Chemical and Magnetic Inequivalence of Glycerol Protons in Individual Subclasses of Choline Glycerophospholipids: Implications for Subclass-Specific Changes in Membrane Conformational States

High-resolution ¹H NMR spectroscopy and subsequent computer simulations of experimental spectra of multiple molecular species in each choline glycerophospholipid subclass demonstrated that (1) the two diastereotopic sn-3 methylene protons of the glycerol backbone in ether-linked phospholipids are more chemically and magnetically inequivalent than comparable protons present in conventionally studied phosphatidylcholine (e.g., Δδ = 0, 0.02, and 0.04 ppm for phosphatidylcholine, plasmenylcholine, and plasmanylcholine, respectively); (2) the two diastereotopic sn-1 methylene protons of the glycerol backbone in ether-linked phospholipids are more chemically and magnetically equivalent than their counterparts in phosphatidylcholine (e.g., Δδ = 0.26, 0.07, and 0.04 ppm and ²J_hh = 12.0, 11.5, and 10.8 Hz for phosphatidylcholine, plasmenylcholine, and plasmanylcholine, respectively); (3) the sn-2 methine proton is more shielded in ether-linked glycerophospholipids in comparisons to diacyl phospholipids; and (4) the vicinal spin coupling constants of protons in the glycerol backbone are distinct in each subclass. Analysis of this data utilizing a three staggered state conformational model demonstrated that modest alterations in the molecular geometry of the proximal portion of the sn-1 aliphatic chain in each choline glycerophospholipid subclass result in changes in the distribution of the conformational states of the glycerol backbone. Such alterations in the distribution of conformational states at the aqueous interface may be important determinants of the specific functional characteristics of subcellular membranes enriched in ether-linked phospholipids.