2H NMR detection of transmembrane potential-driven tetraphenylphosphonium transbilayer redistribution

2H NMR of specifically choline-deuterated phosphatidylcholine incorporated into giant unilamellar vesicles (GUVs), composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) plus 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) plus cholesterol (CHOL), was shown to detect a transmembrane potential-driven redistribution of the potential-sensitive, surface- binding dye tetraphenylphosphonium (TPP+) across the GUV lipid bilayer. The method is based on resolving differences in the surface charge at the inner versus the outer monolayer of the vesicle's bilayer using the so-called 2H NMR "molecular voltmeter" technique. A mathematical model to describe the 2H NMR results was derived by combining the Nernst, Boltzmann, Langmuir, and Gouy-Chapman equations with the established sensitivity of deuterium quadrupolar splittings from choline- deuterated POPC to surface electrostatic charge effects. This model identified experimental factors likely to yield enhanced sensitivity and resolution of the inner versus outer monolayer surface charges via 2H NMR. The predictions of the model were then confirmed experimentally. The improvement in resolution resulting from these studies removes a major hindrance to the general exploitation of 2H NMR for monitoring transbilayer surface charge asymmetrics.