HISE patch

Lawrence McIntosh mcintosh at OTTER.BIOCHEM.UBC.CA
Sat Nov 9 23:43:55 EST 1996


In calculating a structure recently, we had some problems
specifying the tautomeric state of the Histidine ring.
XPLOR defaults to the Nd1H form yet in fact the He2H
tautomer is thermodynamically prefered (for free imidazole, 
at least). It is relatively straight-forward to identify
the tautomeric state of a His by NMR methods.

As outlined in the manual, the HISE patch correctly redefines
the bonds, angles, and charges for the Ne2H form.

However, we found that the parallhdg.pro file from 3.1 was missing
the corresponding information. This can be fixed with:

	bonds     H    NB                 $kbon     0.98
	angle     H    NB   CV            $kang     127.3   
        angle     H    NB   CR            $kang     127.3

	improper   NB   CR   CV   H       $kpla    0    0.0 

I noticed that these statements are in the v3.8 parallhdg.pro
file, except that the two angles are defined as 120. I arrived at the 
values of 127.3 based on the "internal" ring angle:

	angle     CR   NB   CV            $kang     105.3000

(e.g. (360-105.3)/2).

Hopefully this reasoning is correct.


Lawrence McIntosh
Dept. of Biochemistry
University of British Columbia
mcintosh at otter.biochem.ubc.ca

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