A structure determination protocol based on combined analysis of 3D-ED data,powder XRD data,solid-state NMR data and DFT-D calculations reveals the structure of a new polymorph of l-tyrosine

We report the crystal structure of a new polymorph of l-tyrosine (denoted the β polymorph), prepared by crystallization from the gas phase following vacuum sublimation. Structure determination was carried out by combined analysis of three-dimensional electron diffraction (3D-ED) data and powder X-ray diffraction (XRD) data. Specifically, 3D-ED data were required for reliable unit cell determination and space group assignment, with structure solution carried out independently from both 3D-ED data and powder XRD data, using the direct-space strategy for structure solution implemented using a genetic algorithm. Structure refinement was carried out both from powder XRD data, using the Rietveld profile refinement technique, and from 3D-ED data. The final refined structure was validated both by periodic DFT-D calculations, which confirm that the structure corresponds to an energy minimum on the energy landscape, and by the fact that the values of isotropic ¹³C NMR chemical shifts calculated for the crystal structure using DFT-D methodology are in good agreement with the experimental high-resolution solid-state ¹³C NMR spectrum. Based on DFT-D calculations using the PBE0-MBD method, the β polymorph is meta-stable with respect to the previously reported crystal structure of l-tyrosine (now denoted the α polymorph). Crystal structure prediction calculations using the AIRSS approach suggest that there are three other plausible crystalline polymorphs of l-tyrosine, with higher energy than the α and β polymorphs.

A new polymorph of l-tyrosine is reported, with the crystal structure determined by combined analysis of 3D-ED data and powder XRD data, augmented by information from periodic DFT-D calculations and solid-state ¹³C NMR data.     

A comparison of quantitative nuclear magnetic resonance methods: internal,external, and electronic referencing

The performance of three quantitative NMR methods was compared in terms of short‐term and long‐term precision and accuracy, robustness, linear range, and general applicability. The Internal Reference method employs a reference material co‐dissolved with sample; the External Reference method employs a reference material contained in a separate solution; and the third method, known as Electronic REference To access In vivo Concentrations (ERETIC), employs an externally calibrated digital reference peak. The Internal Reference method results were the most precise and remained stable within 0.1% for at least 4 weeks. The results from the External Reference and ERETIC methods were practically equivalent to each other during this time. These methods exhibited small differences relative to the standard set by the Internal Reference method and slightly lower precision, establishing them as practical alternatives to the Internal Reference method. In contrast to the Internal Reference method, the External Reference and ERETIC methods possess several advantages that address peak overlap, flexibility of calibration, and duration of applicability. The study was designed such that each spectrum contained the information needed to compare the three methods while all other variables were kept constant. Applicability of pulse width compensation is addressed. ERETIC software compensation and minor adjustments to 90° pulse width were concluded to be unnecessary for this system. Although each of the methods was applied here to specifically calculate and compare chemical purity values, this evaluation applies generally to absolute quantitation by NMR. Copyright © 2013 John Wiley & Sons, Ltd.     

Comparison of the growth and degradation of poly(glycolic acid) and poly(ε‐caprolactone) brushes

The growth and degradation of poly(glycolic acid) (PGA) and poly(ε‐caprolactone) (PCL) brushes were compared. Using tin (octanoate) as the catalyst, optimal conditions were found for growth of each polyester brush from the hydroxy‐terminated silicon surface via ring‐opening polymerization. PCL brushes grew thicker at elevated temperatures but the thickest PGA brushes grew at room temperature. Unlike bulk polyesters that can degrade under both acidic and basic conditions, the confined surface polyester brushes only degraded under neutral or basic conditions. The degradation mechanism of grafted polyester brushes was probed through a blocking test. It was shown that the terminal hydroxy groups of these polyester brushes were essential to the degradation process indicating a preferential backbiting mechanism. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4643–4649     

Synthesis of mixed cyclotriveratrylenes

Cyclotriveratrylenes containing one benzene ring and two indole rings linked through N1 and C2 can be prepared by acid-catalysed reactions of formaldehyde and aryl aldehydes with 1,2-di-(1-indolylmethyl)benzene compounds.     

Transformation of macromonomers into ring‐opening polymerization macroinitiators: A detailed initiation efficiency study

In the present study, n‐butyl acrylate macromonomer (BAMM) (M_n = 1900 g mol^?1; PDI = 1.96) has been synthesized via a high‐temperature polymerization process. Subsequently, the olefinic termini of the BAMM have been transformed into a diol via a dihydroxylation process using KMnO₄ as an oxidizing agent. The OH‐terminated macroinitiator pBA(OH)₂ has subsequently been employed for the ring‐opening polymerization (ROP) of ε‐caprolactone via various catalytic systems, that is, organo‐(1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene), metal (tin(II) 2‐ethylhexanoate), and enzymatic catalysis (Novozym® 435). The obtained pBA‐b‐pCL block copolymers and the initiation efficiency of the BAMM macroinitiator have been investigated via size exclusion chromatography (SEC), electrospray ionization–mass spectrometry (ESI‐MS) hyphenated with SEC and liquid chromatography at the critical conditions of both poly(ε‐caprolactone) (pCL) and pBA. The in vitro enzyme catalysis (eROP) approach proved to be the most efficient catalysis system due to minor transesterification side reactions during the polymerization process. However, side reactions such as transesterifications occur in each catalytic system and—while they cannot be suppressed—they can be minimized. The species generated during the eROP process include the desired block copolymer pBA‐b‐pCL as main species as well as pCL homopolymer and residual macroinitiator pBA(OH)₂. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Single‐pulse pulsed laser polymerization–electron paramagnetic resonance investigations into the termination kinetics of n‐butyl acrylate macromonomers

The termination of model mid‐chain radicals (MCRs), which mimic radicals that occur in acrylate polymerization over a broad range of reaction conditions, has been studied by single‐pulse pulsed laser polymerization (SP‐PLP) in conjunction with electron paramagnetic resonance spectroscopy. The model radicals were generated by initiator‐fragment addition to acrylic macromonomers that were preformed prior to the kinetic experiments, thus enabling separation of termination from the propagation reaction, for these model radicals propagate sparingly, if at all, on the timescale of SP‐PLP experiments. Termination rate coefficients of the MCRs were determined in the temperature range of 0–60°C in acetonitrile and butyl propionate solution as well as in bulk macromonomer over the range of 0–100 °C. Termination rate coefficients slightly below those of the corresponding secondary radicals were deduced, demonstrating the relatively high termination activity of this species, even when undergoing MCR–MCR termination. For chain length of 10, a reduction by a factor of 6 is observed. Unusually high activation energies were found for the termination rate coefficient in these systems, with 35 kJ mol^?1 being determined for bulk macromonomer. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012