Javier Montenegro

Ion Channel Models Based on Self-Assembling Cyclic Peptide Nanotubes

The lipid bilayer membranes are Natures dynamic structural motifs that individualize cells and keep ions, proteins, biopolymers and metabolites confined in the appropriate location. The compartmentalization and isolation of these molecules from the external media facilitate the sophisticated functions and connections between the different biological processes accomplished by living organisms. However, cells require assistance from minimal energy shortcuts for the transport of molecules across membranes so that they can interact with the exterior and regulate their internal environments. Ion channels and pores stand out from all other possible transport mechanisms due to their high selectivity and efficiency in discriminating and transporting ions or molecules across membrane barriers. Nevertheless, the complexity of these smart membrane holes has driven researchers to develop simpler artificial structures with comparable performance to the natural systems. As a broad range of supramolecular interactions have emerged as efficient tools for the rational design and preparation of stable 3D superstructures, these results have stimulated the creativity of chemists to design synthetic mimics of natural active macromolecules and even to develop artificial structures with functions and properties. In this Account, we highlight results from our laboratories on the construction of artificial ion channel models that exploit the self-assembly of conformationally flat cyclic peptides (CPs) into supramolecular nanotubes. Because of the straightforward synthesis of the cyclic peptide monomers and the complete control over the internal diameter and external surface properties of the resulting hollow tubular suprastructure, CPs are the optimal candidates for the fabrication of ion channels. The ion channel activity and selective transport of small molecules by these structures are examples of the great potential that cyclic peptide nanotubes show for the construction of functional artificial transmembrane transporters. Our experience to date suggests that the next steps for achieving conceptual devices with better performance and selectivity will derive from the topological control over cyclic peptide assembly and the functionalization of the lumen.

Coupling of Carbon and Peptide Nanotubes

Two of the main types of nanotubular architectures are the single-walled carbon nanotubes (SWCNTs) and the self-assembling cyclic peptide nanotubes (SCPNs). We here report the preparation of the dual composite resulting from the ordered combination of both tubular motifs. In the resulting architecture, the SWCNTs can act as templates for the assembly of SCPNs that engage the carbon nanotubes noncovalently via pyrene paddles, each member of the resulting hybrid stabilizing the other in aqueous solution. The particular hybrids obtained in the present study formed highly ordered oriented arrays and display complementary properties such as electrical conductivity. Furthermore, a self-sorting of the cyclic peptides toward semiconducting rather than metallic SWCNTs is also observed in the aqueous dispersions. It is envisaged that a broad range of exploitable properties may be achieved and/or controlled by varying the cyclic peptide components of similar SWCNT/SCPN hybrids.

In Situ Functionalized Polymers for siRNA Delivery.

A new method is reported herein for screening the biological activity of functional polymers across a consistent degree of polymerization and inu2005situ, that is, under aqueous conditions and without purification/isolation of candidate polymers. In brief, the chemical functionality of a poly(acryloyl hydrazide) scaffold was activated under aqueous conditions using readily available aldehydes to obtain amphiphilic polymers. The transport activity of the resulting polymers can be evaluated inu2005situ using model membranes and living cells without the need for tedious isolation and purification steps. This technology allowed the rapid identification of a supramolecular polymeric vector with excellent efficiency and reproducibility for the delivery of siRNA into human cells (HeLa-EGFP). The reported method constitutes a blueprint for the high-throughput screening and future discovery of new polymeric functional materials with important biological applications.

Hiyama Cross-Coupling Reaction in the Stereospecific Synthesis of Retinoids

The first application of the Hiyama reaction to the synthesis of retinoids is reported. A range of organosilicon moieties (siloxanes, silanols and three kinds of safety-catch silanols) were successfully coupled, under activation, to obtain trans-retinol or 11-cis-retinol with high yield and stereoselectivity. The advantageous properties of the silicon-based coupling partners and the mild reaction conditions firmly establish the Hiyama reaction as a viable (even superior) alternative to the traditional Suzuki and Stille couplings in the retinoid field.

Cross‐Coupling Reactions of Organosilicon Compounds in the Stereocontrolled Synthesis of Retinoids

This paper presents a full account of the use of Hiyama cross-coupling reactions in a highly convergent approach to retinoids in which the key step is construction of the central C10-C11 bond. Representatives of two families of oxygen-activated dienyl silanes (ethoxysilanes and silanols) and of all reported families of safety-catch silanols (siletanes, silyl hydrides, allyl-, benzyl-, aryl-, 2-pyridyl- and 2-thienylsilanes) were regio- and stereoselectively prepared and stereospecifically coupled to an appropriate electrophile by treatment with a palladium catalyst and a nucleophilic activator. Both all-trans and 11-cis-retinoids, and their chain-demethylated analogues, were obtained in good yields regardless of the geometry (E/Z) and of the steric congestion in each fragment. This comprehensive study conclusively establishes the Hiyama cross-coupling reaction, with its mild reaction conditions and stable, easily prepared, ecologically advantageous silicon-based coupling partners, as the most effective route to retinoids reported to date.

Dynamic polythioesters via ring-opening polymerization of 1,4-thiazine-2,5-diones.

We describe the preparation and characterization of polythioesters composed of alternating alpha-amino acid and alpha-thioglycolic acid residues that undergo dynamic constitutional exchange under mild conditions. The polymers are assembled via reversible ring-opening polymerizations of 1,4-thiazine-2,5-diones and related monomers in solution-phase conditions that do not require the use of transition metal catalysts. Because 1,4-thiazine-2,5-diones can be derived in part from alpha-amino acids, a variety of side chain functionalized monomers in optically pure forms could readily be accessed. In addition, the resulting polythioesters have the potential for intra- and inter-chain hydrogen bonding, which is known to impart materials properties to other previously studied polyamides. The studies reported here could be useful in advancing a new class of biodegradable polymers and furthermore suggest that dynamic constitutional exchange could be exploited to modify many known synthetic and natural polythioesters.

Self-Assembly of Silver Metal Clusters of Small Atomicity on Cyclic Peptide Nanotubes

Subnanometric noble metal clusters, composed by only a few atoms, behave like molecular entities and display magnetic, luminescent and catalytic activities. However, noncovalent interactions of molecular metal clusters, lacking of any ligand or surfactant, have not been seen at work. Theoretically attractive and experimentally discernible, van der Waals forces and noncovalent interactions at the metal/organic interfaces will be crucial to understand and develop the next generation of hybrid nanomaterials. Here, we present experimental and theoretical evidence of noncovalent interactions between subnanometric metal (0) silver clusters and aromatic rings and their application in the preparation of 1D self-assembled hybrid architectures with ditopic peptide nanotubes. Atomic force microscopy, fluorescence experiments, circular dichroism and computational simulations verified the occurrence of these interactions in the clean and mild formation of a novel peptide nanotube and metal cluster hybrid material. The findings reported here confirmed the sensitivity of silver metal clusters of small atomicity toward noncovalent interactions, a concept that could find multiple applications in nanotechnology. We conclude that induced supramolecular forces are optimal candidates for the precise spatial positioning and properties modulation of molecular metal clusters. The reported results herein outline and generalize the possibilities that noncovalent interactions will have in this emerging field.

Hydrazone-modulated peptides for efficient gene transfection

Gene transfection continues to be a major challenge in chemistry, biology and materials sciences. New methodologies and recent breakthroughs have renewed the interest in the discovery and development of new tools for efficient gene transfection. Hydrazone formation between a cationic head and hydrophobic tails has emerged as one of the most promising techniques for nucleotide delivery. In this contribution, we have exploited hydrazone formation to modulate the transfection activity of a parent linear peptide in combination with a plasmid DNA cargo. This strategy allowed the straightforward preparation, under physiologically compatible conditions, of a discrete library of amphiphilic modulated penetrating peptides. Without the requirement of any isolation or purification steps, these modulated amphiphilic peptides were combined with a plasmid DNA and screened in transfection experiments of human HeLa cells. Three of these hydrazone-conjugated peptides were identified as excellent vectors for plasmid delivery with comparable, or even higher, efficiencies and lower toxicity than the commercial reagents employed in routine transfection assays.

Single-Nucleotide-Resolution DNA Differentiation by Pattern Generation in Lipid Bilayer Membranes

Pattern generation/recognition in lipid bilayers is introduced for the differentiation of anionic biological relevant polymers. The amplification of the polymer differences during transport events allows the straightforward identification of a wide range collection of anionic polymers. The introduced approach displays excellent resolution even for single mutations in short single-stranded oligonuclotides.

Synthesis of N-Heteroaryl Retinals and their Artificial Bacteriorhodopsins

N‐Heteroaryl retinals derived from indole, 1‐indolizine and 3‐indolizine (10u2009a–c) have been synthesized after their UV/Vis red‐shifted absorption properties had been predicted by time‐dependent density functional theory (TD‐DFT) computations. The three new analogues form artificial pigments upon recombination with bacterioopsin: indolyl retinal 10u2009a undergoes fast and efficient reconstitution to form a species with a UV/Vis absorbance maximum similar to that of wild‐type bacteriorhodopsin, whilst the indolizinyl retinals 10u2009b and 10u2009c also reconstitute in significant proportion to give noticeably red‐shifted, although unstable, pigments. Significant changes in the pKa values of these artificial bacteriorhodopsins are interpreted as arising from nonoptimal binding‐site occupancy by the chromophore due to steric constraints.