Subject: Re: CCL: W:Solvation free energy for aromatic hydrocarbon

From: Andreas Klamt <klamt _()_ cosmologic.de>
Date: Fri, 08 Jul 2005 07:33:25 +0200

Hi Bond,

the concepts of the solavtion models are right. You must take into 
account the van der Waals (vdW) energies, which to first order can be 
considered as a kind of vdWs surface energy.

More or less accidentally, for water vdWs energy and surface tension 
(cavity formation free energy) almost cancel, so that hydration free 
energies of alkanes are very slightly positive. Since aromatics (due to 
the quadrupole moment) are slightly more polar than alkanes, hydration 
free energies of aromatics are slightly negative, as you say.

In a less polar solvent than water the cavitation free energy is much 
less positive (typically close to zero). Thus the solavtion free energy 
in such solvent would be much more negative that it is into water, and 
thus the ballance of the transfer of  aromatics from water to less polar 
solvents is much in favor of the non-polar solvent.

You can study these terms in detail in our solvation model COSMO-RS (see 
www.cosmologic.de).

Regards

Andreas

Chemical, , Bond wrote:

>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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