How long can IPTG be stored in -20 after preparing 1M mother liquor?

Our 1M IPTG mother liquor is filtered, sterilized, and packaged in -20 dark containers. Generally, there is no significant decrease in activity after one to one and a half years of use. It's a good thing that it hasn't turned yellow or settled for two years, but it doesn't necessarily mean it's effective. Long term storage of IPTG aqueous solution may lead to slow degradation or repeated freeze-thaw cycles, resulting in local concentration changes. It is recommended not to use this tube for critical induction for now. Use the same batch of bacteria and plasmids as a small control: the old tube and the newly prepared mother liquor should be induced at the same concentration to observe the target band. If there is not much difference, continue to use them. I am used to buying powder for self preparation, which has low cost and is suitable for use in small quantities.

If you induce frequently, buying powder and preparing it yourself is more flexible. Prepare 20-50 mL of stock solution each time and aliquot at -20. Ready-to-use solutions are expensive and also require attention to shelf life after opening, making them suitable for occasional use. Regarding the two-year-old tube, it is not recommended to use it without validation. IPTG aqueous solution is generally stable when stored at -20, but repeated retrieval can cause freeze-thaw cycles. It is recommended to aliquot 200 μL per tube and use each tube once. If the two-year-old tube appears normal, first perform a small-scale induction. If conditions permit, measure the OD600 induction curve to check for any abnormalities.

The key is to protect from light and avoid repeated freeze-thaw cycles. After preparation, filter-sterilize and aliquot 0.5-1 mL per tube into sterile EP tubes, store at -20. For light protection, use brown tubes or wrap in aluminum foil; ordinary transparent tubes stored in a drawer are also acceptable. After two years, if there is no contamination, no yellowing, and no precipitation, activity may decrease, but the extent of decrease may not necessarily affect routine induction. We once had a stock solution stored for 18 months that still worked normally for reporter gene and target protein induction. However, it is not recommended to use two-year-old tubes directly for important purification batches; perform a small-scale test first. If the stock solution becomes turbid or has an off odor, discard it directly.

A rapid parallel validation can be performed: the same batch of BL21 (DE3) competent state can be transformed into the same plasmid and divided into three bottles, each containing newly prepared mother liquor, old tube mother liquor, and no induction control. 0.5 mM at 37 ° C for 3 hours or overnight at 16 ° C can be added, and the target protein band can be observed by SDS-PAGE. If the target band of the old tube is significantly weak, it indicates low potency; If the difference is not significant, it can be used up. Long term storage is recommended to be divided into single doses each time to avoid repeated freezing and thawing of the entire tube. Ready to use solutions are usually labeled with an expiration date, but after opening, they should also be packaged and the opening date recorded. Avoiding light has benefits, but more importantly, avoiding pollution.