Iminothiol/thiourea tautomeric equilibrium in thiourea lipids impacts DNA compaction by inducing a cationic nucleation for complex assembly

Our research on lipidic vectors for transfection led us to develop thiourea lipids able to interact with DNA. Hence, we developed a series of lipopolythioureas based on the strong hydrogen bond donor ability of thiourea. More recently we have reported a branched hydroxylated bis-thiourea derivative with interesting transfecting properties. The last step of the syntheses involved a strong acidic condition, leading to an unstable product upon storage. Therefore we designed a new synthesis in mild acidic conditions. Though they exhibit the same mass, the lipids obtained in the two different conditions differ by their interaction with DNA. We therefore explored the physicochemical properties of these two lipids by different means that we describe in this article. In order to insure easier and reliable 13C-NMR studies of the thiourea group we have designed the synthesis of the corresponding 13C-labeled thiourea lipids. We have thus shown that when the lipid was submitted to mildly acidic medium; only the thiourea group was observed; while a thiourea/charged and/or uncharged iminothiol tautomeric equilibrium formed when the last step of the synthesis was submitted to low pH. NMR experiments showed that this tautomeric equilibrium could not form in polar solvents. However, UV experiments on the liposomal form of the lipopolythiourea showed the presence of the tautomers. Lipid/DNA interaction consequently differed according to the acidic treatment applied. Eventually, these results revealed that on this particular thiourea lipid, electrostatic interactions due to cationic thioureas are likely to be responsible for DNA compaction and that this tautomeric form of the thiourea could be stabilised by hydrogen bonds in a supramolecular assembly. Nevertheless, this does not reflect a general thiourea lipid/DNA interaction as other thiourea lipids that are able to compact DNA do not undergo an acidic treatment during the final stage of their synthesis.