Ionization Behavior of Polyphosphoinositides Determined via the Preparation of pH Titration Curves Using Solid-State 31P NMR

Detailed knowledge of the degree of ionization of lipid titratable groups is important for the evaluation of protein–lipid and lipid–lipid interactions. The degree of ionization is commonly evaluated by acid–base titration, but for lipids localized in a multicomponent membrane interface this is not a suitable technique. For phosphomonoester-containing lipids such as the polyphosphoinositides, phosphatidic acid, and ceramide-1-phosphate, this is more conveniently accomplished by ³¹ P NMR. Here, we describe a solid-state ³¹ P NMR procedure to construct pH titration curves to determine the degree of ionization of phosphomonoester groups in polyphosphoinositides. This procedure can also be used, with suitable sample preparation conditions, for other important signaling lipids. Access to a solid-state, i.e., magic angle spinning, capable NMR spectrometer is assumed. The procedures described here are valid for a Bruker instrument, but can be adapted for other spectrometers as needed.