Why is solubility phe > tyr ?

[Achim Recktenwald]

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 answer, therefore, I am 
trying again, and to more Newsgroups.


I am working with an enzyme that accepts phenylalanine (phe) and tyrosine (tyr) as 
substrate.
When I prepared the first time stock solutions I was surprised to notice that phe is 
soluble in water at pH 7 +/- 1 pH to approx. 180 mM, while with tyr I can prepare at the 
same conditions only a 10 mM solution.

Tyr differs from phe only in an additional hydroxyl-group.
I had always assumed an increase in polarity would also make a substance more water 
soluble.

Can anybody explain to me, why the p-OH group in tyrosine has such a strong effect on the 
solubility of the amino acid, nin effect decreasing it almost to 1/20 of phe.

Is this only valid for the free enzyme, or would this also be the case for the amino acid 
side-chains in proteins?  Does this mean, phe-side-chains have a more hydrophilic 
character than tyr¹s in proteins?


Achim