Subject: OXIDATION POTENTIAL

From: "E. Lewars" <[email-masked]>
Date: Wed, 11 Feb 1998 15:11:12 -0500 (EST)

11 Feb 1998

Hello,

Zhang X-D posed the question:

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                 11-FEB-1998 09:55:37.66
>From:  IN%"[email-masked]"  "zhang"
To:     IN%"[email-masked]"  "PAUL_MATHIAS.ASPENTECH.COM"
CC:     IN%"[email-masked]", IN%"[email-masked]", IN%"clive@
msi.com"
Subj:   CCL:help

Hi,netters

I have an question as follows:

how to calculate the oxidated electric potential of a large molecule,
such as polymer, protein etc with quantum chemical methods
such as GS ?

Thanks in advances!

zhangxd
-----------------

  How to calc the oxidation potential of a protien or polymer using a
  QM method:
  The oxidation potential is essentially the solution ionization potential.

  There are references in the literature to the calc of oxidation potential,
  but my ideas are:

  Ox pot correlates with ionization pot;
  In both cases we are removing an electron from the molecule.  The IP is just
the E needed to remove an electron in the gas phase, but the oxidation
potential is more complicated,describing a process occurring in solution,
often at a surface, and it is defined relative to an arbitrary reference, H2.
However, IP and OP should be at least approximately correlated.

  To calculate _relative_  OP's, _approximately_, you can you Koopmans'
theorem and equate the negative of the highest occupied MO with the IP.
For a protein or other polymer an ab initio calc is not possible on the whole molecule.
So :

(1)  Try a semiempirical cal. You may not be able to do a SE geometry optimizaytion,
     and may have to do a SE single-point calc on a molecular mechanics geom.

(2)  Even a single-pt. SE calc may be to big a job; if so, look at the molecule,
     decide which regions are likely to be the ones of lowest IP, and do
     calcs on small model compounds with the same structure (e.g. a di-
     tri- or tetrapepetide for a protein).

(3)  Ab initio single-pt or even optimization jobs may be possible on model
     compounds (i.e. on small regions of the polymer).
------------
  A plot of calc IP' (done by you) vs. experimental oxidation potentials
  (find some compounds of your kind whose ox pot's are known) will show you how
  good your chosen procedure is.

       E. Lewars
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