3 research outputs found
Vibrational Circular Dichroism Shows Reversible Helical Handedness Switching in Peptidomimetic lāValine Fibrils
We elucidate the supramolecular organization in the form of microsize fibrils of gels formed by a l-Valine peptidomimetic compound. Analysis was based on circular dichroism spectroscopies, vibrational (VCD) and electronic (CD), supported by microscopy (atomic force and scanning electron). We show how the VCD spectra give account of the micrometric structure of the fibrils formed by the helicoidal arrangement of simpler proto-fibrils, which are organized in a lower hierarchical level. This ability is used to monitorize a fully reversible change in the handedness of the helix by modulating different external stimuli as pH or ionic strength, thus providing the first observation by VCD of such a phenomenon in a short peptide
Mode Robustness in Raman Optical Activity
By
reformulating Raman and ROA invariants we provide ground for the definition
of robust modes in ROA spectroscopy. Introduction of two parameters
defining robustness helps characterization and assignment of ROA bands.
Application and use of robustness parameters to [<i>n</i>]Āhelicenes and oxirane/thiirane derivatives are presented
Tuning Proton Conductivity in Alkali Metal Phosphonocarboxylates by Cation Size-Induced and Water-Facilitated Proton Transfer Pathways
The structural and functional chemistry
of a family of alkali-metal
ions with racemic <i>R</i>,<i>S</i>-hydroxyphosphonoacetate
(<b>M-HPAA</b>; M = Li, Na, K, Cs) are reported. Crystal structures
were determined by X-ray data (Li<sup>+</sup>, powder diffraction
following an ab initio methodology; Na<sup>+</sup>, K<sup>+</sup>,
Cs<sup>+</sup>, single crystal). A gradual increase in dimensionality
directly proportional to the alkali ionic radius was observed. [Li<sub>3</sub>(OOCCHĀ(OH)ĀPO<sub>3</sub>)Ā(H<sub>2</sub>O)<sub>4</sub>]Ā·H<sub>2</sub>O (<b>Li-HPAA</b>) shows a 1D framework built up by
Li-ligand āslabsā with Li<sup>+</sup> in three different
coordination environments (4-, 5-, and 6-coordinated). <b>Na-HPAA</b>, Na<sub>2</sub>(OOCCHĀ(OH)ĀPO<sub>3</sub>H)Ā(H<sub>2</sub>O)<sub>4</sub>, exhibits a pillared layered āhouse of cardsā structure,
while <b>K-HPAA</b>, K<sub>2</sub>(OOCCHĀ(OH)ĀPO<sub>3</sub>H)Ā(H<sub>2</sub>O)<sub>2</sub>, and <b>Cs-HPAA</b>, CsĀ(HOOCCHĀ(OH)ĀPO<sub>3</sub>H), typically present intricate 3D frameworks. Strong hydrogen-bonded
networks are created even if no water is present, as is the case in <b>Cs-HPAA</b>. As a result, all compounds show proton conductivity
in the range 3.5 Ć 10<sup>ā5</sup> S cm<sup>ā1</sup> (<b>Cs-HPAA</b>) to 5.6 Ć 10<sup>ā3</sup> S cm<sup>ā1</sup> (<b>Na-HPAA</b>) at 98% RH and <i>T</i> = 24 Ā°C. Differences in proton conduction mechanisms, Grothuss
(Na<sup>+</sup> and Cs<sup>+</sup>) or vehicular (Li<sup>+</sup> and
K<sup>+</sup>), are attributed to the different roles played by water
molecules and/or proton transfer pathways between phosphonate and
carboxylate groups of the ligand HPAA. Upon slow crystallization,
partial enrichment in the <i>S</i> enantiomer of the ligand
is observed for <b>Na-HPAA</b>, while the <b>Cs-HPAA</b> is a chiral compound containing only the <i>S</i> enantiomer