pH Dependent Chiroptical Properties of (1<i>R</i>,2<i>R</i>)- and (1<i>S</i>,2<i>S</i>)-<i>trans</i>-Cyclohexane Diesters and Diamides from VCD, ECD, and CPL Spectroscopy

Diesters of (1<i>R</i>,2<i>R</i>)- and (1<i>S</i>,2<i>S</i>)-cyclohexanediols and diamides of (1<i>R</i>,2<i>R</i>)- and (1<i>S</i>,2<i>S</i>)-diaminocyclohexane with <i>p</i>-hydroxycinnamic acid have been known for some time to exhibit intense bisignate electronic circular dichroism (ECD) spectra in CH₃OH. It was also known that added NaOH causes a bathochromic shift of ∼50 nm in CH₃OH, and an even higher one in DMSO. We have measured vibrational circular dichroism (VCD) spectra both for neutral compounds and in the presence of NaOH and other bases. The VCD and IR spectra in the mid-IR region for CD₃OD and DMSO-<i>d</i>₆ solution exhibit high sensitivity to the charged state for the diesters. They possess two strong bisignate features in the presence of bases in the mid-IR, which are interpreted in terms of vibrational exciton couplets, while this phenomenon is less evident in diamides. VCD allied to density functional theory (DFT) calculations allows one to shed some light on the spectral differences of diesters and diamides by studying their conformational properties. Optical rotatory dispersion (ORD) curves confirm the ECD data. Circularly polarized luminescence (CPL) data have been also acquired, which are rather intense in basified solution: the CPL spectra are monosignate and are as intense in the diester and in the diamide case

Looking at Human Cytosolic Sialidase NEU2 Structural Features with an Interdisciplinary Approach

Circular dichroism (CD) spectra at variable temperatures have been recorded for human cytosolic sialidase NEU2 in buffered water solutions and in the presence of divalent cations. The results show the prevalence of β-strands together with a considerable amount of α-helical structure, while in the solid state, from both previous X-ray diffraction analysis and our CD data on film samples, the content of β-strands is higher. In solution, a significant change in CD spectra occurs with an increase in temperature, related to a decrease in α-helix content and a slight increase in β-strand content. In the same range of temperatures, the enzymatic activity decreases. Although the overall structure of the protein appears to be particularly stable, molecular dynamics simulations performed at various temperatures evidence local conformational changes possibly relevant for explaining the relative lability of enzymatic activity

Helical Sense-Responsive and Substituent-Sensitive Features in Vibrational and Electronic Circular Dichroism, in Circularly Polarized Luminescence, and in Raman Spectra of Some Simple Optically Active Hexahelicenes

Four different hexahelicenes, 5-aza-hexahelicene (<b>1</b>), hexahelicene (<b>2</b>), 2-methyl-hexahelicene (<b>3</b>), and 2-bromo-hexahelicene (<b>4</b>), were prepared and their enantiomers, which are stable at r.t., were separated. Vibrational circular dichroism (VCD) spectra were measured for compound <b>1</b>; for all the compounds, electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) spectra were recorded. Each type of experimental spectrum was compared with the corresponding theoretical spectrum, determined via Density Functional Theory (DFT). Following the recent papers by Nakai et al., this comparison allowed to identify some features related to the helicity and some other features typical of the substituent groups on the helical backbone. The Raman spectrum of compound <b>1</b> is also examined from this point of view