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birefringence

Matthias Dreyer dreyer at bio5.chemie.uni-freiburg.de
Mon Sep 6 07:37:52 EST 1993


Hi everybody!
I have made the following funny observation with crystals of a 24 kDa
protein: the spacegroup of the crystals is P42(1)2, but under the 
polarizing microscope they show birefringence along the 4-fold axis,
exhibiting a checked pattern like this:
              
             |XXXXXXXXXXXXXXX|   
             |  XXXXXXXXXXX  |
             |    XXXXXXX    |
             |      XXX      |
             |       X       |
             |      XXX      |
             |    XXXXXXX    |
             |  XXXXXXXXXXX  |
             |XXXXXXXXXXXXXXX|

Can anyone give me an explanation for that? I'm quite sure that the 
spacegroup is correct since I could solve and refine the structure to 
an r-factor of well below 20% at 2.1 A resolution. Furthermore, after 
statistical analysis of the observed intensities I'm sure not to have 
twinning in my crystals, which could give rise to a higher apparent 
symmetry.

          Thanks for any comments!


********************************************
Matthias Dreyer
Institut fuer Org. Chemie und Biochemie
Albertstr. 21
D - 79104 Freiburg
Germany 
e-mail: dreyer at bio5.chemie.uni-freiburg.de
********************************************  




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