by Jim Caryl
[Promoted from my Posterous ‘Overflow‘ blog…]
LATELY I have been wresting with a particular problem in the lab. I have been trying to put a rather complex DNA molecule down on a gold surface with the aim of having it perform the same chemistry on a surface as it does in solution.
Generally, when putting DNA down on a gold surface, we synthesise our DNA with a particular chemical modification, a thiol, on one end of the DNA strand. A thiol is essentially a sulphur atom, usually together with a hydrogen (-SH), and sulphur forms a strong bond with gold, so this is we want.
Unfortunately, we live in a very oxidizing world (basically, everything rusts), so my thiol gets oxidized to a rather less useful S=O. There are numerous other atoms that can reduce my thiol too, such as other thiols, or metals such as magnesium, zinc, copper etc.
This isn’t generally a problem as an oxidized thiol can be reduced back to -SH, making it ready to react with my gold surface.
Only, it turns out that my DNA adheres to the gold at too great a density. Like a mosh pit at a concert, the poor blighters are unable to move, and this results in them being unable to perform the particular reaction that they normally manage when not constrained.
So I modify my approach and use another layer between the gold and the DNA. This layer can be used to dictate how much of the gold surface is actually available to be bound. The chemistry is quite complicated, with several steps, all of which degrade quite rapidly, so it has me running around to make sure I get all the steps completed in time.
First a layer of long floppy carbon chains go down on the gold, and the ends of these molecules are reacted with another chemical that makes them able to react with yet another chemical, this latter chemical being capable of reacting with a thiol. Phew.
I have spent more time than I care to mention trying to get this approach up and running, all the while having to deal with the fiddly, slow and rather cumbersome thiol-based chemistry, which has a propensity to rust.
Then it occurs to me. The only reason I was using a thiol on my DNA in the first place was to stick my DNA directly to a gold surface. As I’ve already determined that we can’t do this (too dense), I really don’t know why I’ve spent time running around testing numerous crosslinking agents that can link the surface to my thiol-DNA.
Time to ditch the thiol and use something that connects DIRECTLY to the first layer of floppy carbon chains.