Surfactant Assemblies and their Various Possible Roles for the Origin(S) of Life

Abstract

A large number of surfactants (surface active molecules) are chemically simple compounds that can be obtained by simple chemical reactions, in some cases even under presumably prebiotic conditions. Surfactant assemblies are self-organized polymolecular aggregates of surfactants, in the simplest case micelles, vesicles, hexagonal and cubic phases. It may be that these different types of surfactant assemblies have played various, so-far underestimated important roles in the processes that led to the formation of the first living systems. Although nucleic acids are key players in the formation of cells as we know them today (RNA world hypothesis), it is still unclear how RNA could have been formed under prebiotic conditions. Surfactants with their self-organizing properties may have assisted, controlled and compartimentalized some of the chemical reactions that eventually led to the formation of molecules like RNA. Therefore, surfactants were possibly very important in prebiotic times in the sense that they may have been involved in different physical and chemical processes that finally led to a transformation of non-living matter to the first cellular form(s) of life. This hypothesis is based on four main experimental observations: (i) Surfactant aggregation can lead to cell-like compartimentation (vesicles). (ii) Surfactant assemblies can provide local reaction conditions that are very different from the bulk medium, which may lead to a dramatic change in the rate of chemical reactions and to a change in reaction product distributions. (iii) The surface properties of surfactant assemblies that may be liquid- or solid-like, charged or neutral, and the elasticity and packing density of surfactant assemblies depend on the chemical structure of the surfactants, on the presence of other molecules, and on the overall environmental conditions (e. g. temperature). This wide range of surface characteristics of surfactant assemblies may allow a control of surface-bound chemical reactions not only by the charge or hydrophobicity of the surface but also by its "softness". (iv) Chiral polymolecular assemblies (helices) may form from chiral surfactants. There are many examples that illustrate the different roles and potential roles of surfactant assemblies in different research areas outside of the field of the origin(s) of life, most importantly in investigations of contemporary living systems, in nanotechnology applications, and in the development of drug delivery systems. Concepts and ideas behind many of these applications may have relevance also in connection to the different unsolved problems in understanding the origin(s) of life.

Visualization of the spontaneous emergence of a complex, dynamic, and autocatalytic system

“…Autocatalysis occurs in these reactions because the product aggregates take organic precursor molecules into the aqueous phase, allowing more efficient mixing of the reaction components and thereby increasing the rate of reaction. Understanding the dynamics of individual lipid aggregates during growth and division is a long-standing problem in the field of prebiotic chemistry (3-7) because vesicles are widely thought to have compartmentalized and catalyzed reactions in the prebiotic world (8)(9)(10). A full understanding of these dynamics has not yet been achieved in large part owing to analytical limitations.…”

Interactions between Catalysts and Amphiphilic Structures and their Implications for a Protocell Model

“…However, it should be noted that the type and complexity of the catalytic assemblies anchored in vesicle bilayers will be limited compared to the complexity of a transcription-translation apparatus sometimes encapsulated in vesicular and liposomal protocell models. [19,22] To further develop this protocell model, it is necessary to include fully inheritable informational molecules into the catalytic reaction instead of the 8-oxoguanine currently in use. This study proves that when vesicles are present, it should be possible to separate the light harvesting/photosensitizer ruthenium complex from replicable nucleic acid polymers as long as they are colocalized using a lipophilic anchor while preserving catalytic activity…”

Life Began When Evolution Began: A Lipidic Vesicle-Based Scenario

“…These compounds, which are amphiphilic or hydrophobic molecules, were probably abundant (Segré et al 2001;Rasi et al 2004) and their mixtures were likely to compose the membranes of relatively stable vesicles (Namani and Deamer 2008). Experimentally, such vesicles are shown to be able to self-reproduce Luisi et al 2004;Walde 2006). However, these vesicles are not able to evolve for lack of some stabilized structural/functional membrane properties independent of the environmental parameters.…”