Artificial nanomachines based on interlocked molecular species: recent advances

The bottom-up construction and operation of nanoscale machines and motors, that is, supramolecular systems wherein the molecular components can be set in motion in a controlled manner for ultimately accomplishing a function, is a topic of great interest in nanoscience and a fascinating challenge of nanotechnology. The field of artificial molecular machines and motors is growing at an astonishing rate and is attracting a great deal of interest. Research in the last decade has shown that species made of interlocked molecular components like rotaxanes, catenanes and related systems are most attractive candidates. In recent times, the evolution of the structural and functional design of such systems has led to the construction and operation of complex molecular machines that, in some cases, are able to do specific tasks. This tutorial review is intended to discuss the design principles for nanomachines based on interlocked molecules, and to provide a timely overview on representative prototype systems.     

A reduced Newton method for constrained linear least-squares problems

We propose an iterative method that solves constrained linear least-squares problems by formulating them as nonlinear systems of equations and applying the Newton scheme. The method reduces the size of the linear system to be solved at each iteration by considering only a subset of the unknown variables. Hence the linear system can be solved more efficiently. We prove that the method is locally quadratic convergent. Applications to image deblurring problems show that our method gives better restored images than those obtained by projecting or scaling the solution into the dynamic range.     

Co-conformational mechanoisomerism in a calix[6]arene-based [2]rotaxane

The synthesis of a [2]rotaxane, comprising a calix[6]arene-wheel and a dibenzyl-ammonium axle, is here reported. By virtue of its inherent directionality, the calix-wheel makes non-degenerate two equivalent stations of the symmetrical axle. In this way, the neutral rotaxane shows two co-conformations, named endo-alkyl and endo-benzyl, in which an alkyl or benzyl moiety of the axle are included inside the calix-cavity, respectively. NMR and DFT studies showed that the co-conformation preferred by the neutral mechanomolecule is the ‘endo-alkyl’ one, which is more stabilized by C-H···π interactions between the included alkyl chain and the aromatic wall of the calix-cavity.     

Laudal-type theorems in P

In this paper we classify all integral, non-degenerate, locally Cohen-Macaulay subvarieties in P^N, whose general complementary section is a complete intersection set of points: they are either complete intersections or curves on a quadric surface in P³ or degree 4 arithmetically Buchsbaum surfaces in P⁴ (i.e. the Veronese surface or a degeneration of it). As a consequence we show that every locally Cohen-Macaulay threefold in P^S of degree 4 is a complete intersection.Moreover, we obtain a generalization of Laudal's Lemma to threefolds in P⁵ and fourfolds in P⁶, which gives a bound on the degree of a codimension 2, integral subvariety X in P^N, depending both on N and a non-lifting level s of X.     

Separation de semi-algebriques

In this paper we consider the problem of separating by a polynomial function two open disjoint semi-algebraic subsets A and B of a real affine variety M under the assumption that the subsets are already polynomially separated up to a proper algebraic subset. First of all some elementary results in small dimensions are given. When M is non-singular, a hypothesis on the behaviour of the boundaries of A and B is sufficient to obtain the separation. The problem is also analysed if M is singular, and some positive results are obtained in the compact case.

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Les auteurs sont associés au groupe G.N.S.A.G.A. du C.N.R. Recherche partiellement financiée par le M.P.I.     

Determination of field profiles in semiconductors by electroabsorption

The experimental demonstration of the possibility of applying modulation spectroscopy techniques to the microscopic probing of nonuniform electric fields in semiconductors is given. A silicon bar is scanned by a highly focused monochromatic light beam. The sample is sandwiched between two gold electrodes that allow application of an ac field and formation of depletion layers extending over distances of the order of 1 mm. The field intensity is probed by measuring the transverse electroabsorption at a photon energy that is conveniently chosen to produce TO-phonon assisted transitions from the valence band to the n = 1 level of the exciton. At this energy, the modulated signal is proportional to the square of the field because the experiment is done in the limit of Stark perturbation. Mapping of the field is achieved over the whole length of the bar and the expansion of the depletion layer region is followed up to a punch-through condition.     

The gene cluster for the biosynthesis of the glycopeptide antibiotic A40926 by nonomuraea species

The glycopeptide A40926 is the precursor of dalbavancin, a second-generation glycopeptide currently under clinical development. The dbv gene cluster, devoted to A40926 biosynthesis, was isolated and characterized from the actinomycete Nonomuraea species ATCC39727. From sequence analysis, 37 open reading frames (ORFs) participate in A40926 biosynthesis, regulation, resistance, and export. Of these, 27 ORFs find a match in at least one of the previously characterized glycopeptide gene clusters, while 10 ORFs are, so far, unique to the dbv cluster. Putative genes could be identified responsible for some of the tailoring steps (attachment of glucosamine, sugar oxidation, and mannosylation) expected during A40926 biosynthesis. After constructing a Nonomuraea mutant by deleting dbv ORFs 8 to 10, the novel compound dechloromannosyl-A40926 aglycone was isolated.     

Effect of Different Ionic Surfactants on the Structural,Photophysical, and Morphological Properties of Water-Based P3HT:PCBM Nanoparticle Dispersions and Films

Aqueous suspensions of composite nanoparticles of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) are fabricated by miniemulsion method using three different ionic surfactants. The aim is to study how the length and conformation of the surfactants alkyl chains affect the properties of the nanoparticles. While the morphology and dimensions of the nanoparticles are similar, UV–vis spectroscopy evidences that the internal aggregation and ordering of the P3HT chains varies within the three nanoparticle formulations. The surfactant with branched alkyl chains promote the highest degree of ordering of P3HT chains in the nanoparticles (leading to increased conjugation length). In contrast, the lowest ordering is found for the nanoparticles with the surfactant having the shortest linear alkyl chain. The optical/structural properties of nanoparticles are partially retained in the films. Besides, the surfactant with branched alkyl chains favors the strongest coalescence of nanoparticles in the thin film, promoting a further ordering of the polymeric chains in the most external shell of the nanoparticles as evidenced by steady-state and time-resolved UV–vis spectroscopy and confocal fluorescence microscopy. These findings might guide the engineering of new surfactants for composite nanoparticles for optoelectronic applications.     

Studies on puupehenone-metabolites of a Dysidea sp.: structure and biological activity

Puupehenone (1) and a series of its congeners (2-6) have been isolated from a Dysidea sponge. The unprecedented 20-acetoxy-haterumadienone (2) exhibiting a five-membered contracted ring, has been characterized. In addition, stereochemical assignment of two previously reported acetone adducts of puupehenone (5 and 6) has been made. Finally, the inhibition of mitochondrial respiratory chain as well as antibacterial and antifungal activities of all compounds has been evaluated.     

Aplysiols A and B, squalene-derived polyethers from the mantle of the sea hare Aplysia dactylomela

Two novel triterpenoids, aplysiols A (5) and B (6), have been isolated, together with structurally related known metabolites, from a South China Sea collection of the anaspidean mollusc Aplysia dactylomela. The structures of 5 and 6 were determined mainly by spectroscopic NMR techniques. The absolute stereochemistry of compound 5 was deduced by Mosher's method as well as by biogenetical consideration, whereas the absolute stereochemistry of compound 6 was established also using an integrated NMR-QM (Quantum Mechanical) approach, based on the combination of ¹³C NMR chemical shifts and ^2,3J_C-H coupling constant DFT (density functional theory) calculations.