Should be fine. You can always supplement it up to 5% final concentration.   

5% B-mercapto is optimal and ensures complete reduction in disulfide bridge.  2.5% should also be fine... As Andreas said, you can always supplement it to 5%. 

Hi,

I add about 10% of  B-mercapto . I get beautiful westerns with nice separation.

So 2.5% is quite below the line.

Cheers

Hi, we use 5% of b-ME as well. I think it depends on the protein as well since the main function of b-ME is to denature to protein to linear form for SDS to coat. you can always add more b-ME freshly just before use. buffer with b-ME added should only last for at most a day or so. Thus, we always add it freshly to the buffer right before use. =)

Hello

Yes 5% its good ,Look at link here 

Good Luck 

http://openwetware.org/wiki/SDS-PAGE_sample_buffer_(Morris_formulation)

You can also use 50-100 mM DTT final conc. It works as well and is less toxic. 

5% is optimum final concentration, you should also consider the protein concentration and nature of it in the sample you analyses.

If you are not sure about the result maybe you should check two or more version of concentration of B-merkapto. It's one of the factor that influence the final results so it worth. Different material demand different solution. Good luck!

I used 14,4 M of B-mercapto for 95 uL of sample buffer 

Hi,

In my opinion, the major function of 2-mercaptoethanol is to denature protein by cut disulfide. So, it should be 5% for the best result. You can supplement more before using because fresh buffer is best

Yea, 5% is most appropriate and give the best result.

We use 5% 2-ME in 5X SDS-PAGE sample buffer, which means the final conc. will be 1%. However, you shouldn't have any problems even if you use half of that conc.

Your question brought to my attention that I have been using 2.5% for years as I used to prepare my own 5X loading dye. I did not feel any problem with it. Also, I noticed that in BioRad ready made loading dye the final concentration of BME according to their recommendations is 2.5%. I know I am so late now to answer the question so, it is a good chance to see what was your experience? Sumit Mukherjee

I feel like adding my 3 years late answer because I have just seen the question.

Bio-Rad suggests that the beta-mercaptonethanol be 5% of the sample buffer :

http://www.bio-rad.com/webroot/web/pdf/lsr/literature/4006028F.pdf