Subject: Re: CCL:Heme charges

From: Mikael Johansson <mpjohans-.at.-pcu.helsinki.fi>
Date: Wed, 6 Aug 2003 10:58:49 +0300 (EEST)

Hello Emad and All!

On Tue, 5 Aug 2003, Emad Tajkhorshid wrote:

> We just optimized the geometry of a porphyrin ring using DFT and
> calculated ESP charges for the molecule. The iron carries a significant
> amoount of positive charge according to population analyses done on the
> system (it is formally 2+), but the ESP charge for it is negative! The
> iron atom is completely burried inside the molecule, and that may be one
> reason of getting this strange result. Has anybody else had similar
> problems? Any comments, references, or suggestions are
> appreciated.

I had the same "problem" when trying to calculate ESP charges for a
six-coordinated haem. I assume your system is also six-coordinated? As
pointed out by Konrad Hinsen, this is an effect of not probing the ESP
near the iron, which in practice is impossible. Your ligating N's are
probably slightly too positive instead, and to the outside observers, i.e.
the fitting points, it makes no difference. Depending on what you plan to
use the charges for, it might make no difference for you either.

The idea of fixing the iron charge was quite good, thanks Konrad!

For a brief discussion on the iron charge in haems, you could look at
Biochim. Biophys. Acta - Bioenergetics, 1553 (2002) 183-187.

Have a nice day,
    Mikael J.
    http://www.helsinki.fi/~mpjohans/