Base Complementarity and Nucleoside Recognition in Phosphatidylnucleoside Vesicles

Programmed Recognition between Complementary Dinucleolipids To Control the Self-Assembly of Lipidic Amphiphiles

One of the major goals in systems chemistry is to create molecular assemblies with emergent properties that are characteristic of life. An interesting approach toward this goal is based on merging different biological building blocks into synthetic systems with properties arising from the combination of their molecular components. The covalent linkage of nucleic acids (or their constituents: nucleotides, nucleosides and nucleobases) with lipids in the same hybrid molecule leads, for example, to the so-called nucleolipids. Herein, we describe nucleolipids with a very short sequence of two nucleobases per lipid, which, in combination with hydrophobic effects promoted by the lipophilic chain, allow control of the self-assembly of lipidic amphiphiles to be achieved. The present work describes a spectroscopic and microscopy study of the structural features and dynamic self-assembly of dinucleolipids that contain adenine or thymine moieties, either pure or in mixtures. This approach leads to different self-assembled nanostructures, which include spherical, rectangular and fibrillar assemblies, as a function of the sequence of nucleobases and chiral effects of the nucleolipids involved. We also show evidence that the resulting architectures can encapsulate hydrophobic molecules, revealing their potential as drug delivery vehicles or as compartments to host interesting chemistries in their interior.

Adenosine and lipids: A forced marriage or a love match?

Adenosine is a fascinating compound, crucial in many biochemical processes: this ubiquitous nucleoside serves as an essential building block of RNA, is also a component of ATP and regulates numerous pathophysiological mechanisms via binding to four extracellular receptors. Due to its hydrophilic nature, it belongs to a different world than lipids, and has no affinity for them. Since the 1970's, however, new discoveries have emerged and prompted the scientific community to associate adenosine with the lipid family, especially via liposomal preparations and bioconjugation. This seems to be an arranged marriage, but could it turn into a true love match? This review considered all types of unions established between adenosine and lipids. Even though exciting supramolecular structures were observed with adenosine-lipid conjugates, as well as with liposomal preparations which resulted in promising pre-clinical results, the translation of these technologies to the clinic is still limited.

An enzymatic alternative for the synthesis of nucleoside 5'-monophosphates

A new procedure was carried out for the synthesis of nucleoside 5'-monophosphates, involving the use of two enzymes. The first step applied phospholipase D from Streptomyces netropsis and phosphatidylcholine as phosphatidyl donor, to give 5'-(3-sn-phosphatidyl) nucleosides (C, U, A, I). These were selectively hydrolysed in the second step by the action of phospholipase C from Bacillus cereus to produce the respective 5'-nucleotides. Application of this methodology on a preparative scale conducted to 5'-adenosine monophosphate in 63% overall yield from adenosine. The regioselectivity of these enzymes avoids protection steps, the overall synthesis is performed under mild reaction conditions and product isolation is easily achieved.

Enhanced Molecular Recognition between Nucleobases and Guanine-5'-monophosphate-disodium (GMP) by Surfactant Aggregates in Aqueous Solution

Only specific base pairs on DNA can bind with each other through hydrogen bonds, which is called the Watson-Crick (W/C) pairing rule. However, without the constraint of DNA chains, the nucleobases in bulk aqueous solution usually do not follow the W/C pairing rule anymore because of the strong competitive effect of water and the multi-interaction edges of nucleobases. The present work applied surfactant aggregates noncovalently functionalized by nucleotide to enhance the recognition between nucleobases without DNA chains in aqueous solution, and it revealed the effects of their self-assembling ability and morphologies on the recognition. The cationic ammonium monomeric, dimeric, and trimeric surfactants DTAB, 12-3-12, and 12-3-12-3-12 were chosen. The surfactants with guanine-5'-monophosphate-disodium (GMP) form micelles, vesicles, and fingerprint-like and plate-like aggregates bearing the hydrogen-bonding sites of GMP, respectively. The binding parameters of these aggregates with adenine (A), uracil (U), guanine (G), and cytosine(C) indicate that the surfactants can promote W/C recognitions in aqueous solution when they form vesicles (GMP/DTAB) or plate-like aggregates (GMP/12-3-12) with proper molecular packing compactness, which not only provide hydrophobic environments but also shield non-W/C recognition edges. However, the GMP/12-3-12 micelles with loose molecular packing, the GMP/12-3-12 fingerprint-like aggregates where the hydrogen bond sites of GMP are occupied by itself, and the GMP/12-3-12-3-12 vesicles with too strong self-assembling ability cannot promote W/C recognition. This work provides insight into how to design self-assemblies with the performance of enhanced molecule recognition.

Synthetic biology outside the cell: linking computational tools to cell-free systems

As mathematical models become more commonly integrated into the study of biology, a common language for describing biological processes is manifesting. Many tools have emerged for the simulation of in vivo synthetic biological systems, with only a few examples of prominent work done on predicting the dynamics of cell-free synthetic systems. At the same time, experimental biologists have begun to study dynamics of in vitro systems encapsulated by amphiphilic molecules, opening the door for the development of a new generation of biomimetic systems. In this review, we explore both in vivo and in vitro models of biochemical networks with a special focus on tools that could be applied to the construction of cell-free expression systems. We believe that quantitative studies of complex cellular mechanisms and pathways in synthetic systems can yield important insights into what makes cells different from conventional chemical systems.

Self-assembled nucleolipids: from supramolecular structure to soft nucleic acid and drug delivery devices

This short review aims at presenting some recent illustrative examples of spontaneous nucleolipids self-assembly. High-resolution structural investigations reveal the diversity and complexity of assemblies formed by these bioinspired amphiphiles, resulting from the interplay between aggregation of the lipid chains and base–base interactions. Nucleolipids supramolecular assemblies are promising soft drug delivery systems, particularly for nucleic acids. Regarding prodrugs, squalenoylation is an innovative concept for improving efficacy and delivery of nucleosidic drugs.

Association of polynucleotides with nucleolipid bilayers driven by molecular recognition

This contribution reports on the interaction of ss-polynucleotides of various length and sequence with liposomal dispersions of anionic lipids. No appreciable structural and morphological variations were detected for POPG (1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-glycerol) liposomes, as expected from the high negative charge density both of liposomal surface and of the poly or oligonucleotide. Conversely, when similarly charged POPN nucleolipids (1-palmitoyl-2-oleoylphosphatidyl-nucleosides) were used, meaningful differences could be observed both on size and morphology of the mixed aggregates. The comparison with POPG/nucleic acids mixed systems points to the conclusion that the driving force for association of nucleolipid liposomes with nucleic acids can be ascribed to selective interactions at the polar head level which overcome electrostatic repulsion. Dynamic light scattering, Cryo-TEM and circular dichroism provided an ensemble of results where an interesting dependence on the polynucleotide base nature and contour length emerges. The extent of interaction can be modulated, in terms of size of the complexes, by choice of background buffer, ionic strength and polynucleotide length.

Interaction of oligonucleotide-based amphiphilic block copolymers with cell membrane models

Oligonucleotides have unique molecular recognition properties, being involved in biological mechanisms such as cell-surface receptor recognition or gene silencing. For their use in human therapy for drug or gene delivery, the cell membrane remains a barrier, but this can be obviated by grafting a hydrophobic tail to the oligonucleotide. Here we demonstrate that two oligonucleotides, one consisting of 12 guanosine units (G(12)), and the other one consisting of five adenosine and seven guanosine (A(5)G(7)) units, when functionalized with poly(butadiene), namely PB-G(12) and PB-A(5)G(7), can be inserted into Langmuir monolayers of dipalmitoyl phosphatidyl choline (DPPC), which served as a cell membrane model. PB-G(12) and PB-A(5)G(7) were found to affect the DPPC monolayer even at high surface pressures. The effects from PB-G(12) were consistently stronger, particularly in reducing the elasticity of the DPPC monolayers, which may have important biological implications. Multilayers of DPPC and nucleotide-based copolymers could be adsorbed onto solid supports, in the form of Y-type LB films, in which the molecular-level interaction led to lower energies in the vibrational spectra of the nucleotide-based copolymers. This successful deposition of solid films opens the way for devices to be produced which exploit the molecular recognition properties of the nucleotides.

Orientation and specific interactions of nucleotides and nucleolipids inside monoolein-based liquid crystals

The entrapment of AMP, GMP, CMP, and UMP nucleotides along with two different AMP-based nucleolipids (hydrophobically functionalized nucleotides) inside the liquid crystalline phases of the monoolein/water system is investigated through optical microscopy, small-angle X-ray diffraction (SAXRD), and nuclear magnetic resonance (NMR) techniques. As ascertained mainly through (31)P NMR experiments, when included within the cubic phase, the various nucleotides undergo a slow hydrolysis of the sugar-phosphate ester bond, induced by specific interactions at the monoolein-water interface. Upon aging, the degradation of the nucleotides causes a cubic-to-hexagonal phase transition. Differently, neither hydrolysis nor alterations of the monoolein self-assembly are observed when the nucleotides are included as lipid derivatives within the cubic liquid crystalline phase. A model that explains both the hydrolysis and the consequent phase transition is presented.

Nucleotide-promoted morphogenesis in amphiphile assemblies: kinetic control of micrometric helix formation

Anionic nucleotides adenosine monophosphate or guanosine monophosphate interact with cationic vesicles, exchange with the counteranions of the amphiphiles in situ, and organize themselves at the membrane surfaces. Such organized nucleotides reciprocally transfer their chirality to membranes of nonchiral amphiphiles to induce the formation of right-handed micrometric helices on the time scale of hours. The kinetics of the nucleotide molecular organization and the formation of supramolecular helices was followed. We have shown that helix formation is a kinetic-dependent process that does not primarily result from ion exchange but from conformational reorganization and formation of weak interactions between confined nucleotides.

Intercalation of single-strand oligonucleotides between nucleolipid anionic membranes: a neutron diffraction study

This contribution presents a neutron diffraction investigation of anionic lamellar phases composed of mixtures of 1-palmitoyl, 2-oleoyl phosphatidyl-nucleosides (POPN, where N is either adenosine or uridine), and POPC (1-palmitoyl,2-oleoyl-phosphatidyl-choline). Their behavior is studied for two different mole ratios and in the presence of nucleic acids. The samples are formed by the evaporation of liposomal dispersions prepared in water or in solutions containing single-strand oligonucleotides. Previous small angle X-ray scattering (SAXS) experiments on the system POPA/polyU (polyuridylic acid, high degree of polymerization, synthetic ribonucleic acid) proved that the insertion and ordering of the biopolymer in the phospholipid lamellae were driven by molecular recognition. In the present study, we extend the previous investigation to single-strand monodisperse oligonucleotides (50-mers). Structural details of the membranes were obtained from the analysis of the neutron diffraction scattering length density profiles. The evidence of direct and specific interactions, driven by molecular recognition between the nucleic polar heads of the nucleolipid and the single-strand nucleic acid, is strengthened by the comparison with identically charged bilayers formed by POPG/POPC. These results contribute to the understanding of the parameters governing the interactions between nucleolipid membranes and oligonucleotides, providing a novel strategy for the design of lipid-based vehicles for nucleic acids.

Receptor-independent modulation of reconstituted Galpha(i) protein mediated by liposomes

A cationic amphiphile, BC5 (N-pentadecylpiperidin-4-amine), was recently designed and tested for its ability to directly stimulate the activity of recombinant Galpha inhibitory subunits. However, amphiphilic drugs can self-associate and bind to plasma membranes, causing undesired side effects. In this contribution, we report on the incorporation of BC5 in 1,2-dipalmytoyl-sn-glycerophosphocoline (DPPC) liposomes and on the characterization of the mixed DPPC/BC5 systems at various lipid/drug mole ratios by means of dynamic light scattering, differential scanning calorimetry and fluorescence spectroscopy. The myristoylated Galpha(i) subunit (Galpha-mir) was reconstituted in 1,2-dimiristoyl-sn-glycerophosphocoline (DMPC) bilayers, as a mimic of the drug target. We compare several reconstitution procedures in liposomes and present for the first time a complete characterization of a Galpha subunit reconstitution in model membranes in terms of protein activity as a function of the reconstitution protocol. The incorporation of the drug in DPPC bilayers resulted in enhanced Gi-modulating efficiency (evaluated in terms of binding to GTPgammaS (guanosine-5'-(gamma-thio)-triphosphate)). A correlation of the physico-chemical features and binding activity of protein-containing membrane model is proposed.

Intra- and intermembrane pairwise molecular recognition between synthetic hydrogen-bonding phospholipids

Multivalency and preorganization are fundamental aspects of molecular recognition at the lipid membrane-water interface and can render weak monomeric binding interactions selective and robust; this concept is important throughout biology, biotechnology, and materials science. Though hydrogen bonding is typically weakened in water, intramembrane hydrogen bonding between native lipids has been well-studied and is thought to contribute to lipid bioactivity and membrane function. We hypothesized that avidity and preorganization effects at the lipid-water interface could overcome solvent competition and allow for selective hydrogen-bond recognition between small, unstructured components. We have found that electrostatically identical vesicular membranes composed of cyanuric acid and melamine functionalized phospholipids 1 and 2 undergo selective apposition, fusion and adhesion in suspension and on solid support, indicating that their well-known low-dielectric hydrogen bonding properties translate effectively to the lipid-water interface. This work is notable and of general interest given the few detailed studies of aqueous phase hydrogen-bonding systems; we have extensively characterized this system, gaining structural, functional, and thermodynamic data. Furthermore, we have found that the designed lipid-lipid headgroup interactions result in dramatic alteration of the lipid phase morphology, providing insight into the coupling of molecular interactions with assembly state. As such, this work contributes to our understanding of fundamental phenomena such as molecular recognition at the lipid-water interface membrane chemistry and further illustrates the general possibility of designing selective hydrogen-bonding adhesive interactions from simple starting materials at other polar-apolar interfaces; this could have numerous materials and biotechnological applications.

Phospholipid membranes decorated by cholesterol-based oligonucleotides as soft hybrid nanostructures

DNA monomers and oligomers are currently showing great promise as building blocks for supramolecular arrays that can self-assemble in a fashion preprogrammed by the base pairing code. The design and build-up of hybrid DNA/amphiphilic self-assemblies can expand the range of possible architectures and enhance the selectivity toward a well-specified geometry. We report on the self-assembly properties in aqueous solution of a cholesteryl-tetraethylenglycol single stranded 18-mer oligonucleotide (ON 1TEG-Chol) and on its spontaneous insertion in fluid phospholipid membranes. Up to 500 units of these lipophilic ss-oligonucleotides can be incorporated in the outer leaflet of 350 A radius POPC vesicle. The insertion and hybridization with the complementary oligonucleotide are monitored through light scattering as an increase of hydrodynamic thickness, which is interpreted in terms of average distance between anchoring sites. The conformation of the ss-oligonucleotidic portion is strongly dependent on surface coverage, passing from a quasi-random coil to a more rigid configuration, as concentration increases. Interestingly, conformational details affect in a straightforward fashion the hybridization kinetics. Liposomes with single- and double-strand decorations remain stable within the experimental time window (about one week). The structure represents an example of successful and stable amphiphile/DNA supramolecular hybrid, where a DNA guest is held in a membrane by hydrophobic interactions. The lipophilic oligonucleotide under investigation is therefore a suitable building block that can effectively serve as a hydrophobic anchor in the fluid bilayer to assemble supramolecular constructs based on the DNA digital code.

Collective headgroup conformational transition in twisted micellar superstructures

Predictions on amphiphilic self-assemblies traditionally rely on considerations on molecular shape and charge of the surfactant. In the case of functional surfactants a more sophisticated toolbox becomes necessary to design amphiphiles encoding chemical functionalities that provide additional responsive properties to their self-assemblies. Here we report on a comprehensive and combined structural-spectroscopic characterization of 1,2-dilauroyl-phosphatidyl-adenosine (DLPA) micelles in phosphate buffer. The temperature dependence, more precisely the thermal history of the sample, is explicitly taken into account. The experimental data, supplemented with MD simulations, indicate the presence of two possible states at room temperature, characterized by distinctly different structural properties that depend on the thermal history of the sample. The twisted superstructures, produced by aging DLPA micelles through intermicellar assembly of locally cylindrical aggregates at room temperature, collapse upon warming at 35 °C, yielding aligned filaments and/or wormlike structures. The initial superstructures cannot be recovered by thermal inversion. The reason for this behaviour is that the thermal activation causes a redistribution of syn-anti conformations of adenosine headgroups, as indicated by spectroscopic results (NMR, CD, FTIR), which is then collectively frozen thanks to molecular constraints present in the aggregate.

Nucleoside, nucleotide and oligonucleotide based amphiphiles: a successful marriage of nucleic acids with lipids

Amphiphilic molecules based on nucleosides, nucleotides and oligonucleotides are finding more and more biotechnological applications. This Perspective highlights their synthesis, supramolecular organization as well as their applications in the field of biotechnology.

Nucleotides and nucleolipids derivatives interaction effects during multi-lamellar vesicles formation

In this paper a micellar interface, constituted by the cationic surfactant CTAB, in presence of 1,2-epoxydodecane and nucleotides was used for catanionic multi-lamellar vesicles (MLVs) formation. The micellar solution of CTAB is able to disperse the 1,2 epoxydodecane in the micellar core promoting the reaction of this reagent with the nucleotide attracted by the positive surface charge of the micellar aggregates. The alkylation of AMP and UMP nucleotides leads to the synthesis of nucleolipids. The behaviour of the supramolecular structures formed depends on the starting reagents (AMP, UMP and AMP+UMP) and on the assembly capabilities of the products. In particular nucleotides and nucleotides derivatives interaction effects are evaluated during the multi-lamellar vesicles formation. NMR spectroscopy and UV-vis measurements performed on MLVs showed strong aryl interactions. Interestingly, NMR spectra revealed prevailing stacking interactions between complementary nucleolipids. The assembly of complementary nucleotides affects the course of the reaction during the MLVs formation. Moreover the MLVs supramolecular stability has been tested by means of turbidity and UV-vis measurements. In particular, an enhanced stability has been found in systems prepared with complementary nucleotides confirming that in these systems the self-assembly process is influenced by nucleolipids interactions. Furthermore by following the hypocromic effect during the micellar catalysis, we showed that even in the earlier stages of the reaction significant differences are detectable.

Self-assembly of nucleoamphiphiles: investigating nucleosides effect and the mechanism of micrometric helix formation

A new family of self-assembling systems based on nucleoamphiphiles is described. Nano to micrometric left-handed helix formation in aqueous solution was induced simply by complexing a GMP or an AMP with a nonchiral monocationic amphiphile. The assembling behavior such as micellar formation, monolayer at air-water interface, as well as the aggregates in solution of these nucleoamphiphiles are strongly influenced by the presence of nucleosides in solution. The observed effects depend on the properties of complexed nucleotides and nucleosides with a complex mixture of pi stacking, hydrophobicity of the bases, and hydrogen bonding.

Amphiphilic Self-Assemblies Decorated by Nucleobases

Phosphatidyl-nucleosides are a class of functional amphiphiles, where a nucleic acid monomer is conjugated to a lipid skeleton. These derivatives self-organize in aqueous solution as assemblies of various size, shape, and interfacial curvature. This paper presents a comparison of the aggregation behavior of different 1-R,2-R-sn-glycero-3-phosphatidyl-nucleosides, where R = 8 (DiC8PN) or R = 12 (DLPN) and N is either adenosine (a purine) or uridine (a pyrimidine), a complementary pair in RNA. Surface tension, small angle neutron scattering, cryo-TEM, and circular dichroism are used to highlight and distinguish the impact of the hydrophobic assembler and of the base substitution on the solution phase behavior. Our main conclusion is that the nucleic functionalization provides an additional parameter to control self-assembly through specific interactions among the polar heads. Further nonideal effects are induced by mixing nucleolipids with complementary base substitution. We show that these contributions alter the aggregation thresholds and modulate properties of the aggregates on the mesoscale.

Nucleolipids: natural occurrence, synthesis, molecular recognition, and supramolecular assemblies as potential precursors of life and bioorganic materials

Nucleolipids are hybrid molecules composed of a nucleobase, a nucleoside, a nucleotide or an oligonucleotide (either DNA or RNA), and a lipophilic moiety, which might be either simply a single- or double-chained alkyl (or alkenyl) moiety or a carbocyclic hydrocarbon such as cholesterol, a vitamin, or a bile acid. This review covers all aspects of nucleolipids, namely their natural occurrence, their synthesis, their molecular recognition, as well as aggregation behavior, either in aqueous or non-aqueous solution. Potential future aspects of nucleolipids in material sciences and for the elucidation of biochemical reactions in living cells are discussed.

Aggregation behaviors of gemini nucleotide at the air-water interface and in solutions induced by adenine-uracil interaction

Cationic gemini surfactants having nucleotides as counterions (called nucleo-gemini hereafter) were synthesized and their aggregation behavior at air-water surfaces as well as in bulk solutions were studied. Fluid solutions of these nucleo-gemini surfactants show transitions to hydrogels upon addition of complementary nucleoside bases or other nucleo-gemini surfactants having complementary bases as counterions. The FTIR-ATR measurements show that the carboxylate groups of uridine form hydrogen bonds with the amine groups of adenosine. The aggregation behavior was also confirmed at the air-water interface by Brewster angle microscopy as well as surface pressure measurements; the monolayer of a gemini nucleotide was observed to undergo a transition to multilayers when nucleosides with complementary bases were added into the subphase. Isotherm curves of surface pressure monitored in parallel show a decrease in molecular area upon addition of such nucleosides.

Molecular Recognition of Cytosine- and Guanine-Functionalized Nucleolipids in the Mixed Monolayers at the Air−Water Interface and Langmuir−Blodgett Films

Molecular recognition of mixed nucleolipids of 1-(2-octadecyloxycarbonylethyl)cytosine and 7-(2-octadecyloxycarbonylethyl)guanine in the monolayers at the air-water interface and Langmuir-Blodgett (LB) films has been investigated in detail using surface pressure/potential-area isotherms, infrared reflection-absorption spectroscopy (IRRAS), and Fourier transform infrared (FTIR) transmission spectroscopy, respectively. Prior to molecular recognition, the cytosine moieties in the monolayer were hydrogen bonded with an almost flat-on orientation, the alkyl chains were uniaxially oriented with respect to the film normal, the guanine moieties in the monolayer were stacked probably through pi-pi interaction with an end-on orientation, and the C-C-C planes of the alkyl chains were preferentially oriented parallel to the water surface. In the monolayer of equimolar mixture, molecular recognition between the cytosine and guanine moieties occurred together with the ring planes of base pairing and the C-C-C planes of the alkyl chains favorably oriented parallel to the water surface. The guanine moieties underwent an orientation change from an end-on mode before molecular recognition to a flat-on one after molecular recognition. The base pairing between the cytosine and guanine moieties in the monolayers was achieved since the N7-substituted guanine derivatives suppressed the formation of guanine tetramers. Both the IRRAS spectra of the monolayers and the FTIR spectra of the LB films presented the exact sites in the cytosine and guanine moieties for the formation of triple hydrogen bonds. The base pairing resulted in a change in molecular orientation and interaction, and the corresponding LB film exhibited a different phase transition behavior from a typical crystal transition for the cytosine-functionalized nucleolipids and an analogous glass transition for the guanine-functionalized nucleolipids. The thermal stability of the mixed LB film was improved in comparison to the LB films of pure components.

Structural investigation of bilayers formed by 1-palmitoyl-2-oleoylphosphatidylnucleosides

Bilayers of palmitoyl-oleoylphosphatidylnucleoside derivatives (1-palmitoyl-2-oleoyl-phosphatidyl-adenosine and 1-palmitoyl-2-oleoyl-phosphatidyl-uridine) were synthesized and investigated in the low-water content regime by a combination of neutron diffraction and Fourier transform infrared linear dichroism (LD-FTIR). Attention was focused on the modulation of structural properties operated by the presence of nucleic acid bases (either adenosine or uridine, a purine and a pyrimidine that are complementary in RNA). Base substitution causes major differences in phase behavior of the phospholipids, i.e., water sorption from a controlled humidity atmosphere and smectic periodicity. The profile of scattering length density can be inferred from five diffraction orders for 1-palmitoyl-2-oleoyl-phosphatidyl-uridine lamellar phase. 1-Palmitoyl-2-oleoyl-phosphatidyl-adenosine is characterized by lower and less ready hydration, giving rise to a powder-like sample. A linear dichroism FTIR investigation on the same lamellar phases was undertaken with the purpose of gathering details at the submolecular level on different portions of the molecule. 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayers were also investigated with the same technique for the sake of comparison. Besides a confirmation of the diffraction data interpretation, FTIR has provided evidence that the same chemical groups at the bilayer interface (namely the sugar-phosphate) have a different orientation depending on whether the base is a purine or a pyrimidine. A very simple geometrical optimization agrees with this observation. This indicates that a different pattern of base interaction is operating in the two cases and that base substitution acts as a modulator of the phase properties.

Self-Adhesion among Phospholipid Vesicles

A compound was synthesized that binds to a phospholipid bilayer via a hydrophobic steroid thereby projecting a strong multi-hydrogen bonding unit into the surrounding water. As shown by light scattering, light microscopy, and cryo-HRSEM, this latter unit self-adheres and induces membrane-membrane attachments, as found in many biological systems.

Self-assembled microspheres from f-block elements and nucleoamphiphiles

Hollow microspheres featuring a hybrid lipid-cation multilamellar shell are prepared by hydration of a nucleoside based amphiphile with an aqueous solution containing either actinide or lanthanide salts. The physico-chemical data collected clearly indicate that the formation of these microspheres is a consequence of the following concomitant stabilizing factors: (i) hydrophobic interactions, (ii) nucleobase dimer formation and (iii) phosphate/f-block element salt binding.

Fusogenic supramolecular vesicle systems induced by metal ion binding to amphiphilic ligands

The incorporation of lipophilic ligands into the bilayer membrane of vesicles offers the possibility to induce, upon binding of suitable metal ions, a variety of processes, in particular vesicle aggregation and fusion and generation of vesicle arrays, under the control of specific metal-ligand recognition events. Synthetic bipyridine lipoligands Bn bearing a bipyridine unit as head group were prepared and incorporated into large unilamellar vesicles. The addition of Ni2+ or Co2+ metal ions led to the formation of complexes MBn and MBn2 followed by spontaneous fusion to generate giant multilamellar vesicles. The metal ion complexation was followed by UV spectroscopy and the progressive fusion could be visualized by optical dark-field and fluorescence microscopies. Vesicle fusion occurred without leakage of the aqueous compartments and resulted in the formation of multilamellar giant vesicles because of the stacking of the lipoligands Bn. The fusion process required a long enough oligoethylene glycol spacer and a minimal concentration of lipoligand within the vesicle membrane. Metallosupramolecular systems such as the present one offer an attractive way to induce selective intervesicular processes, such as vesicle fusion, under the control of molecular recognition between specific metal ions and lipoligands incorporated in the bilayer membrane. They provide an approach to the design of artificial "tissue-mimetics" through the generation of polyvesicular arrays of defined architecture and to the control of their functional properties.

Supramolecular Assemblies of Nucleoside Phosphocholine Amphiphiles

A family of new uridine phosphocholine amphiphiles that were prepared using a convenient four-step synthetic route is described. Physicochemical studies (differential scanning calorimetry, small-angle X-ray scattering, UV-vis and circular dichroism spectroscopies, light microscopy, transmission electronic microscopy, and scanning electron microscopy) show that these amphiphiles spontaneously assemble into supramolecular structures including vesicles, fibers, hydrogels, and organogels. In aqueous solution, the amphiphiles possessing saturated alkyl chains self-assemble into DNA-like helical fibers in the crystalline state below T(m) and compact bilayers above the melting temperature (T(m)). The transition from bilayers to fibers is thermally reversible. Above a threshold concentration (>6% w/w), a hydrogel is formed due to an entangled network of the fibers. A therapeutic agent such as DNA can be entrapped within the hydrogel structure. In addition to forming bilayer vesicles and hydrogels in aqueous solution, these nucleoside amphiphiles also form organogels in cyclohexane above T(m). Scanning electron microscopy shows a continuous multilamellar phase in the organogels.

Multilamellar liposomes formed by phosphatidyl nucleosides: an NMR-HR-MAS characterization

We present an NMR investigation of multilamellar vesicles (MLVs) obtained from phosphatidyl nucleosides, 5'-(1-palmitoyl-2-oleoyl-sn-glycero(3)phospho)cytidine (1), 5'-(1-palmitoyl-2-oleoyl-sn-glycero(3)phospho)inosine (2), and their mixtures. Because of the lower stability of liposomes obtained from 2, studies have been preferentially performed in this case with mixed liposomes 2/POPC (4:1). The investigation is conducted mostly via the HR-MAS technique and the general observation is that the resolution achieved in this way is superior to that obtained in the past with small unilamellar vesicles (SUVs). A full assignment is now possible, which includes the spectral region of the ribose ring and part of the glycerol moiety. Also in the case of MLVs, both for 1 and 2, a stacking between the aromatic bases of the same liposome layer seems to be ruled out, although in both cases the nucleobases appear to be exposed to the aqueous phase. The splitting of both aromatic H-5cyt and H-6cyt is ascribed to the presence of two aggregate populations that may correspond to the two syn and anti conformations observed for cytidine monophosphate in aqueous solution. On the basis of NOESY cross-peaks, it is not always possible to discriminate between inter- and intramolecular interactions; however, the distances found for 1 appear to be compatible with the intramolecular contacts in the anti conformation of the cytidine and also with intermolecular interactions between neighboring molecules of 1. We also find that the glycerol moiety does not seem to interact with the cytidine; however, part of the ribose ring seems to be close to the glycerol moiety. More generally, the interaction of one base with the sugar moiety of a neighboring base, previously observed for SUVs, still appears to be true for MLVs. Studies have been performed also for mixed liposomes obtained from the mixture of 1 and 2, where it is observed that the HR-MAS spectra of the corresponding MLVs are not simply the sum of the spectra of the two isolated components. In particular, there is the presence of a NOESY cross-peak between the aromatic protons H-6cyt and H-2ino, and this permits us to rule out large patchwork domains containing only one nucleoside components in the mixed liposomes. Finally, a study is performed on the time evolution of the system obtained by mixing the previously prepared liposomes of 1 and 2. No interaction is obtained in this case, i.e., the spectra are constitutive, which is consistent with the general picture of liposomes as kinetic traps that are not fusing with each other.

Fourier transform surface-enhanced Raman scattering of single-layer nucleolipid Langmuir–Blodgett films on silver island film substrates

Surface-enhanced Raman scattering (SERS) spectra of four amphiphilic nucleolipids in single-layer Langmuir-Blodgett (LB) films deposited on silver island film substrates from pure water and complementary nucleotide-containing subphase and corresponding powder normal Raman spectra were obtained. The analysis of these spectra indicates that the SERS effect is mainly caused by a charge-transfer mechanism, and only the nucleobase headgroup moieties and complementary bases combined with them through hydrogen bonds, which are directly in contact with the silver island film substrates, could be enhanced. For the amphiphilic nucleolipids with the identical nucleobase headgroups, the SERS spectra of the LB films are similar, implying that the orientations of these nucleobase moieties on the silver substrates are analogous. However, the nucleobase takes different orientations on the silver substrates before and after complementary binding. The nucleobases in the LB films deposited from pure water are nearly lying flat on the silver surface, while the complementary binding pairs transferred from the air/water interface tend to take an end-on orientation on the metal surface.

Molecular recognition of 7-(2-octadecyloxycarbonylethyl)guanine to cytidine at the air/water interface and LB film studied by Fourier transform infrared spectroscopy

Monolayer behavior of a nucleolipid amphiphile, 7-(2-octadecyloxycarbonylethyl)guanine (ODCG), on aqueous cytidine solution was investigated by means of surface-molecular area (pi-A) isotherms. It indicates that molecular recognition by hydrogen bonding is present between ODCG monolayer and the cytidine in subphase. The Fourier transform infrared (FTIR) transmission spectroscopic result indicates that the cytidine molecules in the subphase can be transferred onto solid substrates by Langmuir-Blodgett (LB) technique as a result of the formation of Watson-Crick base-pairing at the air/water interface. Investigation by rotating polarized FTIR transmission also suggests that the headgroup recognition of this amphiphile to the dissolved cytidine influence the orientation of the tailchains.

Spectroscopic studies on molecular recognition capabilities of a nucleolipid bearing thymine headgroup to adenosine

The Langmuir-Blodgett (LB) films of octadecanoyl ester of 1-(2-carboxyethyl) thymine deposited from pure water and aqueous adenosine subphases were investigated by ultraviolet-visible (UV-vis), Fourier transform infrared-attenuated total reflection (FTIR-ATR), and Fourier transform surface-enhanced Raman scattering (FT-SERS) spectroscopy. The obtained spectral results indicate that the adenosine molecules in the subphase can be transferred onto solid substrates by LB techniques as a result of the formation of base pairs at the air/water interface. UV-vis spectra alternations indicated that, with increasing adenosine concentration in subphase, more adenosine molecules were recognized by nucleolipid monolayer and were transferred onto the quartz substrates. The closed-packing of the constituent molecules facilitates the photodimerization of the thymine moieties in the headgroup under ultraviolet irradiation. FTIR-ATR results suggest that the hydrocarbon chains of nucleolipid in the LB films deposited from pure water and aqueous adenosine take on a close-packed all trans conformation. By analyzing the FT-SERS spectra results, it can be deduced that the orientation of nucleobase in the headgroup is different before and after the recognition effect occurred. For LB film deposited from pure water, the nucleobases are lying flat on the silver substrates; whereas for LB film deposited from aqueous adenosine, the base pairs take an end-on adsorption on silver substrate.