Vibrational Circular Dichroism Shows Reversible Helical Handedness Switching in Peptidomimetic l‑Valine 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>]­helicenes 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⁺, powder diffraction following an ab initio methodology; Na⁺, K⁺, Cs⁺, single crystal). A gradual increase in dimensionality directly proportional to the alkali ionic radius was observed. [Li₃(OOCCH­(OH)­PO₃)­(H₂O)₄]·H₂O (<b>Li-HPAA</b>) shows a 1D framework built up by Li-ligand “slabs” with Li⁺ in three different coordination environments (4-, 5-, and 6-coordinated). <b>Na-HPAA</b>, Na₂(OOCCH­(OH)­PO₃H)­(H₂O)₄, exhibits a pillared layered “house of cards” structure, while <b>K-HPAA</b>, K₂(OOCCH­(OH)­PO₃H)­(H₂O)₂, and <b>Cs-HPAA</b>, Cs­(HOOCCH­(OH)­PO₃H), 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 × 10^–5 S cm^–1 (<b>Cs-HPAA</b>) to 5.6 × 10^–3 S cm^–1 (<b>Na-HPAA</b>) at 98% RH and <i>T</i> = 24 °C. Differences in proton conduction mechanisms, Grothuss (Na⁺ and Cs⁺) or vehicular (Li⁺ and K⁺), 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