) depends on three factors. Interaction of 
(solvent_solvent molecules)+ (solvent_solute)+(solute_solute).
In this case assuming water as solvent, the solvent_solute factor is same 
for both Phe and Tyr.  The solvent_solute is more favorable for Tyr since 
it has a polar group (-OH).  The third factor solute_solute is the one 
that creates confusion here.  This energy depends on crystal lattice.  
The energy of crystallization for Tyr is much larger than that of Phe, 
ie. the O..H--O interaction  and stacking of the hydrophobic rings 
results in stronger interaction in the solid state.  Thus more energy 
must be provided to move the molecules from a solid_solid state to a 
solvent_solid state.  Thus poor solubility for tyr.

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