This may be a silly question, but is it feasible that Isopropyl β-D-1-thiogalactopyranoside (IPTG) when acted upon by a beta-galactosidase could produce 1-deoxy-galactose or just galactose?

The reason I am asking is because I have a crystal of a beta-galactosidase that was exposed to 5mM IPTG for ~6 months at 4 degrees C for inhibitor studies.I have X-ray crystallographic data (Beta-galactosidase + IPTG) to ~1.8 angstroms and can easily place the galactosyl ring into my electron density, however, I cannot place the sulfur atom or the isopropyl group.

This would normally indicate that the sulfur and isopropyl atoms are disordered, however, the possible conformations are sterically restricted to one direction.

As such I am wondering if IPTG could, in fact, have been turned into either 1-deoxy galactose or galactose?


1 Answer 1


I'm not sure if I understood your question correctly, and my thought is based on this assumed reaction:

enter image description here

Your indication of a resolution of about 1.8 Å reads like the data were collected by Cu K$\alpha$ radiation, rather than by Mo K$\alpha$. If sulfur was split off from your molecule, than (even more with softer Cu than Mo X-rays) this should have decreased a lot the contribution of anomalous scattering in the X-ray data, as CHO's atomic scattering factors -- in comparsion -- are mainly described by $f'$, and not so much about $f''$. What about looking on value .and. esd of the Flack parameter of your model for your new product, compared to a "typical / reference" model of IPTG?

(By the way, Howard Flack passed away just at the beginning of this year.)

  • $\begingroup$ The data was collected at 1.000A at a synchrotron. I have succesfully phased the data by molecular replacement and built a publication ready model. I do not have the required redundancy needed for anomalous diffraction maps. The reaction is what I had in mind. $\endgroup$ Commented Mar 29, 2017 at 18:48
  • $\begingroup$ @JeppeNielsen Your experiment was approved by the gate keepers, and now there could be a crystal-to-crystal reaction. So I assume, there is no resonable quantity of substance left (assuming Murphy) to eventually pursue further spectroscopy or GC-MS, or even to collect additional diffraction data of high brilliance again. Ok, "true biochemists" should look at the question. Still, with invested work & advanced X-ray data in hand, may I hint you to other experts on xrayforum.co.uk or Olex2's olexsys who may provide additional hints regarding "where is the sulphur gone"? Good luck! $\endgroup$
    – Buttonwood
    Commented Mar 29, 2017 at 19:26

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