Journal of Lipid Research, Vol. 20, 663-668, July 1979
Copyright © 1979 by Lipid Research, Inc.

Effects of ionization and counterion binding on the surface areas of phosphatidic acids in monolayers

Gajanan S. Patil , Nancy J. Dorman , and David G. Cornwell

Department of Physiological Chemistry, The Ohio State University, Columbus, OH 43210

At 24-26°C, force-area isotherms show that unionized dipalmitoyl phosphatidic acid forms a solid-condensed film while unionized egg and dioleoyl phosphatidic acids form liquid-expanded films. Surface area is a characteristic feature of a specific phosphatidic acid and the purity of a phosphatidic acid preparation can be established by the surface area of the unionized phosphatidic acid (acid subphase) at 17 dynes/cm (castor oil piston). Ionized dipalmitoyl phosphatidic acid desorbs from a monolayer at a measurable rate while ionized egg and dioleoyl phosphatidic acids desorb too slowly for rate studies. The apparent surface pK₂ for dipalmitoyl phosphatidic acid, calculated from desorption rates, is 9.4. Surface areas of the phosphatidic acids expand with ionization. Solid dipalmitoyl phosphatidic acid films expand only in the pK₂ region, showing one inflection point which indicates that the K₁/K₂ ratio is less than 100 and that, as a consequence of this ratio, the apparent surface pK₁ is greater than 7.4. Liquid egg and dioleoyl phosphatidic acid films have two inflection points, expanding in both the pK₁ and pK₂ regions. The apparent surface pK₁ and pK₂ values, calculated from inflection points in surface area data, are 3.5 and 8.0, respectively. Film expansion with phosphatidate anions is less than anticipated, showing the presence of weak transient hydrogen bonds. Expanded phosphatidate anion films are condensed by alkaline earth cations. The Ca²⁺ and Ba²⁺ salts of completely ionized phosphatidic acids collapse from monolayers, showing that the phosphatidate anion may function as an ionophore for the transport of alkaline earth ions.—Patil, G. S., N. J. Dorman, and D. G. Cornwell. Effects of ionization and counterion binding on the surface areas of phosphatidic acids in monolayers.

Supplementary key words dioleoyl • dipalmitoyl • egg • Ca²⁺ • Ba²⁺ • desorption • collapse • hydrogen bonding • acid-soap • apparent surface pK

Submitted on January 10, 1979
Revised on March 19, 1979
Accepted on April 2, 1979

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