Chapter 4 - Phospholipid Monolayers
Introduction
Why study phospholipid monolayers? For most of the readers of this book a convincing motivation may be the membrane biophysical aspect. The monolayer, being half of a membrane, is a very well-defined planar system to study intermolecular interactions between lipids and also between lipids and proteins. This was also my perspective when entering the field a decade ago. However, I then realized the many interesting aspects of physics in two dimensions as well as some technological relevance. Hopefully the reader will also grasp some of these newer aspects which are related to areas of future research interest.
Most of the basic principles of a phospholipid monolayer are typical for other insoluble monolayers [1] and hence one may find many ideas now becoming fashionable already in Langmuir’s earlier work 2, 3. However, whereas these original ideas were based only on indirect experimental observation and thus were close to speculations there has been a tremendous development of experimental tools to investigate monolayer structure. These techniques have been to some extent applied to the best defined monolayers of glycerophosphatidyls with saturated aliphatic tails, and therefore this chapter will concentrate mostly on these. The discussion of these results will hopefully help the reader to conclude at least tentatively on other systems which are not explicitly mentioned here.
This chapter is organized as follows: The main body will contain a description of experimental and theoretical techniques. It will be shown how they were applied to phospholipids, what one could learn and where the limitations are. Being myself involved in some very recent developments I feel competent also to comment on future directions and improvement of our understanding. The next chapter will then briefly discuss theoretical developments. In a separate chapter the reader will find an extraction of our present knowledge on phases, phase transitions and on the structure at length scales between molecular and macroscopic dimensions. I will also try to correlate results on phospholipid monolayers with those on other surfactant films which will lead to the elaboration of general physical principles and also suggest extrapolation to other phospholipids and complexer systems not yet studied as extensively.
Section snippets
Thermodynamic measurements
The very first and most simple measurement to characterize a surfactant monolayer is that of the lateral pressure π as a function of molecular area A. π is measured as the difference in surface pressure comparing the value in the absence (π0) and presence (π1) of surfactant at the surface [4]
Since π is the derivative of the surface free energy with respect to the intrinsic variable A it is a thermodynamic variable.
Measuring π versus A, the so-called pressure/area (π, A)-isotherms, one
Theoretical calculations
Stimulated by the possibilities of more refined structural analysis and by the interest in different aspects of low-dimensional systems there has been an increasing activity to theoretically describe surfactant films. These shall be summarized below classified not according to the tools used but with respect to the aims.
Thermodynamic equilibrium?
Attempts to describe a monolayer by equilibrium thermodynamics are frequently criticized since in most cases the system is not in thermodynamic equilibrium:
- (i)
The equilibrium spreading pressure generally amounts to at most a few mN/m, often to the pressure corresponding to the main phase transition [152]. Hence for higher lateral pressures the monolayer would prefer to form a three-dimensional crystal on the water surface. From this follows that generally the ordered monolayer phases are
System diversity
Above we have concentrated on showing basic features of well-defined phospholipid monolayers and will subsequently try to conclude on more complex systems. For these there do not exist too many structural data and the extrapolation therefore is not based on many facts.
Future outlook
This contribution has concentrated on our understanding of some of the most simple phospholipid systems, and I have to apologize with those colleagues working on complexer systems who are therefore not well represented here. The latter is unfortunate since the biologically more relevant systems are not the simple ones. However, lack of space and partly also of knowledge prevented me from simply listing the wealth of nature. Instead I wanted to spend more effort to elaborate on physical
Acknowledgement
In writing this chapter I have profited from numerous experimental contributions from my skillful and eager co-workers. However, this review would never have been completed without the competent and elaborate literature research by Andrea Dietrich and Ulrike Höhne, by the patient and careful preparation of this manuscript by Helga Resch.
References (209)
- et al.
Phase transitions in monolayers of saturated lipids. Exact results and Monte Carlo simulations
J. Colloid Interface Sci.
(1982) - et al.
Electrostatic interactions in phospholipid membranes, II. Influence of divalent ions on monolayer structure
J. Colloid Interface Sci.
(1989) - et al.
The miscibility of dipalmitoyl phosphatidylcholine and cholesterol in monolayers
J. Colloid Interface Sci.
(1976) - et al.
Impurity controlled phase transitions of phospholipid mono-layers
Eur. Biophys. J.
(1984) - et al.
Electrostatic interactions at charged lipid membranes, I. Effects of pH and univalent cations on membrane structure
Biophys. Chem.
(1976) Electrostatic potentials at membrane-solution interfaces
Curr. Top. Membr. Transp.
(1977)- et al.
The colloidal nature of phospholipid monolayers
J. Phys. (Paris)
(1987) - et al.
Surface films of vinyl stéarate, cetyl-vinyl ether, and their polymers
J. Colloid Interface Sci.
(1959) - et al.
Preferred conformation and molecular packing of phosphatidylethanolamine and phosphatidylcholine
Biochim. Biophys. Acta
(1981) - et al.
Local surface potentials and electric dipole moments of lipid monolayers: Contributions of the water/lipid and the lipid/air interfaces
J. Colloid Interface Sci.
(1988)et al.Thin Solid Films
(1989)
A fluorescence microscopic study concerning the phase diagram of phospholipids
Ber. Bunsenges. Phys. Chem.
Translational diffusion in phospholipid monolayers measured by fluorescence microphotolysis
Proc. Nat. Acad. Sci. USA
Surface-plasmon microscopy
Nature
Crystalline two-dimensional domains of cyanine dyes at interfaces
Chem. Phys. Lett.
Direct visualization of monolayers at the air-water interface by Brewster Angle Microscopy
J. Phys. Chem.
Fluorescence properties of Langmuir-Blodgett films of Chlorophyll a mixed with membrane lipids
Thin Solid Films
Effects of molecular organization on photophysica! and photochemical behavior: Quenching by subphase iodide of fluorescence in pyrene-bearing probes at the nitrogen-water interface
Thin Solid Films
Photobleaching measurements of the lateral diffusion of lipids and proteins in artificial phospholipid bilayer membranes
Translational molecular diffusion in phospholipid monolayers: Substrate coupling and phase transitions
J. Phys. (Paris)
Ellipsometry study of two-dimensional phase transitions
Phys. Rev.
Ellipsometric study of the physical states of phosphatidylcholines at the air-water interface
J. Phys. Chem.
Static and dynamic properties of pentadecanoic acid monolayers at the air-water interface
Langmuir
Ordering in lipid monolayers studied by synchrotron X-ray diffraction and fluorescence microscopy
Phys. Rev. Lett.
X-ray diffraction studies of organic monolayers on the surface of water
Phys. Rev. Lett.
Phospholipid monolayers between fluid and solid states
Biophys. J.
Nature of the thermal pretransition of synthetic phospholipids: Dimyristoyl- and dipalmitoyllecithin
Biochemistry
Structure of the Lb phases in a hydrated phosphatidylcholine multimembrane
Phys. Rev. Lett.
A refinement analysis of the crystallography of the phospholipid, 1,2-dilauroyl-DL-phosphatidylethanolamine, and some remarks on lipid-lipid and lipid-protein interactions
Proc. R. Soc. London
X-ray and neutron reflectivity from spread monolayers
Thin Solid Films
Insoluble Monolayers at Lipid-Gas Interfaces
The constitution and fundamental properties of solids and liquids, II. Liquids
J. Am. Chem. Soc.
Oil lenses on water and the nature of monomolecular expanded films
J. Chem. Phys.
Polymorphism of phospholipid monolayers
J. Phys. (Paris)
Phase transitions in insoluble one and two-component films at the air/water interface
Theoretical models for quasi-two-dimensional mesomorphic monolayers and membrane bilayers
Can. J. Phys.
Liquid-expanded to liquid-condensed transitions in lipid monolayers at the air/water interface
Langmuir
Diffusion limited growth of crystalline domains in phospholipid monolayers
J. Chem. Phys.
A mono- and bilayer study of homologous branched-chain lecithins
Liq. Cryst.
Correlations between chemical structure and chain packing in two- and three-dimensional systems
Makromol. Chem. Macromol. Symp.
Pressure-composition phase diagrams of cholesterol/lecithin, cholesterol/phosphatidic acid, and lecithin/phosphatidic acid mixed monolayers: A Langmuir film balance study
J. Colloid Interface Sci.
Critical mixing in monolayer mixtures of phospho-lipid and cholesterol
J. Phys. Chem.
Equilibrium and metastable states in lecithin films
Biophys. J.
Lipoid pH indicators as probes of electrical potential and polarity in micelles
J. Phys. Chem.
Electrical interactions in phospholipid layers
Thin Solid Films
Electric field effects on monolayers at the air-water interface
J. Chem. Soc. Faraday Symp.
Monolayer characteristics of some glycolipids at the air-water interface
Biochim. Biophys. Acta
Monolayer characteristics of some 1,2-Diacyl, l-Alkyl-2-Acyl and 1,2-Dialkyl phospholipids at the air-water interface
Biochim. Biophys. Acta
Hydrated polar groups in lipid monolayers: Effective local dipole moments and dielectric properties
Thin Solid Films
Two-dimensional chiral crystals of phospholipid
Nature (London)
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