As written in the beginning of the project, "[t]he ultimate goal of this blog is to find an atomistic (preferably united-atom) force field that reproduces the experimental properties discussed in the manuscript [headgroup and glycerol backbone order parameters and their responses to ions, dehydration and cholesterol]. Naturally the optimal situation would be that some of the already available force fields would fulfill this goal. If this, however, turns out not to be the case, the goal will be to find the appropriate modifications."
In NMRlipids projects I and II it turned out that none of the available force fields fully satisfied these goals. Especially the ion binding affinities and detailed structure of the glycerol backbone and headgroup posed major challenges for the current force fields. On the other hand, qualitative response to dehydration and bound charge were well reproduced by all the models.
The attempts to improve force field parameters by Antti Lamberg in March 2014 revealed the importance of signs and stereospecifity of the order parameters. The necessary details are now reviewed in NMRlipids V publication and the development process can be resumed using better defined experimental numbers. Inspired by this, we have made concrete plans to expand the NMRlipids project towards systematic force field improvement and have build a prototype of an automatic force field quality assessment tool. However, we are still looking for ways to organize appropriate human resources to run this extension of the project.
Samuli Ollila has now received IOCB fellow funding from Institute of Organic chemistry and Biochemistry in Prague, Czech Rebublic to focus on ion interactions with zwitterionic and charged lipid bilayers in Pavel Jungwirth's group. For this reason, the main focus of NMRlipids IV will be in charged lipids and their interactions with ions (new post will follow soon). Also NMRlipids VI, to develop PC lipid model with correct Ca2+ and Na+ binding behaviour, will be lauched in the near future.