The molecular electrometer at 40

In 1987 Seelig and colleagues proposed that the phosphocholine headgroup of phosphatidylcholine behaved as a universal sensor of surface electrostatic charge, both cationic and anionic, in lipid bilayers (J. Seelig, P.M. Macdonald, P.G. Scherer, Phospholipid Head Groups as Sensors of Electric Charge in Membranes. Biochemistry, 26 (1987) 7535-7541.) Changes in the deuterium NMR quadrupolar splitting measured with specifically-deuterated positions within the choline headgroup in response to surface charges were attributed to a conformational change within the phosphocholine group corresponding to a "tilt" of the choline group towards or away from the direction of the bilayer normal as the PN dipole sought to align with the surface electrostatic field. In the ensuing nearly 4 decades this so-called "Molecular Electrometer" concept has become accepted doctrine in membrane science and has been employed to examine lipid bilayer surface electrostatics in a host of situations involving multiple membrane- associating biologically significant factors from ions, to anesthetics, to peptides and proteins. In this review, I describe the history of the science behind the Molecular Electrometer, the evolution of methods for examining the Molecular Electrometer response and provide a survey of its application in the myriad instances of membrane-associating molecules affecting and being affected by surface electrostatics. Lastly, I include an overview of the efforts of molecular dynamics simulations to be guided by and to account for the Molecular Electrometer effect in simulations of lipid bilayers.