Going Off the Grid: Iterative Model Selection for Biclustered Matrix Completion

We consider the problem of performing matrix completion with side information on row-by-row and column-by-column similarities. We build upon recent proposals for matrix estimation with smoothness constraints with respect to row and column graphs. We present a novel iterative procedure for directly minimizing an information criterion to select an appropriate amount of row and column smoothing, namely, to perform model selection. We also discuss how to exploit the special structure of the problem to scale up the estimation and model selection procedure via the Hutchinson estimator, combined with a stochastic Quasi-Newton approach. Supplementary material for this article is available online.     

Photografting as a versatile,localizable, and single‐step surface functionalization of silica‐based monoliths dedicated to microscale separation techniques

In this work, we developed a surface functionalization way of silica monoliths with a rapid, simple, versatile, and localizable photografting step. The elaboration of a photoreactive layer at the surface of monoliths was first optimized. The functionalization with [γ‐(methacryloyloxy)propyl]trimethoxysilane at 80°C in a hydro‐organic solution containing triethylamine as catalyst allows reachng the highest density of methacrylate photoactive moieties on silica surfaces. These methacrylate reactive surfaces were subsequently photografted within few minutes with acrylate monomers bearing alkyl chains (C₁₂ and C₁₈). The photografting efficiency was determined by monitoring the retentive properties of monoliths in the RP mode. The retention factors are of the same order of magnitude as highly retentive columns obtained by modification of silica surface with long‐alkyl chain silanes or by thermal polymerization of long‐alkyl chain monomers. It was also verified that such grafting neither impaired the efficiency of the monolithic stationary phase (H_min = 6–8 μm in nano‐LC) nor its permeability (about 6 × 10^?14 m²). Further, it was also demonstrated that photografting is localizable in nonmasked defined areas. Results obtained in anion‐exchange chromatography after photopolymerization of [2‐(methacryloyloxy)ethyl]trimethylammonium chloride are presented as well to demonstrate the versatility of the developed approach.     

Retention of β blockers on native titania stationary phase

In recent years, metal oxides such as titania have been commercially available as chromatographic beds that can potentially be used to achieve novel separations of polar compounds. For example β blockers, which are more often encountered in environmental sciences, have a wide range of polarity, and their basic character leads to difficult sample treatment and separation on conventional silica‐based sorbents. The contribution of titania to the selective analysis of nine β blockers was evaluated in terms of retention mechanisms observed in hydrophilic interaction LC using acetonitrile/water mobile phases with various additives. The mobile phase additives enabled to control the β blocker charge as well as the titania surface charge. Depending on their respective ionic state, various retention mechanisms were identified at low water contents (<40%), including mainly adsorption mixed with hydrophilic interaction LC partition, ion exchange and ion exclusion. An unexpected retention was also observed for high water content and high pH, changing the selectivity of the support.     

Efficient Krylov subspace methods for uncertainty quantification in large Bayesian linear inverse problems

Uncertainty quantification for linear inverse problems remains a challenging task, especially for problems with a very large number of unknown parameters (e.g., dynamic inverse problems) and for problems where computation of the square root and inverse of the prior covariance matrix are not feasible. This work exploits Krylov subspace methods to develop and analyze new techniques for large‐scale uncertainty quantification in inverse problems. In this work, we assume that generalized Golub‐Kahan‐based methods have been used to compute an estimate of the solution, and we describe efficient methods to explore the posterior distribution. In particular, we use the generalized Golub‐Kahan bidiagonalization to derive an approximation of the posterior covariance matrix, and we provide theoretical results that quantify the accuracy of the approximate posterior covariance matrix and of the resulting posterior distribution. Then, we describe efficient methods that use the approximation to compute measures of uncertainty, including the Kullback‐Liebler divergence. We present two methods that use the preconditioned Lanczos algorithm to efficiently generate samples from the posterior distribution. Numerical examples from dynamic photoacoustic tomography demonstrate the effectiveness of the described approaches.     

Use of N-trifluoroacetyl-protected amino acid chlorides in peptide coupling reactions with virtually complete preservation of stereochemistry

The use of protected amino acid chlorides for peptide coupling reactions has long been avoided due to the extensive racemization that commonly occurs during either the acid chloride formation or the coupling reaction itself. Conditions are described which allow N-trifluoroacetyl-protected amino acid chlorides to be generated in high purity and with high retention of stereochemical integrity. Control of temperature is the predominant factor in controlling racemization, and rapid formation of acid chlorides under low temperature can be conveniently achieved using Vilsmeier reagent. Stereochemical integrity is further maintained when coupling of N-trifluoroacetyl acid chlorides is carried out with amino acid esters under Schotten-Baumann conditions using specific controls on pH, temperature, and agitation. Second order rate constants for coupling and the azlactone formation associated with racemization were measured to be 4260 and 3.6 L/mol s, respectively. This high rate differential allows for the reaction to be run with a minimum excess of amine ester, and makes it suitable for continuous processing. The applicability of the preferred coupling conditions to a range of amino acid couplings is described.     

In vivo metabolic investigation of silodosin using UHPLC–QTOF–MS/MS and in silico toxicological screening of its metabolites

Silodosin (SLD) is a novel α1‐adrenoceptor antagonist which has shown promising clinical efficacy and safety in patients with benign prostatic hyperplasia (BPH). However, lack of information about metabolism of SLD prompted us to investigate metabolic fate of SLD in rats. To identify in vivo metabolites of SLD, urine, feces and plasma were collected from Sprague–Dawley rats after its oral administration. The samples were prepared using an optimized sample preparation approach involving protein precipitation followed by solid‐phase extraction and then subjected to LC/HR‐MS/MS analysis. A total of 13 phase I and six phase II metabolites of SLD have been identified in rat urine which includes hydroxylated, N‐dealkylated, dehydrogenated, oxidative, glucosylated, glucuronide and N‐sulphated metabolites, which are also observed in feces. In plasma, only dehydrogenated, N‐dealkylated and unchanged SLD are observed. The structure elucidation of metabolites was done by fragmentation in MS/MS in combination with HRMS data. The potential toxicity profile of SLD and its metabolites were predicted using TOPKAT software and most of the metabolites were proposed to show a certain degree of skin sensitization and occular irritancy. Copyright © 2016 John Wiley & Sons, Ltd.     

Supramolecular Frameworks Based on Rhenium Clusters Using the Synthons Approach

The reaction of the K₄[{Re₆Sⁱ₈}(OH)^a₆]·8H₂O rhenium cluster salt with pyrazine (Pz) in aqueous solutions of alkaline or alkaline earth salts at 4 °C or at room temperature leads to apical ligand exchange and to the formation of five new compounds: [trans-{Re₆Sⁱ₈}(Pz)^a₂(OH)^a₂(H₂O)^a₂] (1), [cis-{Re₆Sⁱ₈}(Pz)^a₂(OH)^a₂(H₂O)^a₂] (2), (NO₃)[cis-{Re₆Sⁱ₈}(Pz)^a₂(OH)^a(H₂O)^a₃](Pz)·3H₂O (3), [Mg(H₂O)₆]_0.5[cis-{Re₆Sⁱ₈}(Pz)^a₂(OH)^a₃(H₂O)^a]·8.5H₂O (4), and K[cis-{Re₆Sⁱ₈}(Pz)^a₂(OH)^a₃(H₂O)^a]·8H₂O (5). Their crystal structures are built up from trans- or cis-[{Re₆Sⁱ₈}(Pz)^a₂(OH)^a_4−x(H₂O)^a_x]^x−2 cluster units. The cohesions of the 3D supramolecular frameworks are based on stacking and H bonding, as well as on H₃O₂₋bridges in the cases of (1), (2), (4), and (5) compounds, while (3) is built from stacking and H bonding only. This evidences that the nature of the synthons governing the cluster unit assembly is dependent on the hydration rate of the unit.     

One‐pot synthesis of a new high vinyl content hybrid silica monolith dedicated to nanoliquid chromatography

A new vinyltrimethoxysilane‐based hybrid silica monolith was developed and used as a reversed‐phase capillary column. The synthesis of this rich vinyl hybrid macroporous monolith, by cocondensation of vinyltrimethoxysilane with tetramethoxysilane, was investigated using an unconventional (formamide, nitric acid) porogen/catalyst system. A macroporous hybrid silica monolith with 80% in mass of vinyltrimethoxysilane in the feeding silane solution was obtained and compared to a more conventional low vinyl content hybrid monolith with only of 20% vinyltrimethoxysilane. Monoliths were characterized by scanning electron microscopy, ²⁹Si nuclear magnetic resonance spectroscopy and N₂ adsorption–desorption. About 80% of the vinyl precursor was incorporated in the final materials, leading to 15.9 and 61.5% of Si atoms bonded to vinyl groups for 20% vinyltrimethoxysilane and 80% vinyltrimethoxysilane, respectively. The 80% vinyltrimethoxysilane monolith presents a lower surface area than 20% vinyltrimethoxysilane (159 versus 551 m²/g), which is nevertheless compensated by a higher vinyl surface density. Chromatographic properties were evaluated in reversed‐phase mode. Plots of ln(k) versus percentage of organic modifier were used to assess the reversed‐phase mechanism. Its high content of organic groups leads to high retention properties. Column efficiencies of 170 000 plates/m were measured for this 80% vinyltrimethoxysilane hybrid silica monolith. Long capillary monolithic columns (90 cm) were successfully synthesized (N = 120 000).     

High temperature drop calorimetry measurements of enthalpy increment in Ti–xTa (x=5, 10, 15, 20 mass%) alloys

The enthalpy increment for a series of Ti–xTa (x=5, 10, 15, 20 mass%) alloys, having α(hcp)+β(bcc) two phase microstructure has been measured using inverse drop calorimetry in the temperature range 463–1257 K. The studies clearly revealed the occurrence of α→β diffusional phase transformation. Both the α→β transformation onset temperatures and the measured transformation enthalpy Δ⁰H_tr^α→β exhibit progressively lower values with increasing Ta content. It is found that the measured enthalpy in the transformation region is constituted of two principal contributions namely, (i) the enthalpy due to untransformed α and coexisting β phases and (ii) the transformation enthalpy due to α→β phase change. Since the fractional extent of α→β transformation varies continuously with temperature, the measured enthalpy variation in the transformation domain has been modeled using Kolmogorov–Johnson–Mehl–Avrami formalism for the diffusional transformation kinetics. The thermodynamic quantities for all the alloys have been derived.