Models for phospatidylglycerol lipids put to structural test

Jérôme Hénin, Wataru Shinoda, and Michael L. Klein

J. Phys. Chem. B 113, 6958-6963 (2009).

Three atomistic, empirical models for phosphatidylglycerol (PG) lipids are tested against structural data in the crystal and liquid crystal states. Simulations of the anhydrous crystal of dimyristoyl-phosphatidylglycerol (DMPG) show that only the CHARMM force field describes the conformation and interactions of PG headgroups accurately. The other two models do not reproduce the native network of hydrogen bonds, suggesting the presence of biases in their conformational and nonbonded interaction properties. The CHARMM model is further validated in the biologically relevant liquid crystal phase by comparing experimental smallangle X-ray scattering spectra from DMPG unilamellar vesicles with data calculated from fluid bilayer simulations. The good agreement found in this model-free comparison implies that liquid crystal PG bilayers as described by CHARMM exhibit realistic bilayer thickness and lateral packing. Last, this model is used to simulate a fluid bilayer of palmitoyl-oleoylphosphatidylglycerol (POPG). The resulting view of POPG bilayer structure is at variance with that proposed previously based on simulations, in particular with respect to lateral packing of headgroups and the role of counterions.