[ccp4bb]: C-H...O type interaction

[Narayanan Ramasubbu <ramasun1@UMDNJ.EDU>]

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Hello all:
This is not a CCP4 question but I would like to find out opinion about a
peculiar interaction that I notice in my protein.
There is a C-H...O type interaction (H bond perhaps?) from an indole ring
to a catalytic Asp (Trp to Asp). Mutation of Trp to either Tyr or Leu or
Ala reduces activity. Other than this interaction, Trp is part of a
classic hydrophobic interaction with a His and Trp. 
1) Has anybody come across this type of situation in other enzymes?
2) How does the negative density of a Asp carboxyl group get affected
 by an aromatic residue? 
3) Can the pKa of the carboxyl group be affected in the wild type or
in the mutants?
The resolution is 2.0 A and the distance is 3.2 A. 
thanks in advance
subbu

subbu@xray.UMDNJ.EDU