Why does this region fit NMR spectroscopy technique? I was reading a paper and came across a sentence (highlighted one) about NMR spectroscopy.


  Several Leishmania Rabs, like Rab1, Rab5a, Rab5b, and Rab7, have been functionally characterized. LdRab5a is a GTPase that has been shown to
  bind to GTP and hydrolyze it to GDP. It specifically regulates
  fluid-phase endocytosis. Although the G1–G5 loop sequences
  are well conserved in LdRab5, it has unique
  interswitch region with substitutions and a 20-residue insertion
  with respect to the human Rab5. The unique insertion
  feature of LdRab5a prompted us to choose NMR spectroscopy as the most suitable technique for determining
  its structure. 
  
  It is pertinent to mention that NMR solution
  structure has not been determined for any Rab protein
  from any organism. This would also provide the opportunity
  for exploring the residue-specific dynamics for LdRab5a in
  particular and reveal the general implications for the Rab
  family of proteins. Hence, we have undertaken the determination
  of solution structure and characterization of backbone
  relaxation dynamics for LdRab5a by using NMR
  spectroscopy.
  However, stabilization of LdRab5a through systematic
  and specific mutations and deletions was necessary to arrive
  at a construct that was stable enough for recording long
  multidimensional NMR experiments.


Why do we need to have a unique interswitch region so as to use the NMR spectroscopy technique?
The insert in the Ld ortholog occurs in a beta sheet and suggests the presence of additional strands in that sheet compared to the human variant. The structure of the human protein has been determined by crystallography. It would therefore appear that it should be possible to determine the structure of the Ld protein the same way (via crystallography), provided a crystallizable construct is identified. 

Based on the available reference provided, the insertion feature from LdRab5a appears to make the protein unstable in vitro and difficult to crystallize, leaving NMR as the only good alternative for a structure determination. However, it seems the insertion feature also rendered the protein too unstable to be able to complete long NMR experiments, so the authors chose to remove the insert in order to stabilize the protein and perform NMR.