Chiral ligand-exchange separation and resolution of extremely rigid glutamate analogs: 1-aminospiro[2.2]pentyl-1,4-dicarboxylic acids

Owing to their chelation ability, a series of fully constrained l-Glu analogs formed by the spiro-union of two cyclopropane rings (1-aminospiro[2.2]pentyl-1,4-dicarboxylic acids, ASPED A–D), was submitted to chiral ligand-exchange chromatographic (CLEC) analysis. As the initial step, two methodologically different chiral devices were evaluated. A chiral stationary phase (CSP) obtained by dynamic coating of C₁₈ chains with the S-trityl-(R)-cysteine ((R)-STC) was used first with this objective. The lack of separation of the enantiomers of ASPED C and D prompted us to utilize the chiral mobile phase (CMP) prepared from O-benzyl-(S)-serine ((S)-OBS). The latter afforded complete separation of the four pairs of enantiomers. For all the pairs, quantum mechanical investigations shed light on the main features responsible for the different enantiomer recognition mechanism with (S)-OBS. The validated analytical method was then fruitfully adopted for semi-preparative-scale isolation of the enantiomers of ASPED C.     

A new procedure for exact ring closure

An algorithm is presented that very efficiently generates all feasible values for the six torsions constrained by the condition of ring closure. Furthermore, the algorithm is straightforward to implement, thus providing an alternative to the more common and less efficient conformational search technique of breaking the ring and rejecting trial conformers with an unacceptable length for the broken bond. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 870–881, 2000     

On the motion of a vortex ring with a sharply concentrated vorticity

We study an incompressible non‐viscous fluid with axial symmetry without swirl, in the case when the vorticity is supported in an annulus. It is well known that there exist particular initial data for which the Euler evolution reduces to a translation with a constant speed. In this paper we prove a similar property for any initial condition in the limit situation in which the initial vorticity is sharply concentrated. Copyright © 2000 John Wiley & Sons, Ltd.     

Cathodic chloride extraction treatment of a late bronze-age artifact affected by bronze disease in room-temperature ionic-liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI)

This paper concentrates on a novel approach to the electrochemical treatment of bronze disease, based on the use of room-temperature ionic liquids (RTIL). In particular, we employed 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide as the electrolyte for the galvanostatic cathodic treatment of a late bronze-age artifact that had been exposed to marine environment during its history, dating back to ca. 1100 B.C. After an accurate metallographic and structural analysis of the as-found object—proving, among other findings, that bronze disease is essentially related to the presence of nantokite locked inside subsurface pits of typical equivalent diameter of several hundred micrometers, we subjected it to optimal electrochemical conditions, showing—on the basis of X-ray diffraction—that nantokite could be effectively removed and Cu(I) reduced to metallic Cu. Numerical computations in the full three-dimensional pit geometry, with realistic nonlinear electrochemical boundary conditions, provide the theoretical framework for the choice of RTIL—as opposed to aqueous solutions—and for the quantitative evaluation of Cl⁻ removal rates.     

Solid-phase synthesis of cyclic PNA and PNA-DNA chimeras

[structure: see text] A new and versatile on-line automated solid-phase approach to obtain cyclic PNA (I and III) and cyclic PNA-DNA chimeras (II) in highly pure form has been developed. Starting from a Tentagel matrix functionalized with a 3-chloro-4-hydroxyphenylacetic linker, the synthesis of representative, new cyclic molecules by standard peptide and phosphoramidite-based chemistry has been achieved.     

Fixed points in lambda calculus. An eccentric survey of problems and solutions

The fact that every combinator has a fixed point is at the heart of the $\lambda$-calculus as a model of computation. We consider several aspects of such phenomenon; our specific, perhaps eccentric, point of view focuses on problems and results that we consider worthy of further investigations. We first consider the relation with self application, in comparison with the opposite view, which stresses the role of coding, unifying the first and the second fixed point theorems. Then, we consider the relation with the diagonal argument, a relation which is at the origin of the fixed point theorem itself. We also review the Recursion Theorem, which is considered a recursion theoretic version of the fixed point theorem. We end considering systems of equations which are related to fixed points.     

Besov Spaces for the Schrödinger Operator with Barrier Potential

Let H = ?d ²/dx ² + V be a Schrödinger operator on the real line, where \({V=c\chi_{[a,b]}}\) , c > 0. We define the Besov spaces for H by developing the associated Littlewood–Paley theory. This theory depends on the decay estimates of the spectral operator \({{\varphi}_j(H)}\) for the high and low energies. We also prove a Mihlin multiplier theorem on these spaces, including the L ^p boundedness result. Our approach has potential applications to other Schrödinger operators with short-range potentials.     

Pointwise Comparison of PCM and Σ Δ Quantization

A quantitative comparison of Pulse Code Modulation (PCM) and Sigma–Delta (Σ Δ) quantization methods is made in the setting of finite frames. Frames allow for redundant, overcomplete signal decompositions. PCM and Σ Δ are two industry-standard quantization methods, and the setting of finite frames is appropriate for a host of modern applications. Previous results for this comparison are known for upper error bounds, where Σ Δ performs better in the setting of frames, as opposed to orthonormal bases, where PCM is optimal. We answer the following question: For which signals x is the PCM error, that is, the norm of the difference between x and its PCM approximant, less than the Σ Δ error? We prove that, typically, in the setting of frames, Σ Δ outperforms PCM, but not always.