A Monolayer Model Study of Surface Tension-Associated Parameters of Membrane Phospholipids: Effect of Unsaturation of Fatty Acyl Tails

Degree of unsaturation of sn-2 located fatty acyl side-chainsin typical membrane phospholipids has a marked effect on surfaceten si on-associated parameters of monolayers of these lipidsover aqueous sub-phases. For a fixed area monolayer in a completelyexpanded state, an increase in the number of cis double bondswas found to cause a concomitant increase in surface tension.For the same fatty acids incorporated into phosphatidylcholinemolecules, surface pressure/molecular area isotherms show thatsn-2 linoleoyl monolayers manifest markedly higher surface pressuresthan sn-2 oleoyl ones in the fully compressed state. Furthermore,the isotherms and monolayer collapse points indicate a greaterrigidity of the oleoyl species monolayer. The strong correlationof these effects with molecular radius, rotational diffusionconstant and total molecular peripheral properties of the monolayermay be determined by rotational micro viscosity experiencedby the molecules. This in turn is determined by molecular radiusthrough the total molecular peripheral length. It is suggestedthat a contributing factor to membrane bioregulation may besuch changes in surface tension-associated parameters arisingfrom the structure of the unsaturated fatty acyl phospholipidside-chains. In a biological membrane the surface tension would,therefore, be an average of headgroup effects and of degreeof fatty acyl side-chain unsaturation. Key words: Fatty acyl unsaturation, membrane, phosphatidylcholine