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The formation of phosphatidic acid (PA) from lysophosphatidic acid (LPA), diacylglycerol, or phosphatidylcholine plays a key role in the regulation of intracellular membrane fission events, but the underlying molecular mechanism has not been resolved. A likely possibility is that PA affects local membrane curvature facilitating membrane bending and fission. To examine this possibility, we determined the spontaneous radius of curvature (R_0p) of PA and LPA, carrying oleoyl fatty acids, using well-established X-ray diffraction methods. We found that, under physiological conditions of pH and salt concentration (pH 7.0, 150 mM NaCl), the R_0p values of PA and LPA were −46 Å and +20 Å, respectively. Thus PA has considerable negative spontaneous curvature while LPA has the most positive spontaneous curvature of any membrane lipid measured to date. The further addition of Ca²⁺ did not significantly affect lipid …