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Vaporization free energy of water. (fwd)
Sender: stoutepf@chemsci1.es.dupont.com (Pieter Stouten)
Subject: WSN: Vaporization free energy of water.
I had developed an effective solvation term for use in macromolecular
simulations. Now I want to add parameters for water so that we can
incorporate some crucial crystal waters explicitly. In order to do so, I
need the free energy of vaporization. That seems easy: based on vapor
pressure data I calculate 2.00 kcal/mol, while the CRC Handbook of
Chemistry and Physics gives 2.05 kcal/mol.
However, atom/fragment based schemes of Forsythe & Hopfinger, and Hine &
Mookerjee give values between 8.5 and 9.0 kcal. Using my own parameters
based on protein side chain analogs, I find values between 6.2 and 7.3
kcal/mol. Their and my schemes are optimized for organic molecules and may
not be transferable.
I am puzzled as to why experimental data give 2 kcal/mol while any
group-based scheme gives values between 6 and 9 kcal/mol. I know that
group-based schemes do not always work that well, but the large discrepancy
I see is hard to explain. I must be missing something glaringly obvious.
Initially, I thought that maybe the work upon volume change should be
corrected for (leading to a Helmholtz rather than a Gibbs free energy), but
that contribution is only RT (0.6 kcal/mol) *AND* works in the wrong
direction.
I would appreciate any help in explaining the observed differences.
References to papers that have effective solvation parameters for water
would be very welcome, too, as would any random thoughts on this and
related topics.
Thanks very much for your help !
Cheers, Pieter.
Pieter Stouten, Senior Research Scientist ||
Computer Aided Drug Design Group ||
The Du Pont Merck Pharmaceutical Company || Adventures get spoiled
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