Dear Esra,
I do not believe that a transition from H3 to I213 is possible, because these two crystal forms contain different oligomeric forms of the protein in the asu. The rhombohedral form contains a hexamer (dimer in the asu), while the cubic one contains a dimer (monomer in the asu). Since these two oligomeric forms are not crystallographically related, I doubt you can induce such a rearrangement in the unit-cell just by temperature.
My suspicion is that the 100 K double c axis is due to lattice strains during freezing. Speaking from experience, insulin crystals are a bit finicky to work with, they are very fragile and these axis doubling effects are very common when the crystal is not cryoprotected well enough or not flash frozen fast enough, therefore it's more of a handling artifact. Now if all these datasets are from the same crystals and you first measured at 100 K and then increased the temperature, that would explain why the lattice strains are relieved as the crystal slowly heats up. If not, it's hard to tell without further details on the crystallization condition or collection on more crystals whether the double c axis cell at 100 K is a fluke or not.
Now, regarding your MR issue, there are a few double c insulin structures in the PDB (human, bovine or porcine) that you could use instead for starting models:
- H3 double c [78.6 78.6 79.1 90 90 120 / R6 conformation] : 1XDA, 3P33, 5URU
- H3 double c [80.4 80.4 72.0 90 90 120 / T3R3 conformation] : 1FU2, 1G7A, 1G7B
- H3 double c [82.7 82.7 68.1 90 90 120 / T6 conformation] : 3W80
Judging from the provided lattice parameters, you're probably in the R6 conformation. Note that some of these entries have mutations and/or other chemical modifications that you'll probably need to get rid off eventually during model building - still could be helpful as starting models.
Hope this helps!
Best of luck!
Dimitris
______________________________
Dimitris P. Triandafillidis
Hamburg Centre for Ultrafast Imaging
Universität Hamburg
HARBOR (Building 610)
Luruper Chaussee 149, DE-22761
Hamburg, Germany
