RE: W:Solvation free energy for aromatic hydrocarbon

1. Although benzene and water are immiscible in the sense that if you put
 50 mL H2O and 50 mL C6H6 in a vessel, close it, and shake vigorously, you will
 get two liquid phases, not one.   However, there is an appreciable amount of
 water in the benzene phase, and an appreciable amount of benzene in the water
 phase. This  experimental evidence suggests that the free energy change from
 vapor phase* to a dilute solution in water is negative, but the free energy
 change from the liquid benzene phase to a more concentrated aqueous solution is
 positive.
 2. The movement of the phenyl group from solution to protein is not DEsolvation;
 instead solvation by water is replaced with more favorable solvation by the
 protein.
 3. Phenyl groups (in this context) don't usually migrate by themselves; they are
 attached to some molecule.  So the positive solvation energy of the phenyl group
 may actually represent the difference between the solvation energy of alanine v.
 phenylalanine, or of some other pair of molecules differing by a phenyl group.
 --David Shobe, Ph.D., M.L.S.
 S|d-Chemie, Inc.
 phone (502) 634-7409
 fax (502) 634-7724
 Don't bother flaming me: I'm behind a firewall.
 *The experiment uses liquid benzene but under ambient conditions the liqud-vapor
 equilibrium favors the liquid.  If vapor to liquid is negative and liquid to
 dilute aq. soln. is negative, then vapor to dilute aq. soln. is negative also.
 -----Original Message-----
 From: Computational Chemistry List [mailto:chemistry-request|at|ccl.net] On Behalf Of Chemical, , Bond
 Sent: Thursday, July 07, 2005 4:08 PM
 To: chemistry|at|ccl.net
 Subject: CCL: W:Solvation free energy for aromatic hydrocarbon
 Hi there,
 I am confused with a single concept: what should be the sign for the solvation
 free energy for a fragment like phenyl?
 There are many experimental data of solvation free energy for aromatic
 hydrocarbons, such as benzene, naphthalene, etc, but they are all negative
 surprisingly.  I looked at several papers with models of solvation energy, and
 their predictions all all have negative solvation energies for those compounds,
 just like the experiments.
 Typically when a phenyl group is transfered from water into protein, we would
 like to say there is a negative(beneficial) desolvation energy, or the usual
 called hydrophobic effect.  And this has been used in tons of models for protein
 simulation or protein-ligand binding.  Note, in this context, the solvation
 energy (from gas into water) is positive!
 Is there something wrong over here?  Or just some artifact from the fitting?
 Hope there would be some good comments.
 Thanks a lot,
 Bond
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