Charge pairing of headgroups in phosphatidylcholine membranes: A moleculardynamics simulation study

Molecular dynamics simulation of the hydrated dimyristoylphosphatidylcholin e (DMPC) bilayer membrane in the liquid-crystalline phase was carried out f or 5 ns to study the interaction among DMPC headgroups in the membrane/wate r interface region. The phosphatidylcholine headgroup contains a positively charged choline group and negatively charged phosphate and carbonyl groups , although it is a neutral molecule as a whole. Our previous study (Pasenki ewicz-Gierula, M., Y. Takaoka, H. Miyagawa, K. Kitamura, and A. Kusumi. 199 7. J. Phys. Chem. 101:3677-3691) showed the formation of water cross-bridge s between negatively charged groups in which a water molecule is simultaneo usly hydrogen bonded to two DMPC molecules. Water bridges link 76% of DMPC molecules in the membrane. In the present study we show that relatively sta ble charge associations (charge pairs) are formed between the positively an d negatively charged groups of two DMPC molecules. Charge pairs link 93% of DMPC molecules in the membrane. Water bridges and charge pairs together fo rm an extended network of interactions among DMPC headgroups linking 98% of all membrane phospholipids. The average lifetimes of DMPC-DMPC association s via charge pairs, water bridges and both, are at least 730, 1400, and ove r 1500 ps, respectively. However, these associations are dynamic states and they break and re-form several times during their lifetime.