If you read the previous post where we looked at a recent review on fluorous linkers, one of the nits picked was that the review really didn’t look at linkers as is in the title, since the fluorous domains on all of the molecules reviewed were terminal. While an analogy was attempted with Wang, Rink, and other types of linkers used commonly in solid phase supported synthesis, the reality was the fluorous domains didn’t really “link” anything. They were attachments. Calling these fluorous domains “linkers” didn’t change their utility or effectiveness in fluorous separations, obviously, but it is somewhat a misnomer.
Hot on the heels of that, however, is a recently available J. Org. Chem. paper from the University of Queensland and Institut Curie which actually does describe fluorous linkers in the true sense. The researchers’ goal was to design lipid monolayers using lipids with a His-tag binding polar head group. This nickel containing head group could then bind to proteins that had been labeled with a His-tag for 2-D crystallization purposes.
One problem with this approach is when one goes to study membrane proteins which are much more hydrophobic. The study of these proteins often requires large concentrations of surfactants, which can solubulize the lipid and destroy your monolayer. The researchers, therefore, decided to modify their lipids by adding a fluorous linker between the lipid tails and the polar head group. Fluorous domains are both hydrophobic and lipophobic and should lead to a lipid which is more resistant to detergent solubulization.
The structure shown above is one of the lipids that they ended up making and indeed it behaved as they anticipated. Much of the paper, however, is not about the physical attributes of these lipids, but rather describes the difficulties they encountered in making these molecules. The difficulties that they describe is one of the reasons fluorous domains have not been used much as true linkers. As seen in the reaction below they began with the fluorous alpha, omega-diol which they hoped to be able to monoprotect. Benzylation was “successfully” carried out in 41% yield using NaH in DMF in the presence of a tetrabutylammonium iodide under sonication. Not exactly the first conditions I would have tried for a seemingly simple alkylation. I’m sure that there were a lot of frustrations leading up to that modest result. Alkylation of the other oxygen didn’t prove much better.
The worst of it came, however, when they tried to conduct an acylation reaction on the lipid-fluorous-carboxylic acid molecule to finally attach the His-tag binding piece. The authors cite a “modest” yield in this reaction, and by modest they mean a lousy 28%, which I’m sure was the best they got.
This is no knock on the researchers, either. These are just difficult molecules to deal with. Trust us, we know. The presence of the electron withdrawing fluorous chains, surely diminishes the nucleophilicity of the oxygen anions. They might be better served trying to use a fluorous diol with a propylene spacer between the CF2 and the hydroxyl group rather than a methylene group. Those aren’t commercially available, however, so how you make them is the next problem.
Now you have an idea why true fluorous linkers are not that common. The chemistry can be tough. (Note: This isn’t true for most fluorous chemistry. In general, the tags are fully soluble and the chemistry quite straightforward. ) However, for applications such as this one which uses very small amounts in a high value application, these molecules could have great utility. The efforts and the struggles that Hussein et al went through to get these molecules is greatly appreciated.