Configurational assignment of alpha-chiral carboxylic acids by complexation to dimeric Zn-porphyrin: host-guest structure,chiral recognition and circular dichroism

A circular dichroic (CD) excition chirality method based on host-guest chiral recognition has been developed to determine the absolute configuration of carboxylic acids with an alpha-stereogenic center; an amide C = O-->Zn coordination, identified by infrared spectroscopy and computations, is involved in this complexation.     

Absolute configurational assignments of secondary amines by CD-sensitive dimeric zinc porphyrin host

A general chiroptical protocol for determination of absolute configuration of secondary amines including acyclic and cyclic aliphatic amines, aromatic amines, amino acids, and amino alcohols is described. The chiral substrate is linked to the achiral carrier moiety (3-N-Boc-amino-propyl-N-Boc-amino)acetic acid 1 (BocHNCH(2)CH(2)CH(2)BocNCH(2)COOH), which after deprotection, yields a bidentate conjugate, capable of forming a 1:1 host/guest complex with dimeric zinc porphyrin host 2. As in the cases of primary amines and secondary alcohols reported earlier, the complexation of secondary amine conjugates to porphyrin tweezer host 2 represents a stereodifferentiating process, where the large (L) group at the stereogenic center (assigned on the basis of conformational energies A value) protrudes from the porphyrin binding pocket. This leads to formation of host/guest complexes with a preferred porphyrin helicity that exhibit intense exciton split CD spectra. It was found that the chiral sense of porphyrin twist is clearly controlled by the stereogenic center despite the Z/E conformational complexity around the tertiary amide bond of secondary amine conjugates that has greatly hampered previous configurational assignments. Thus, in cases where there is no ambiguity regarding the relative steric size of substituents, the observed CD couplet can be applied for straightforward assignment of absolute configurations. In addition, to extend the application to more difficult cases a molecular mechanics calculation approach using the Merck Molecular Force Field (MMFFs) was developed; this provides conformational information of host/guest complexes and leads to prediction of preferred porphyrin helicity independent of conformational A values. This chiroptical protocol in combination with molecular modeling represents a general method for configurational assignments of secondary amines.     

A qualitative model for the growth mechanism of silver clusters in polymer solution

A simple and high-reproducible method for the synthesis of polymer-protected silver cluster of controlled size is described. UV-visible spectroscopy has been used for investigating the influence of the aging of the protective poly(vinylpyrrolidone) layer on the cluster growth rate at different reaction temperatures and poly(vinylpyrrolidone)/ethylene glycol weight ratios. The obtained results show that the aging time of the polymeric stabilizer solution plays a fundamental role in the reproducibility of the cluster growth process. A model for the metal cluster formation-grow process is also proposed. Received 18 July 2001 and Received in final form 3 October 2001     

PMMA Based Nanocomposites Filled with Modified CaCO3 Nanoparticles

PMMA based nanocomposites filled with calcium carbonate nanoparticles (CaCO₃) have been prepared by in situ polymerization approach. In order to improve inorganic nanofillers/polymer compatibility, PBA chains have been grafted onto CaCO₃ nanoparticle surface. Morphological analysis performed on nanocomposite fractured surfaces has revealed that the CaCO₃ modification induces homogeneous and fine dispersion of nanoparticles into PMMA as well as strong interfacial adhesion between the two phases. Mechanical tests have shown that both unmodified and modified CaCO₃ are responsible for an increase of the Young's Modulus, whereas only PBA-grafted nanoparticles allow to keep unchanged impact strength, strongly deteriorated by adding unmodified CaCO₃. Finally, the presence of CaCO₃ nanoparticles significantly improves the abrasion resistance of PMMA also modifying its wear mechanism.     

Erratum to “Heavy neutral gauge bosons at the LHC in an extended MSSM” [Nucl. Phys. B 866 (3) (2013) 293–336]

We correct a Feynman diagram presented in the paper ‘Heavy neutral gauge bosons at the LHC in an extended MSSM’, published in Nuclear Physics B 866 (2013) 293.     

Insights About CHAPS Aggregation Obtained by Spin Label EPR Spectroscopy

CHAPS is a zwitterionic surfactant derivative of bile salts, widely used in membrane protein isolation. While some studies regarding CHAPS self-aggregation suggest continuous increase in micelle size at increasing CHAPS concentration, other works point to the existence of two definite micelle types. In this work, stearic acid spin labels (5, 12, or 16-SASL) were added to CHAPS solutions to obtain information about the micellar structure using electron paramagnetic resonance spectroscopy. The spectra of 12-SASL were processed using Principal Factor Analysis, and at all concentrations they could be reproduced as linear combinations of only three fundamental spectra, the first one corresponding to free 12-SASL in aqueous solution. This fact suggests only two hydrophobic environments that host 12-SASL, assigned to primary and secondary CHAPS micelles. The relative populations of the label in each environment were obtained as a function of CHAPS concentration. Our results suggest barrel-shaped primary micelles with a minimum mean radius of 1.46 nm, and a critical micelle concentration cmc_I = 4 mM. Secondary micelles are formed by aggregation of primary ones, with cmc_II = 10 mM. They have several elongated hydrophobic pockets, with similar dimensions for all aggregate sizes. Our results contribute to the understanding of the mechanism of interaction of chain amphiphiles with CHAPS micelles.     

Self detachment of free-standing porous silicon membranes in moderately doped n-type silicon

In this article we describe a reliable etching method to fabricate porous silicon free-standing membranes (FSMs) based on a self detachment of the porous layer in moderately doped n-type silicon substrates. We found that stable growth of smooth and straight pores is restricted to a narrow range of etching conditions and, unlike p-type substrates, the lift-off of the membrane is a self-limited process that does not require a large burst of current. The detachment of the porous membrane is independent of the structure of the already porosified layer, meaning that the average pore diameter can be tuned from nano to macro size within the same membrane. We also demonstrate that, despite their limited thickness, FSMs are quite robust and can sustained further processing. Thus, the etching receipt we are proposing here extends the range of sensors and filters that can be fabricated using porous silicon technology.     

Noether–Lefschetz theory with base locus

Let \(Z\) be a closed subscheme of a smooth complex projective variety \(Y\subseteq \mathbb {P}^N\) , with \(\dim \,Y=2r+1\ge 3\) . We describe the intermediate Néron–Severi group (i.e. the image of the cycle map \(A_r(X)\rightarrow H_{2r}(X;\mathbb {Z})\) ) of a general smooth hypersurface \(X\subset Y\) of sufficiently large degree containing \(Z\) .