Conformational structure of glycerol trivalerate and its relation to phospholipids: studies by NMR and potential energy calculations

Abstract

The structure of a model lipid (glycerol trivalerate, GTV) has been investigated by ¹H and ¹³C NMR, and energy calculations based on classical potential functions (CPF). The structure of glycerol-ester pivot in GTV is characterized by a dynamic equilibrium between two dominant conformations. One of the two conformations (SA) is similar to the reported crystal structure of triglycerides β-triaurin and β-tricaprin and has the following conformational angles: θ₁ = 52°, θ₃ = 46#176;, α₁ = 208#176;, β₁ = 156° and γ₁ = 186°. The other conformer (SB) is similar to the reported crystal structure of β and γ chains in 1,2-dilauroyl-D,L-phosphatidylethanolamine and has the conformational angles: θ₁ = 287°, θ₃ = 155°, α₁ = 195°, β₁ = 85° and γ₁ = 194°. The ester groups acquire a rigid planar conformation (α₂, β₂, γ₂ = 180°), while the potential energy curves for rotation around the chemical bonds O---CO---C---C---C(α₃, β₃ and γ₃) indicate a high degree of flexibility. The hydrocarbon chains show a distribution of the gauche and trans conformations around C---C---C---C bonds with a preference for the trans arrangement by almost 0.8 kcal/mol. The conformational picture of the hydrocarbon chains and glycerol moiety in GTV is very similar to that in phospholipids. These results indicate caution in using the reported crystal structure of 1,2-dilauroyl-D,L-phosphatidylethanolamine as the only model for the organization of lipid molecules in biological membranes.