How to make IPTG Stock Solution - Banner Image

How to Make an IPTG Stock Solution

by Simon Currie

Making IPTG stock solution involves weighing out IPTG powder, pouring it into a conical tube or cylinder, and adding deionized water to the desired volume. After that, solubilize it by inversion, double check the volume and finally filter the solution.

IPTG is frequently used for blue/white colony screening and to induce protein expression in bacterial cultures. Fortunately, IPTG is very stable and easy to work with. For maximum convenience it is a good idea to make an IPTG stock solution and pipette it into single-use aliquots that you can quickly pull out of the freezer, one at a time.

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Step-By-Step IPTG Stock Solution Instructions

What concentration for IPTG stock solutions?

Step-By-Step IPTG Stock Solution Instructions

1) Weigh out the amount of IPTG that you’re going to use based on how much and what concentration of stock solution you want. For example, in Figure 1 we give an example calculation for making 10 mL of 1 M IPTG stock solution.

2) Pour the weighed IPTG into a conical tube or a graduated cylinder and add deionized water up to the desired volume.

3) Solubilize the IPTG. If using a conical tube, cap it and invert several times until the IPTG goes into solution. If using a graduated cylinder, add a stir bar and allow the solution to stir until IPTG is solubilized.

4) Re-check the volume and add more deionized water if solubilizing the IPTG left you with less volume than you were aiming for.

5) Filter the stock solution with a syringe filter or a bottle-top filter. This is important for sterilizing the IPTG solution to make sure that you don’t contaminate your cultures.

6) Pipet the solution into convenient single-use volumes.

7) Freeze the IPTG stock aliquots in the -20 °C freezer.

Figure 1. Example calculation for how much IPTG you need to weigh out to make 10 mL of a 1 M stock solution. 1 mol per L is the definition of 1 M. 238.3 g/mol is the molar mass of IPTG. And 0.01 L is equal to 10 mL.

That’s all there is to making IPTG stock solutions. When stored at -20 °C and used without repeated freeze-thaw cycles, these aliquots are good for at least one year. Check out this video if you want to see how exactly each of those steps are performed.

What concentration for IPTG stock solutions?

For inducing protein expression, I generally make IPTG stocks at 1 M, which is a 1000x concentration for using 1 mM IPTG to induce bacterial cultures. I also typically make 1 mL aliquots, so that when I add the 1 mL of 1 M IPTG to a 1 L culture it is diluted to the final 1 mM concentration.

A final IPTG concentration around 1 mM is also used for blue-white colony screening. However, in this case you’re typically working with much smaller volumes, so it is more common to make a 100 mM stock solution of IPTG. Check out our website for a protocol to make 100 mM IPTG stocks, our use this nifty calculator.

Of course, you can adjust the concentration and volume of your IPTG stock aliquots to whatever is convenient for your experimental needs.

If all of that sounds like too much work, then lucky for you GoldBio sells IPTG EZ Pak^TM, which has the IPTG already weighed out for you, and it also comes with a sterile filter that’s ready to use. See below for that and other great products, as well as related resources if you’re interested in learning more about IPTG.