I have run a SDS-PAGE with a protein molecular weight marker with a size range from 14.4 to 116 kDa. I get a band lying under 14,4 kDa. Is it possible to determine the size of proteins in this band? If yes, how? Can I be sure this is all the same size proteins or can it be different sizes that have just all ended here because the electrophoresis was stopped?
Yes you can calculate it by making a plot. You essentially measure the distance the markers run and you know their size so you generate a line and then extrapolate to the distance your protein migrated. See the following:
https://www.bio-rad.com/webroot/web/pdf/lsr/literature/Bulletin_6210.pdf
It is generally not advisable to use data outside the range of available calibration standards, that is, to extrapolate rather than to interpolate. In electrophoresis, especially, the Rf vs log(mass) plot becomes non-linear for very large and very small proteins.
In addition, I'd consider tricine-based electrophoresis for such small proteins. Together with standards that bracket my protein of interest.
I cannot even see the migration front in your gel, so i am unable to tell whether your protein was separated from the mix or not. Is it a western blot that you are showing? And secondly, the plot tends to become non-linear in its extrema, so you can try to plot for the markers and see, but be sure that your protein is separated from the rest.
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