Spiers memorial lecture: Experimental discovery of asymmetric bilayers, and a recent asymmetry example

This introductory <i>Faraday Discussions</i> lecture considers the wide range of subjects that involve asymmetric bilayer membranes. The <i>Discussions</i> organizers have chosen four themes that describe the current interest in asymmetric bilayers. The main part of this introductory lecture starts with the discovery from over 50 years ago that the plasma membrane of mature, human erythrocytes has different composition and properties in each leaflet of the plasma membrane. We also comment on what is newly recognized in the asymmetric membrane field. The asymmetric bilayer is a remarkable state of matter. Newly recognized, we can describe it as a "new state of matter". Like other biological matter, it evolved to be in its particular form. There is new appreciation for the properties of the asymmetric bilayer, and there has been progress in understanding these emerging properties. Asymmetric lipid bilayers have a remarkable range of characteristics that involve the physically and chemically special connection of the two different monolayer leaflets. Much of this work is an explanation of how the "van Deenen researchers", over 50 years ago, quantitatively measured the phosphatidylcholine, sphingomyelin, and phosphatidylethanolamine content of the erythrocyte plasma membrane exoplasmic leaflet. Their use of surface chemistry principles was important in proper quantitation, and this introductory lecture explains these research findings in detail. We also present a detailed discussion of one remarkable example of a newly-discovered behavior of asymmetric bilayers, which we term "induced order". This discovery has an absolute requirement for microscopy image data to reveal the superposition of induced order with the thermodynamic order of liquid-ordered + disordered phase separation.