Alfa and beta refer to the configuration of the anomeric carbon at the glucose moiety (See the attached picture). In solution, both anomers coexist in equilibrium. In other words, as soon as you dissolve alfa (or beta) lactose in water, it starts converting to the other anomer until an equilibrium between both is reached. Nitpicking-wise, they are not exactly the same, but for autoinduction media, it does not matter which one you pick, and it is more cost-efficient to buy plain D-lactose.

Yes, they are the same. Two descriptions for the same molecule. However, beta-Lactose would describe the anomer (OH-group at the reducing end).

Thank you so much, Peter! 

Alfa and beta refer to the configuration of the anomeric carbon at the glucose moiety (See the attached picture). In solution, both anomers coexist in equilibrium. In other words, as soon as you dissolve alfa (or beta) lactose in water, it starts converting to the other anomer until an equilibrium between both is reached. Nitpicking-wise, they are not exactly the same, but for autoinduction media, it does not matter which one you pick, and it is more cost-efficient to buy plain D-lactose.

Good morning Shanice,

Great question and excellent answers. You learn something everyday.

I was familiar with the alpha and beta anomers of lactose but the use of D lactose is new to me. Could the followers of this question provide a definition of when the D lactose term should be used.

Following on from Alejandro's excellent response. I have done a bit of work on the kinetics of epimierisation of lactose, the conversion of alpha to beta.

In general at room temperatures the equilibrium is reached at approximately 4hours and 4'C it takes about a day, and at 60'C just a few minutes. It is a first order reversible reaction. The actual equilibrium constant, the amount of alpha and beta present at equilibrium, depends on conditions, pH, conc, temp etc. However as a rule of thumb at equilibrium you can expect around 40% alpha and 60% beta at 25'C. For the materials properties when the lactose is recrystallised from such mixtures the amounts of alpha and beta are critical. Apologies I don't know enough about your e-coli experiment but to take out a potential variable, either work very very quickly with your aqueous lactose systems once prepared, or leave them for a morning, so equilibrium reached, and thus the composition of alpha and beta will be constant.

If you want to read further I have attached the titles of two papers on the conversion of alpha to beta.

Regards Paul

Article The Measurement of the β/α Anomer Composition Within Amorpho...

Article Stability of Sugar Solutions: A Novel Study of the Epimeriza...

'D-lactose' is just plain lactose (D-glucose+D-galactose). I'm guessing the problem is that Sigma offers 'D-lactose', hence people simply get used to the name.

Good morning Alejandro & Shanice,

Marvellous thanks for sorting this, in terms of pharmacy we prefer solids (tablets etc) thus when purchasing lactose the anomeric composition is really important. I hope the papers were of interest.

I have also just found an excellent article on the anomeric composition of lactose, see attached.

Regards Paul

Article Influence of supersaturation level on the morphology of α-la...

For the purposes of protein production in E coli it doesn't matter - just use the plain D-lactose which is cheap. (I've used just Lactose 1-hydrate (Reag. USP) PA-ACS from Pancreac, cat. no. 141375).

As already noted above, alpha/beta- D-lactose quickly reaches equilibrium, especially during the autoclaving of the 50x5052 stock (as it was in the original Studier's paper, 2005).

Actually it's a pity that almost everyone is still using IPTG even after all these years.