The lipid bilayer concept: Experimental realization and current applications

H.T. Tien; A. Ottova    Physiology Department, Biomedical and Physical Sciences Building Michigan State University, East Lansing, MI 48824 (USA)

1 INTRODUCTION

It may sound far fetched but it is nonetheless true that the concept of the lipid bilayer of cell or biological membranes began with the observations of R. Hooke of Hooke's law fame, who in 1672 coined the word ‘cell’ to describe the array of a cork slice under a microscope he constructed. Using the microscope, Hooke discovered ‘black holes’ in soap bubbles and films. Years later, Isaac Newton estimated the blackest soap film to be about 3/8 × 10−6 inch thick. Modern measurements give thickness between 5 and 9 nm, depending on the soap solution used.

Question: How does one embark from black soap films to black lipid membranes (BLMs) as models of biological membranes? Here we must go back in time to the early 1960s. A good starting point, perhaps, is the conference held in New York City.

1.1 Symposium on the Plasma Membrane

In December 1961, at the meeting sponsored by the American and New York Heart Association, when a group of unknown researchers reported the reconstitution of a bimolecular lipid membrane in vitro, the account was met with skepticism. Those present included some of the foremost proponents of the lipid bilayer concept, such as Danielli, Davson, Stoeckenius, Adrian, Mauro, Finean, and many others [1]. The research group led by Donald Rudin began the report with a description of mundane soap bubbles, followed by ‘black holes’ in soap films, etc. ending with an invisible ‘black’ lipid membrane, made from extracts of cow's brains. The reconstituted structure (60-90 Å thick) was created just like a cell membrane separating two aqueous solutions. The speaker then said:

As one member of the amused audience remarked, “… the report sounded like… cooking in the kitchen, rather than a scientific experiment!” That was in 1961, and the first report was published a year later [2]. In reaction to that report, Bangham, the originator of liposomes [3], wrote in a 1995 article entitled ‘Surrogate cells or Trojan horses’:

Indeed, the Rudin group, then working on the 9th floor of the Eastern Pennsylvania Psychiatric Institute (now defunct) in Philadelphia, Pa., was playing with soap bubbles with the ‘equipment’ purchased from the local toyshop [4]. While nothing unusual for the researchers at work, it must have been a curious and mysterious sight for the occasional visitors who happened to pass through the corridor of laboratories there! However, playing with soap bubbles scientifically has a long, respectable antiquity, as already mentioned in the introductory paragraph.

1.2 Origins of the Lipid Bilayer Concept

Nowadays it is taken for granted that the lipid bilayer comprises the fundamental structure of all biomembranes. The recognition of the lipid bilayer as a model for biomembranes dates back to the work of Gorter and Grendel published in 1925 [5]. However, the origin of the lipid bilayer concept is much older, and is traceable to black soap bubbles more than three centuries ago!

The early observation of ‘black holes’ in soap films by Hooke and Newton had a profound influence on the development of the lipid bilayer concept of biomembranes and its subsequent experimental realization in planar lipid bilayers and spherical liposomes. In this connection, there has been much discussion lately on self-assemblies of molecules, meaning the aggregation of molecular moieties into thermodynamically stable and more ordered structures. Without question, the inspiration for these exciting developments comes from the biological world, where, for example, Nature uses self-assembly, as a strategy to create complex, functional structures, such as viral protein coatings, and DNA, besides the above-mentioned lipid bilayer of cell membranes. Many researchers have reported self-assembling systems such as Langmuir-Blodgett monolayers and multi-layers [6]. A broad list of man-made, self-assembling systems involving amphiphilic molecules is given in Table 1.

1.3 Early experimental evidence for the lipid bilayer

Prior to the seminal work of Gorter and Grendel on the red blood cells (RBC), several pertinent questions had been raised:

• Was there a plasma membrane (Pfeffer, 1877)?

• What was the nature of the plasma membrane (Overton, 1890)?

• What is the meaning of membrane potential (Bernstein, 1900)?

• How thick is the plasma membrane (Fricke, 1925)?

• What is the molecular organization of lipids in the plasma membrane (Gorter and Grendel, 1925)?

Before answering the last posed question, let us digress for the moment to a different topic, namely, interfacial and colloid chemistry. It is no exaggeration that modern investigation of interfacial and colloid chemistry began in the kitchen sink! Over a span of 4 years (1891-1894), Agnes Pockels (a house wife working over the kitchen sink) reported in Nature (see Ref. [7]) how surface films could be enclosed by means of physical barriers to about 20 A2/molecule. To be more accurate historically, the first landmark is generally bestowed to Benjamin Franklin, who in 1774 demonstrated that a teaspoonful of oil (olive ?) is able to calm a half-acre (~ 2000 m2) surface of a pond. Pockels' observation was followed...