Determination of the absolute configuration of chiral molecules via density functional theory calculations of vibrational circular dichroism and optical rotation: The chiral alkane D3-anti-trans-anti-trans-anti-trans-perhydrotriphenylene

The Absolute configuration (AC) of the chiral alkane D₃-anti-trans-anti-trans-anti-trans-perhydrotriphenylene (PHTP), 1, is determined by comparison of density functional theory (DFT) calculations of its vibrational circular dichroism (VCD) and optical rotation (OR) to the experimental VCD and OR of (+)−1, obtained in high enantiomeric excess using chiral gas chromatography. Conformational analysis of 1 demonstrates that the all-chair (CCCC) conformation is the lowest in energy and that other conformations are too high in energy to be significantly populated at room temperature. The B3PW91/TZ2P calculated IR spectrum of the CCCC conformation of 1 is in excellent agreement with the experimental IR spectrum, confirming the conformational analysis and demonstrating the excellent accuracy of the B3PW91 functional and the TZ2P basis set. The B3PW91/TZ2P calculated VCD spectrum of the CCCC conformation of S-1 is in excellent agreement with the experimental VCD spectrum of (+)−1, unambiguously defining the AC of 1 to be S(+)/R(−). The B3LYP/aug-cc-pVDZ calculated OR of S-1 over the range 589–365 nm has the same sign and dispersion as the experimental OR of (+)−1, further supporting the AC S(+)/R(−). Our results confirm the AC proposed earlier by Farina and Audisio. This study provides a further demonstration of the excellent accuracy of VCD spectra predicted using Stephens’ equation for vibrational rotational strengths together with the ab initio DFT methodology, and further documents the utility of VCD spectroscopy in determining the ACs of chiral molecules.     

Comment on "HCl adsorption on ice at low temperature: a combined X-ray absorption, photoemission and infrared study" by P. Parent, J. Lasne, G. Marcotte and C. Laffon, Phys. Chem. Chem. Phys., 2011, 13, 7142

On the basis of NEXAFS, photoemission and FTIR spectra of ice films with low doses of adsorbed HCl, the authors of the PCCP paper "HCl adsorption on ice at low temperature: a combined X-ray absorption, photoemission and infrared study", Phys. Chem. Chem. Phys., 2011, 13, 7142, have come to conclusions regarding the behavior of submonolayer amounts of HCl at 50 K that contradict published results of the authors of this Comment. Our purpose is to argue that the conclusion, attributed going forward to PLML (authors' initials), that nearly 100% of HCl ionizes for dosage levels near to 0.16 monolayer (ML) or 0.3 Langmuir (L) at 50 K is questionable. Rather, we reaffirm our conclusions of much lower levels of ionization for similar temperatures and HCl dosages based on reactive ion scattering (RIS) and low energy sputtering (LES) data for ice films and FTIR spectra of ice nanocrystals. A second current paper by Ayotte et al., J. Phys. Chem. A, 2011, 115, 6002, that largely parallels in method and results the RAIR spectroscopy of PLML, is also given special notice.     

Highly luminescent biocompatible carbon quantum dots by encapsulation with an amphiphilic polymer

Highly luminescent, water-soluble and biocompatible Carbon Quantum Dots (aqCQDs) were prepared by encapsulating the parent hydrophobic CQDs in an amphiphilic polymer. The resulting aqCQDs were non-toxic to living cells, and were found to cross the cell membrane and localise primarily in the cytosol.     

X-ray scattering study of the effect of hydration on the cross-beta structure of amyloid fibrils

We have investigated the effect of sample hydration on the wide-angle X-ray scattering patterns of amyloid fibrils from two different sources, hen egg white lysozyme (HEWL) and an 11-residue peptide taken from the sequence of transthyretin (TTR105-115). Both samples show an inter-strand reflection at 4.7 A and an inter-sheet reflection which occurs at 8.8 and approximately 10 A for TTR105-115 and HEWL fibrils, respectively. The positions, widths, and relative intensities of these reflections are conserved in patterns obtained from dried stalks and hydrated samples over a range of fibril concentrations. In 2D scattering patterns obtained from flow-aligned hydrated samples, the inter-strand and inter-sheet reflections showed, respectively, axial and equatorial alignment relative to the fibril axis, characteristic of the cross-beta structure. Our results show that the cross-beta structure of the fibrils is not a product of the dehydrating conditions typically employed to produce aligned samples, but is conserved in individual fibrils in hydrated samples under dilute conditions comparable to those associated with other biophysical and spectroscopic techniques. This suggests a structure consisting of a stack of two or more sheets whose interfaces are inaccessible to bulk water.