Strong H-Bonds Form Bilayers: Ochromonas Danica is an Extreme Example

Abstract

Anionic lipid bilayers provide the basic structure for nearly all living membranes. Anion-anion binding in head groups? I suggest a potent H-bond, a quantized H-bond (QHb). Biomembranes use glycerol-phospholipids with vicinal anionic phosphates. Maleate, 2-cis-succinate, is archetypical for a QHb. Its 2 carboxyls trap a proton between pH 2.1 to pH 6.2, contributing quantized bond energy to the molecule. Contrasting that with 2-trans-succinate (No H-bond) it contains an extra ∼15kcal. The carboxyls are forced by the structure to share resonance with each other via the, “perhaps vibrating,” proton. Albeit in a narrow pH range, oleic acid and other unsaturated fatty acids do likewise. Here I report the structure of another anionic natural membrane, the chlorosulfolipid bilayer of Ochromonas danica. Its headgroups are QHb sulfates forced together by the hydrophobic chains of the bilayer. O. danica is an acidophilic (pH 4.3), fresh water alga in acid bogs. Its plasma membrane has two sets of single chain polar lipids: 2, 2, 11, 13, 15, 16, hexachloro-docosane-1,14-disulfate (>80%) and 2, 2, 12, 14, 16, 17-hexachloro-docosane-1, 15-disulfate, (<20%). Its C1-sulfates and C14/C15-sulfates are each arranged in sheets of sulfates within each monolayer. The latter sulfate sheet is made of sulfates separated by QHbs enclosed in a hydronium ion cage in the low dielectric. The chlorogroups bind hydronium ions so strongly that the CL-H3O+ bond has 0.9 the strength of a covalent bond. They are arranged along the hydrocarbon chains to lead hydroniums toward the poly-sulfate cage. They enter the bilayer by chloro pair at C2, and are then passed down to the water cages. Both primary and secondary sulfate sheets are held together by QHbs, the strength of which prevent osmotic bursting. With phospholipid bilayers such H-bonds would be moving on a nanosecond timescale.