Lenka Hanyková

NMR spectroscopic detection of chirality and enantiopurity in referenced systems without formation of diastereomers.

Enantiomeric excess of chiral compounds is a key parameter that determines their activity or therapeutic action. The current paradigm for rapid measurement of enantiomeric excess using NMR is based on the formation of diastereomeric complexes between the chiral analyte and a chiral resolving agent, leading to (at least) two species with no symmetry relationship. Here we report an effective method of enantiomeric excess determination using a symmetrical achiral molecule as the resolving agent, which is based on the complexation with analyte (in the fast exchange regime) without the formation of diastereomers. The use of N,N′-disubstituted oxoporphyrinogen as the resolving agent makes this novel method extremely versatile, and appropriate for various chiral analytes including carboxylic acids, esters, alcohols and protected amino acids using the same achiral molecule. The model of sensing mechanism exhibits a fundamental linear response between enantiomeric excess and the observed magnitude of induced chemical shift non-equivalence in the 1H NMR spectra.

1H NMR study of thermotropic phase transition of linear and crosslinked poly(vinyl methyl ether) in D2O

Abstract The changes in the dynamic structure during temperature-induced phase transition in poly(vinyl methyl ether) (PVME)/D2O solutions and gels in a broad range of concentrations (c=0.1–30 wt %) and crosslinking densities, respectively, were studied by 1H NMR methods. Similar behaviour was found both for linear and crosslinked systems, indicating the formation of compact globular-like structures during the phase transition. The fraction of PVME segments in globular-like structures is ∼0.85 for solutions and ∼1.0 for swollen networks, independent of the polymer concentration or the crosslinking density. While for dilute PVME solutions the thermotropic transition as detected by NMR is virtually discontinuous, for semidilute and concentrated solutions, and for swollen networks, the transition sets in at lower temperatures and its width is several Kelvin broad.

Phase separation in Poly(N, N-diethylacrylamide)/D2O solutions and physical gels as studied by 1H NMR spectroscopy

The structural-dynamic changes during temperature-induced phase separation in poly(N,N-diethylacrylamide) (PDEAAm)/D 2 O solutions and physical gels in a broad range of concentrations (c = 0.5-50 wt.-%) were studied by conventional high-resolution and MAS 1 H NMR spectra. In the whole concentration range, irrespective of whether the studied system is a solution (c 5 wt.-%), the phase transition is manifested by line brondening (linewidth 3.6 kHz) of a major part of PDEAAm anits, evidently due to the formation of compact globular-like structures. 1 H. MAS NMR spectra have shown that this broadening is not due to near-static dipolar interactions. The respective motion is effectively isotropic with correlation time 1 μs, probably corresponding to Brownian tumbling of the whole globular particles. The thermotropic phase transition, as revealed by NMR, is not discontinuous, but 6 K broad (302-308 K). Above the LCST transition, the fraction p * of PDEAAm segments in globular-like structures is between 0.94-1.0, independent of the polymer concentration. At temperatures below the LCST a certain preaggregation of PDEAAm is indicated by smaller line broadening (linewidths 100 Hz) in the 1 H NMR spectra.

Structural Modulation of Chromic Response: Effects of Binding-Site Blocking in a Conjugated Calix[4]pyrrole Chromophore

Abstract Herein, we modulate the chromic response of a highly colored tetrapyrrole macrocycle, namely, tetrakis(3,5‐di‐tert‐butyl‐4‐oxocyclohexadien‐2,5‐yl)porphyrinogen (OxP) by structural modification. N‐Benzylation at the macrocyclic nitrogen atoms leads to stepwise elimination of the two calix[4]pyrrole‐type binding sites of OxP and serial variation of the chromic properties of the products, double N‐benzylated Bz2OxP and tetra N‐benzylated Bz4OxP. The halochromic (response to acidity) and solvatochromic (response to solvent polarity) properties were studied by using UV/Vis spectroscopy and NMR spectroscopy in nonpolar organic solvents. Titration experiments were used to generate binding isotherms to elucidate their binding properties with difluoroacetic acid. Differences in the halochromic properties of the compounds allowed construction of a colorimetric scale of acidity in nonpolar solvents, as the compounds in the series OxP, Bz2OxP, and Bz4OxP are increasingly difficult to protonate but maintain their propensity to change color upon protonation. The concurrent effects of binding‐site blocking and modulation of acidity sensitivity are important new aspects for the development of colorimetric indicators.

Improving the Colloidal Stability of Temperature-Sensitive Poly(N-isopropylacrylamide) Solutions Using Low Molecular Weight Hydrophobic Additives

Poly(N-isopropylacrylamide) (PNIPAM) is an important polymer with stimuli-responsive properties, making it suitable for various uses. Phase behavior of the temperature-sensitive PNIPAM polymer in the presence of four low-molecular weight additives tert-butylamine (t-BuAM), tert-butyl alcohol (t-BuOH), tert-butyl methyl ether (t-BuME), and tert-butyl methyl ketone (t-BuMK) was studied in water (D2O) using high-resolution nuclear magnetic resonance (NMR) spectroscopy and dynamic light scattering. Phase separation was thermodynamically modeled as a two-state process which resulted in a simple curve which can be used for fitting of NMR data and obtaining all important thermodynamic parameters using simple formulas presented in this paper. The model is based on a modified van’t Hoff equation. Phase separation temperatures Tp and thermodynamic parameters (enthalpy and entropy change) connected with the phase separation of PNIPAM were obtained using this method. It was determined that Tp is dependent on additives in the following order: Tp(t-BuAM) > Tp(t-BuOH) > Tp(t-BuME) > Tp(t-BuMK). Also, either increasing the additive concentration or increasing pKa of the additive leads to depression of Tp. Time-resolved 1H NMR spin–spin relaxation experiments (T2) performed above the phase separation temperature of PNIPAM revealed high colloidal stability of the phase-separated polymer induced by the additives (relative to the neat PNIPAM/D2O system). Small quantities of selected suitable additives can be used to optimize the properties of PNIPAM preparations including their phase separation temperatures, colloidal stabilities, and morphologies, thus improving the prospects for the application.

Microphase-Separated PE/PEO Thin Films Prepared by Plasma-Assisted Vapor Phase Deposition

Immiscible polymer blends tend to undergo phase separation with the formation of nanoscale architecture which can be used in a variety of applications. Different wet-chemistry techniques already exist to fix the resultant polymeric structure in predictable manner. In this work, an all-dry and plasma-based strategy is proposed to fabricate thin films of microphase-separated polyolefin/polyether blends. This is achieved by directing (-CH2-)100 and (-CH2-CH2-O-)25 oligomer fluxes produced by vacuum thermal decomposition of poly(ethylene) and poly(ethylene oxide) onto silicon substrates through the zone of the glow discharge. The strategy enables mixing of thermodynamically incompatible macromolecules at the molecular level, whereas electron-impact-initiated radicals serve as cross-linkers to arrest the subsequent phase separation at the nanoscale. The mechanism of the phase separation as well as the morphology of the films is found to depend on the ratio between the oligomeric fluxes. For polyolefin-rich mixtures, polyether molecules self-organize by nucleation and growth into spherical domains with average height of 22 nm and average diameter of 170 nm. For equinumerous fluxes and for mixtures with the prevalence of polyethers, spinodal decomposition is detected that results in the formation of bicontinuous structures with the characteristic domain size and spacing ranging between 5 × 10(1) -7 × 10(1) nm and 3 × 10(2)-4 × 10(2) nm, respectively. The method is shown to produce films with tunable wettability and biologically nonfouling properties.