Hello. Aptamer libraries are usually chosen in the range of 30 to 100 b. There are several factors to be taken into account, but a minimal length is usually necessary in order to have a good folding of aptamers, and thus to have more chance to generate binding sites. Then, it is usually not necessary to order a too long aptamer library, because of several things: what we want to obtain is usually a rather short fragment having one or two binding sites and a stem onto which can be fixed other molecules; and also because the costs of the DNA will increase a lot with the size of the DNA library requested. Also, what is possible to do is to shorten the aptamers afterwards, once the binding sites are defined. But it was sometimes observed that the rest of the aptamer that does not contain the binding sites can still play an essential role in the folding of the aptamer, so that the reduced aptamer could have a different specificity for instance. Finally, considering the size of the target molecule, a protein, I suppose that an aptamer between 40 and 90 bases, should be in the range of size of the molecule of interest. Good luck

Hello. Aptamer libraries are usually chosen in the range of 30 to 100 b. There are several factors to be taken into account, but a minimal length is usually necessary in order to have a good folding of aptamers, and thus to have more chance to generate binding sites. Then, it is usually not necessary to order a too long aptamer library, because of several things: what we want to obtain is usually a rather short fragment having one or two binding sites and a stem onto which can be fixed other molecules; and also because the costs of the DNA will increase a lot with the size of the DNA library requested. Also, what is possible to do is to shorten the aptamers afterwards, once the binding sites are defined. But it was sometimes observed that the rest of the aptamer that does not contain the binding sites can still play an essential role in the folding of the aptamer, so that the reduced aptamer could have a different specificity for instance. Finally, considering the size of the target molecule, a protein, I suppose that an aptamer between 40 and 90 bases, should be in the range of size of the molecule of interest. Good luck

Hello Prof Rousa

Thank you so muxh for the information. Further i have a couple of doubts with respect to the process itself. I have a purified protein which technically does not have any tags or links through which i can immobilize them. Which one would you recommend as a safe and better techniqueo for immobilization? Adding a biotin tag or a his tag or something else?

Hello. If you have a primary amine in your protein, then you can covalently bind it to carboxy-activated magnetic beads for instance, or other types; as there are many that can be used with primary amines and also secondary amines. Otherwise, a thiol group can be fixed onto thiol-activated beads, but the binding is not as strong as a C-C bond. Then, a biotinylated version of the protein will bind to streptavidin beads, but the binding is much weaker than a covalent binding and the biotinylated version can be expensive too. So, I suggest that you use some magnetic beads that can react with the functions you have in the protein, like amines. The biotin/streptavidin binding can be very interesting and very convenient to use later for SPR experiments for instance, because then, it is possible to fix the molecule of interest on the chip in a soft way, thus preventing destroying the machine with chemicals. I hope this will help you. Cheers

Wow. Thanks Elsa

That was definitely helpful. will try to get in touch if I have some more doubts. Thank you.

I hope that is okay