Spontaneous formation of polypeptides in the interfacial thin films of amphiphilic amino acid esters: acceleration of the polycondensation and control of the structure of resultant polymers

Creation of High-Density and Low-Defect Molecular Films with a Flat-on Conformation by Interfacial Organization of Triphosphasumanene Trisulfides

This paper focuses on the formation of high-density, low-defect monolayers of triphosphasumanene trisulfides, which are newly synthesized electronic and geometric Janus-type molecules, in a flat-on conformation. Although the molecules stack easily because of the developed π-conjugated plane, their application as a metal coating in a flat-on conformation via an interfacial molecular film enables the work function to be tuned. Surface pressure-area isotherms of the triphosphasumanene trisulfides show a two-dimensional phase transition at the air/water interface. Atomic force microscopy observations of the transferred monolayer and in- and out-of-plane X-ray diffraction patterns of the corresponding multilayers reveal that this phase transition occurs from the flat-on to the end-on conformation. The X-ray diffraction patterns obtained in the two directions completely reversed before and after the phase transition, indicating that the molecular arrangement that is generated by layers of molecular films and resultant molecular stacking is similar. The flat-on conformation of the molecules was evident from the out-of-plane X-ray diffraction and polarized infrared spectroscopy results, which indicate that a large, low-defect monomolecular film is obtained using a toluene solution with a small diffusion coefficient. The spectroscopic results reveal triphosphasumanene trisulfide aggregation in the organized molecular film, suggesting high-density molecular packing.

Investigation of Spontaneous Polycondensation of N-(O, O-Ditetradecyl) Phosphorylalanine in Highly Ordered Films by Ftir Spectroscopy

Spontaneous polycondensation to polypeptides on both cast films and Langmuir-Blodgett films (LB films) of N-(O, O-ditetradecyl)phosphorylalanine (DNTP-L-Ala) was studied by FTIR transmission spectroscopy.

Control of Fine Structure in "Polymer Nanosphere Multilayered Organization" and Enhancement of Its Optical Property

This paper reports on a new functionality exhibited by "polymer nanosphere multilayered organization", a new type of molecular organization, and the relationship between their structure and function. The polymer nanosphere multilayered organization is a fine structural material formed by the accumulation of single-particle layers of a hydrophobic polymer at the air/water interface; these single-particle layers have uniform height along the c-axis. By employing the "alternate compression-relaxation method", high-density, low-defect particle layers are formed with a clear increase in their crystallite sizes. In the case of a ternary comb copolymer containing a carbazole ring, one particle is formed by the assembly of approximately 60 units of collapsed monolayer-like double layers. This structure is stabilized by the formation of side-chain crystals in the interlayer, with oriented π-π stacking of carbazole rings, resulting in enhanced fluorescence emission intensity.

Dramatic shape modulation of surfactant/diacetylene microstructures at the air-water interface

Langmuir monolayers constitute a powerful platform for self-assembly and organization of amhiphilic molecules. Controlling the structural features of condensed domains formed within Langmuir monolayers, however, is a challenging task. The formation of remarkably diverse condensed microstructures is demonstrated in binary monolayers comprising of a surfactant (octadecylmelamine) and a diacetylene monomer. The mole ratio between the two constituents and composition of the aqueous subphases (specifically pH and which dissolved metal ions are present) dramatically modulated the shapes and dimensions of microstructures formed at the air-water interface. The self-assembled microstructures could be transferred from the water surface onto solid substrates, and subsequently further served as templates for gold coating, yielding electrically conductive microwires.

Ocean-atmosphere interactions in the emergence of complexity in simple chemical systems

The prebiotic conversion of simple organic molecules into complex biopolymers necessary for life can only have emerged on a stage set by geophysics. The transition between "prebiotic soup," the diverse mixture of small molecules, and complex, self-replicating organisms requires passing through the bottleneck of fundamental chemistry. In this Account, we examine how water-air interfaces, namely, the surfaces of lakes, oceans, and atmospheric aerosols on ancient Earth, facilitated the emergence of complex structures necessary for life. Aerosols are liquid or solid suspensions in air with a broad, power law size distribution. Collectively, these globally distributed atmospheric particles have an enormous surface area. Organic films at the interface between water and air offer advantages for biomolecular synthesis compared with the bulk and can simultaneously participate in the folding of biopolymers into primitive enclosed structures. We survey the advantages of the water-air interface for prebiotic chemistry in a geophysical context from three points of view. We examine the formation of biopolymers from simple organic precursors and describe the necessity and availability of enclosures. In addition, we provide a statistical mechanical approach to natural selection and emergence of complexity that proposes a link between these molecular mechanisms and macroscopic scales. Very large aerosol populations were ubiquitous on ancient Earth, and the surfaces of lakes, oceans, and atmospheric aerosols would have provided an auspicious environment for the emergence of complex structures necessary for life. These prebiotic reactors would inevitably have incorporated the products of chemistry into their anhydrous, two-dimensional organic films in the three-dimensional fluids of the gaseous atmosphere and the liquid ocean. The untrammeled operation of natural selection on these aerosols provided the likely location where condensation reactions could form biopolymers by elimination of water. The fluctuating exposure of the large, recycling aerosol populations to radiation, pressure, temperature, and humidity over geological time allows complexity to emerge from simple molecular precursors. We propose an approach that connects chemical statistical thermodynamics and the macroscopic world of the planetary ocean and atmosphere.

In situ observation of peptide bond formation at the water-air interface

We report unambiguous spectroscopic evidence of peptide bond formation at the air-water interface, yielding a possible mechanism providing insight into the formation of modern ribosomal peptide bonds, and a means for the emergence of peptides on early Earth. Protein synthesis in aqueous environments, facilitated by sequential amino acid condensation forming peptides, is a ubiquitous process in modern biology, and a fundamental reaction necessary in prebiotic chemistry. Such reactions, however, are condensation reactions, requiring the elimination of a water molecule for every peptide bond formed, and are thus unfavorable in aqueous environments both from a thermodynamic and kinetic point of view. We use the hydrophobic environment of the air-water interface as a favorable venue for peptide bond synthesis, and demonstrate the occurrence of this chemistry with in situ techniques using Langmuir-trough methods and infrared reflection absorption spectroscopy. Leucine ethyl ester (a small amino acid ester) first partitions to the water surface, then coordinates with Cu(2+) ions at the interface, and subsequently undergoes a condensation reaction selectively forming peptide bonds at the air-water interface.

1H NMR spectroscopic investigations on the conformation of amphiphilic aromatic amino acid derivatives in solution: effect of chemical architecture of amphiphiles and polarity of solvent medium

In this study, the conformation of the amphiphilic lauryl esters of L-tyrosine (LET) and L-phenylalanine (LEP) in water and dimethyl sulfoxide is established. The alkyl chain protons of LEP in D(2)O appear at δ 1.010-1.398 and show an upfield shift and large line width, suggesting the proximity of the phenyl ring to the alkyl chain in contrast to that of LET. Quite interestingly, in DMSO-d(6), the (1)H NMR spectra of LET and LEP show a strong similarity that is suggestive of an orientation that positions the aromatic ring and aliphatic chain away from each other. These results are substantiated with two-dimensional nuclear Overhauser enhancement spectroscopy (2D NOSEY). Theoretical molecular models of the conformation at the interface corroborate the experimental findings. Investigations of the solvent polarity and chemical structure-dependent conformation are discussed.

Controlled assembly of silane-based polymers: chemically robust thin-films

We describe the controlled assembly of silane-based copolymers on various interfaces that have surface silanol groups. This assembly occurs as a result of the formation of very robust siloxane bonds (Si-O-Si) due to a condensation reaction between the alkoxysilane groups of the polymers and surface hydroxyl groups of the substrates. Deposition of these copolymers is not self-limiting; therefore, they could not be assembled into discrete monolayers. However, UV-visible data collected as a function of deposition cycle reveals a linear relationship, confirming the deposition of a constant amount of polymer in each deposition cycle. A linear variation of layer thickness with deposition cycles is also observed. The assembled polymer layers are found to be very robust and resistant even when exposed to piranha solution for several hours.

Amphiphilic lauryl ester derivatives from aromatic amino acids: significance of chemical architecture in aqueous aggregation properties

Lauryl esters of L-tyrosine (LET) and L-phenylalanine (LEP) were, in a previous interface adsorption study, found to adopt very different interfacial conformations. The present study is an investigation of their aqueous aggregation properties with the goal of elucidating the effects of the presence in LET and absence in LEP of the phenolic OH group on their aqueous aggregate structures and micellar conformations of the surfactant monomers. The measured properties included aggregation numbers from time-resolved fluorescence quenching (TRFQ), interface hydration index and microviscosity by electron spin resonance (ESR), chemical shifts of (1)H resonance lines by NMR, and Krafft temperatures and enthalpies of structural transitions by differential scanning calorimetry (DSC). The TRFQ, ESR, and NMR experiments were conducted at various temperatures from 23 to 70 degrees C for various surfactant concentrations from 0.050 to 0.200 M. Markedly different temperature dependences of aggregation number and (1)H NMR chemical shifts are exhibited by LET and LEP micelles. LET and LEP form ionic micelles. The aggregation number of LEP decreases as is characteristic of ionic micelles, but that of LET increases slightly with temperature. The changes with temperature in the NMR chemical shifts and width of the resonance lines are significantly greater for the various LEP protons than for those of LET. The differences in these properties and other fluorescence decay characteristics of fluorophores incorporated into the micelles could be attributed to the difference in the micellar conformations of LET and LEP which are postulated to be similar to that at oil-water interfaces. The phenolic group is hypothesized to be in the micelle-water interface as part of the headgroup in LET micelles, and its location does not change with temperature. On the other hand, in LEP micelles, the phenyl ring is folded into the core overlapping with the flexible hydrophobic chains. The resulting closer proximity between the phenyl ring and the flexible hydrocarbon chain causes interdependence of the phenyl ring and chain proton resonances, leading to the observed temperature dependence of the chemical shifts in LEP. The TRFQ and ESR data are combined together in a molecular space-filling model, referred to as the polar shell model, to derive the geometrical properties of the micelle. The DSC scans in the temperature range 10-55 degrees C showed the presence of distinctly different endotherms for LET and LEP. The Krafft temperatures, K(T), and the enthalpies were determined. The higher K(T) and broader peak of the DSC endotherm of LET as compared to LEP are attributed to the stabilization of fiberlike structures below the Krafft temperature due to its chirality and the hydrogen bonding capability of the phenolic OH and also to the ion-dipole interactions. Thus, all of the observed differences between LET and LEP could be attributed to the difference in their chemical architecture.

Racemic beta-sheets as templates of relevance to the origin of homochirality of peptides: lessons from crystal chemistry

The origin of life is a historical event that has left no relevant fossils; therefore, it is unrealistic to reconstruct the chronology of its occurrence. Instead, by performing laboratory experiments under conditions that resemble the prebiotic world, one might validate feasible reaction pathways and reconstruct model systems of artificial life. Creating such life in a test tube should go a long way toward removing the shroud of mystery over how it began naturally. The riddle of the appearance of natural proteins and nucleic acids--that is, biopolymers wholly consisting of homochiral subunits (L-amino acids and D-sugars, respectively)--from the unanimated racemic prebiotic world is still unsolved. There are two hypotheses concerning the sequence of their emergence: one maintains that long homochiral (isotactic) peptides must have been formed after the appearance of the first living systems, whereas the other presumes that such biopolymers preceded the primeval enzymes. The latter scenario necessitates, however, the operation of nonlinear synthetic routes, because the polymerization of racemates in ideal solutions yields chains composed of residues of either handedness. In this Account, we suggest applying lessons learned from crystal chemistry, in which molecules from isotropic media are converted into crystals with three-dimensional (3D) periodic order, to understand how the generation of homochiral peptides from racemic alpha-amino acids might be achieved, despite its seemingly overwhelming complexity. We describe systems that include the self-assembly of activated alpha-amino acids either in two-dimensional (2D) or in 3D crystals, followed by a partial lattice-controlled polymerization at the crystal-aqueous solution interface. We also discuss the polymerization of mixtures of activated hydrophobic racemic alpha-amino acids in aqueous solutions, as initiated by primary amines or thiols. The distribution of the diastereomeric oligopeptides was analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and MS/MS with monomers enantioselectively tagged with deuterium. The reaction performed in aqueous solutions encompasses the following sequential steps: (i) formation of a library of short racemic peptides enriched with isotactic diastereoisomers during the early stages of the polymerization, and (ii) self-assembly of oligopeptides into racemic beta-sheet colloidal-like aggregates that are delineated by enantiotopic sites or rims; these operate as templates (nuclei) for regio-enantioselective growth in the ensuing steps of chain elongation. Desymmetrization of the racemic mixtures of peptides was achieved with enantiopure alpha-amino acid esters as initiators. The enantiomeric excess of the isotactic peptides, not including the initiator, varies with chain length, the result of a cross-enantiomeric impeding mechanism. Our results suggest a feasible scenario in which primitive homochiral peptides might have emerged early in the prebiotic world.

Condensation properties of vesicles formed from an amphiphilic N-phosphorylamino acid

Stable unilamellar vesicles were formed in water under appropriate pH from dispersions of N-(O,O-di-n-hexadecyl)phosphorylalanine, an amphiphilic N-phosphorylamino acid. We found that condensation occurred in the vesicle solution after incubated at 40 degrees C, which may contribute to the stability of the vesicular system. Dipeptide derivative in the vesicle solution was identified by electrospray ionization mass spectrometry (ESI-MS), which suggests the peptide formation without any coupling reagents. Hydrogen bond and electrostatic interactions play important roles in the process of vesicle formation, while the suitable orientation and packing of the amphiphilic molecules at the vesicle/water interface together with certain conformational freedom in the vesicular bilayer are considered to be most favorable for the condensation in ordered systems as vesicles.

FT–IR Studies on Langmuir–Blodgett Films of Novel Phosphorus Amphiphiles: Spontaneous Polycondensation at the Air/Water Interface

A series of amphiphilic N-(O,O-dihexadecyl)phosphorylamino acids were designed and synthesised, and the transmission FT–IR spectroscopic investigation of their Langmuir–Blodgett films (LB films) on CaF2 substrates indicates the spontaneous polycondensation at the air/water interface before the deposition.

Oligopeptides with homochiral sequences generated from racemic precursors that spontaneously separate into enantiomorphous two-dimensional crystalline domains on water surface

The feasibility of generating oligopeptides with homochiral sequence via lattice-controlled polymerization of racemic mixtures of precursor molecules that undergo spontaneous segregation into two-dimensional (2-D) enantiomorphous domains at the air-aqueous solution interface was analyzed. For model systems, we studied the polymerization reaction within 2-D crystalline domains of mixtures of (R,S)-N(epsilon)-stearoyl-thio-lysine with approximately 10% (R,S)-N(epsilon)-stearoyl-lysine, and (R,S)-N(alpha)-carboxyanhydride of N(epsilon)-stearoyl-lysine. According to in situ grazing incidence X-ray diffraction (GIXD) measurements at the air-water interface, the molecules form 2-D crystallites packing by translation symmetry only. Oligopeptides 4-6 units long were obtained at the air-solution interface after injection of an appropriate catalyst into the subphase. The course of the chemical transformations was monitored by GIXD. The distribution of the diastereoisomeric oligopeptides was determined by matrix-assisted laser-desorption ionization time-of-flight (MALDI-TOF MS) mass spectrometry on samples prepared from precursor molecules enantioselectively labeled with deuterium. The experimental relative abundance of oligopeptides with homochiral sequence was found to be larger than that calculated for a theoretical random process, yielding an excess by a factor of 2.5-3.5 for the tetra- to hexapeptides. The present studies may be relevant for probing the role that might have been played by ordered clusters at interfaces for the generation of homochiral oligopeptides under prebiotic conditions.

Chiral amplification of oligopeptides in two-dimensional crystalline self-assemblies on water

Differences in the two-dimensional packing arrangements of racemic and enantiomeric crystalline self-assemblies on the water surface of amphiphilic activated analogs of lysine and glutamic acid have been used to prepare oligopeptides of homochiral sequence and oligopeptides of single handedness from chiral nonracemic mixtures. The crystalline structures on the water surface were determined by grazing incidence x-ray diffraction and the diastereomeric composition of the oligopeptides by matrix-assisted laser desorption time-of-flight mass spectrometry with enantio-labeling. These results suggest that reactivity of ordered clusters at interfaces might have played a role in the generation of early homochiral biopolymers.