Estimation of parameters of non-linear dynamical systems

A method is proposed to describe non-linear systems without memory, forced by random excitation. The concept of statistical linearization is extended to an approximation with a series of functions. The parameters of these functions describing the non-linearity are estimated in the presence of noise by using correlation techniques. The results apply also if the non-linearity cannot be separated from a linear dynamic model.     

Solution Structure of a RNA Decamer Duplex,Containing 9‐[2‐O‐(β‐D‐ribofuranosyl)‐β‐D‐ribofuranosyl]adenine,a Special Residue in Lower Eukaryotic Initiator tRNAs

The solution structure of the self‐complementary deca‐ribonucleotide 5′‐r(GCGA*AUUCGC)‐3′ containing 9‐[2‐O‐(β‐D ‐ribofuranosyl)‐β‐D ‐ribofuranosyl]adenine (A*), a modified nucleotide that occurs in lower eukaryotic methionine initiator tRNAs (tRNAs_i^Met), was determined by NMR spectroscopy. Unexpectedly, the modification has no effect on the thermal stability of the duplex. However, the extra ribose moiety is in the C(3′)‐endo conformation and takes up a well‐defined position in the minor groove, which is in agreement with its position in tRNAs_i^Met as determined by X‐ray crystallography. Molecular‐dynamics simulations on the RNA duplex in H₂O show that the position of the extra ribofuranose moiety seems to be stabilized by bridged H‐bonds (mediated by two H₂O molecules) to the backbone of the complementary chain.     

The effect of the amido substituent on polymer molecular weight in propene homopolymerisation by titanium cyclopentadienyl‐amide catalysts

In the homopolymerisation of propene by the cyclopentadienyl‐amide titanium catalyst systems [η⁵,η¹‐C₅H₄(CH₂)₂NR]TiCl₂/MAO and [η⁵,η¹‐C₅H₄(CH₂)₂NR]Ti(CH₂Ph)₂/B(C₆F₅)₃ (R = ^tBu, ⁱPr, Me), the catalyst with the smallest substituent (Me) on the amido moiety consistently gives the highest polymer molecular weight. This differs from the trend usually observed in related catalysts with tetramethylcyclopentadienyl‐amide ancillary ligands, where larger amide substituents result in higher molecular weights. Based on the present information a hypothesis is formulated in which an increased cation‐anion interaction for the less sterically hindered catalyst is responsible for disfavouring chain transfer relative to chain growth.     

Mechanistic Implications of the Observation of Kinetic Resolution in a Palladium-Catalyzed Enantioselective Allylic Alkylation

Preferential rotation in substrate—palladium intermediates in a catalyzed asymmetric allylic alkylation is proposed to be responsible for both the observed kinetic resolution of the racemic allylic acetate starting material as well as the high selectivity found in the enantiodiscriminating product-forming step [Eq. (a)].     

Solution Crystallization Analysis by Laser Light Scattering (SCALLS)

Summary: Solution crystallization analysis by laser light scattering offers a direct way of studying the solution crystallization of polyolefins. The technique yields similar results to Crystaf, but in a shorter time and with apparently greater sensitivity in some cases. The use of SCALLS is demonstrated for the study of selected propylene/higher α-olefin copolymers. Some conclusions are also drawn regarding the effect of molecular weight on the solution crystallization of polyolefins.     

Base-base interactions in the minor groove of double-stranded DNA

A method has been developed for the synthesis of bisheaded nucleosides with thymine and adenine base moieties. We have demonstrated that, when incorporated in oligonucleotides, extrahelical A-T base interactions are possible when the bisheaded nucleosides are positioned in opposite strands of the duplex and are separated from each other by one regular base pair.     

Hydrodynamic chromatography separations in micro- and nanopillar arrays produced using deep-UV lithography (J. Sep. Science 35'15)

DOI: 10.1002/jssc.201200076 Micro-pillar array columns with an inter-pillar spacing down to 500 nm have been designed and fabricated enabling hydrodynamic chromatography separations of fluorescently labeled polystyrene nano-particles. Injecting sub-nanoliter sized sample volumes and detecting the fluorescent signal on-chip, fluorescein, 20 and 40 nm diameter particles could be separated in less than 1.5 minutes.     

Photo-orientation of axial molecules

We examine the photo-orientation of molecules in a linearly polarized field and the ensuing optical anisotropy of a sample. We propose a theoretical model that considers both photoinduced reorientation and rotational diffusion, for the case of linear or axial molecules not interacting among them, as in dilute solutions in viscous media. We perform numerical simulations to highlight the dependence on the parameters of the molecular reorientation processes, on the intensity of the exciting light, and on the use of cross polarized pulses. As a realistic example we simulate the photo-orientation of azobenzene in ethylene glycol.