Investigating internal structural differences between micelles and unilamellar vesicles of decanoic acid/sodium decanoate

Dynamics of the vesicles composed of fatty acids and other amphiphile mixtures: unveiling the role of fatty acids as a model protocell membrane

Fundamental research at the interface of chemistry and biology has the potential to shine light on the question of how living cells can be synthesized from inanimate matter thereby providing plausible pathways for the emergence of cellular life. Compartmentalization of different biochemical reactions within a membrane bound water environment is considered an essential first step in any origin of life pathway. It has been suggested that fatty acid-based vesicles can be considered a model protocell having the potential for change via Darwinian evolution. As such, protocell models have the potential to assist in furthering our understanding of the origin of life in the laboratory. Fatty acids, both by themselves and in mixtures with other amphiphiles, can form different self-assembled structures depending on their surroundings. Recent studies of fatty acid-based membranes have suggested likely pathways of protocell growth, division and membrane permeabilisation for the transport of different nutrients, such as nucleotides across the membrane. In this review, different dynamic processes related to the growth and division of the protocell membrane are discussed and possible pathways for transition of the protocell to the modern cell are explored. These areas of research may lead to a better understanding of the synthesis of artificial cell-like entities and thus herald the possibility of creating new form of life distinct from existing biology. Graphical Abstract Table of Content (TOC) only.

Self-Assembly of Amphiphiles into Vesicles and Fibrils: Investigation of Structure and Dynamics Using Spectroscopy and Microscopy Techniques

Amphiphiles are a class of molecules which are known to assemble into a variety of nanostructures. The understanding and applications of self-assembled systems are based on what has been learned from biology. Among the vast number of self-assemblies, in this article, we have described the formation, characterization, and dynamics of two important biologically inspired assemblies: vesicles and fibrils. Vesicles, which can be classified into several categories depending on the sizes and components, are of great interest due to their potential applications in drug delivery and as nanoscale reactors. The structure and dynamics of vesicles can also mimic the complex geometry of the cell membrane. On the other hand, the self-assembly of proteins, peptides, and even single amino acids leads to a number of degenerative disorders. Thus, a complete understanding of these self-assembled systems is necessary. In this article, we discuss recent work on vesicular aggregates composed of phospholipids, fatty acids, and ionic as well as nonionic surfactants and single amino acid-based fibrils such as phenylalanine and tyrosine. Beside the characterization, we also emphasize the excited-state dynamics inside the aggregates for a proper understanding of the organization, reactivity, and heterogeneity of the aggregates.

Proton Transfer Pathways of 2,2'-Bipyridine-3,3'-diol in pH Responsive Fatty Acid Self-Assemblies: Multiwavelength Fluorescence Lifetime Imaging in a Single Vesicle

Fatty acids are known to form different supramolecular aggregates in aqueous solutions depending on the pH of the medium. The dynamics of the transformation of oleate micelles into oleic acid/oleate vesicles has been investigated using a pH-sensitive intramolecular proton transfer fluorophore, 2,2'-bipyridine-3,3'-diol [BP(OH)₂]. Different prototropic forms of BP(OH)₂ exist in different pH values of the system, and thus, the ground state and the excited state dynamics of BP(OH)₂ have been modulated in these confined media. The formation of different tautomeric forms of BP(OH)₂ in oleate micelles (at basic pH) is confirmed using time-resolved emission spectra and fluorescence anisotropy measurements. The hydrophobic environment provided by these assemblies reduces the water-assisted nonradiative decay channels and lengthens the fluorescence lifetime of BP(OH)₂. The rotational relaxation time in the micellar assembly is higher than that in the vesicle, which may be due to the higher microviscosity sensed by the fluorophore in the micelle. Besides, we have shown for the first time that BP(OH)₂ can be used as a membrane-bound fluorophore, using fluorescence lifetime imaging microscopy (FLIM). A broad distribution in the size of the vesicle is observed from the FLIM image. Further, we have used multiwavelength FLIM to collect the FLIM images of a single vesicle at different emission wavelengths, and the lifetime distribution obtained from the FLIM images at different emission wavelengths in a single vesicle correlates well with the lifetime values obtained from the ensemble average measurements in the bulk solution.

A comparison of energy flow in micelle and vesicle structures

We have investigated the vibrational population relaxation dynamics and state-dependent orientational relaxation behavior of perylene in micelles and vesicles formed using the same amphiphile(s). Decanoic acid and its conjugate base sodium decanoate can form either micelle or vesicle structures in aqueous solution depending on amphiphile concentration and solution pH. The issue of interest in this work is whether or not different assemblies of a given amphiphile manifest different efficiencies with the dissipation of energy. Vibrational population relaxation data show that initial energy flow from the chromophore to the amphiphile aliphatic chains is more efficient in micelles than in vesicles. Longer time scale relaxation, gauged by transient local heating induced by the dissipation of excess energy from perylene shows that the local environment formed by micelles experiences greater temperature change than the local environment formed by vesicles. This finding suggests that the strength of coupling between the bath and the amphiphiles differs for the two structural motifs.

Systematical characterization of phase behaviors and membrane properties of fatty acid/didecyldimethylammonium bromide vesicles

Fatty acids (FAs) are known to form vesicle structures, depending on the surrounding pH conditions. In this study, we prepared vesicles by mixing FAs and a cationic surfactant, and then investigated their physicochemical properties using fluorescence spectroscopy and dielectric dispersion analysis (DDA). The assemblies formed from oleic acid (OA) and linoleic acid (LA) were modified by adding didecyldimethylammonium bromide (DDAB). The phase state of FA/DDAB mixtures was investigated with pH titration curves and turbidity measurements. The trigonal diagram of FA/ionized FA/DDAB was successfully drawn to understand the phase behaviors of FA/DDAB systems. The analysis of fluidities in the interior of the membrane with use of 1,6-diphenyl-1,3,5-hexatriene (DPH) indicated that the membrane fluidities of OA/DDAB and LA/DDAB at pH 8.5 slightly decreased in proportion to the molar ratio of DDAB in FA/DDAB systems. The fluorescent probe 6-lauroyl-2-dimethylamino naphthalene (Laurdan) indicated that the LA vesicle possessed a dehydrated surface, while the OA vesicle surface was hydrated. Modification of LA vesicles with DDAB induced the hydration of membrane surfaces, whereas modification of OA vesicles by DDAB had the opposite effect. DDA analysis indicated that the membrane surfaces were hydrated in the presence of DDAB, suggesting that the surface properties of FA vesicles are tunable by DDAB modification.

Constituent-dependent liposome structure and organization

We have used steady state and time-resolved fluorescence spectroscopy in concert with TEM to study organization and dynamics of molecules comprising liposomes, discoidal micelles, and spherical micelles. The lipid aggregates contained controlled amounts of lipids with headgroups modified with a thiol-terminated polyethylene glycol (thio-PEG lipids) and a small amount of 1-palmitoyl-2-(pyrene-1-yl)decanoyl-sn-glycero-3-phosphocholine (pyrene tethered DPPC), pyrene, or perylene as spectroscopic probes. The maximum diameter of the lipid aggregates was controlled by the polycarbonate filter pore size used in the extrusion process. The concentration of thio-PEG lipid in the aggregates determines not only the shape of the lipid assemblies but also the organization of the molecules within the assembly. Fluorescence lifetime and anisotropy decay data show that the immediate environment of pyrene tethered DPPC changes with the addition of thio-PEG lipid. In contrast, the dynamics of free chromophore (perylene) are insensitive to the addition of thio-PEG lipid. The addition of thio-PEG lipid to the lipid assembly produces changes in organization that are most pronounced in the lipid headgroup region. Reorientation dynamics of perylene show that the organization of the lipid bilayer acyl chain region is affected little by the addition of thio-PEG lipid and consequent macroscopic changes in the morphology of the lipid assemblies.

An ESR characterization of micelles and vesicles formed in aqueous decanoic acid/sodium decanoate systems using different spin labels

Aqueous decanoic acid/sodium decanaote systems were studied as a function of pH and concentration, up to 0.3 M decanoic acid/sodium decanoate, by electron spin resonance (ESR) spectroscopy using three different amphiphilic spin labels. The distribution of the spin labels between vesicles and micelles as well as their dynamic properties were determined by quantitative analysis of the ESR spectra using two novel simulation software packages. Rotational correlation time of the labels in micelles was found to increase with decreasing pH, with substantial increase in the region where vesicles were formed (7.8<pH<8). In the interval 6.5<pH<7.8, the coexistence of vesicles and micelles was observed. Presence of vesicles was confirmed by the captured aqueous volume, determined independently with a hydrophilic spin label. The ESR measurements indicate that decanoic acid vesicle formation observed as the concentration is increased between 0.01 M and 0.03 M at pH 7.0 most likely occurs via the formation of micelles which remain in coexistence with the vesicles, even if the concentration is well above these values.