The absorption and circular dichroism spectra of chiral olefins

Continuous-Wave Cavity-Enhanced Polarimetry for Optical Rotation Measurement of Chiral Molecules

Precise optical rotation measurements play an important role in the analysis of chiral molecules in various fields, especially in biological chemistry and pharmacology. In this paper, we demonstrate a new variant of continuous-wave cavity-enhanced polarimetry for detecting the optical activity of two enantiomers of a chiral molecule at 730 nm. It is based on a signal-reversing technique for which the chiral specific rotation is directly determined by the cavity ring-down signal from two counter-propagating beams in a bow-tie cavity. In particular, we ensure reproducible excitation of both modes by broadening the linewidth of a diode laser source by application of a radio frequency perturbation to its injection current. The performance of the polarimeter is demonstrated for the specific rotation of (+)- and (-)-α-pinene in different environments, including the pure vapor, open air, and the liquid phase; the detection precision ranges between 10^-5 and 10^-4 degrees per cavity pass depending on the environment. The apparatus is a robust and practical tool for quantifying chirality and can be developed for the entire visible and near-infrared spectral regions.

Chiral discrimination by recollision enhanced femtosecond laser mass spectrometry

Chiral molecules and their interactions are critical in a variety of chemical and biological processes. Circular dichroism (CD) is the most widely used optical technique to study chirality, often performed in a solution phase. However, CD has low-efficiency on the order of 0.01-1[Formula: see text]. Therefore, there is a growing need to develop high-efficiency chiroptical techniques, especially in gas-phase, to gain background-free in-depth insight into chiral interactions. By using mass spectrometry and strong-field ionization of limonene with elliptically polarized light, we demonstrate an efficient chiral discrimination method that produces a chiral signal of one to two orders of magnitude higher than the conventional CD. The chiral response exhibits a strong dependence on wavelength in the range of 1,300-2,400 nm, where the relative abundance of the ion yields alternates between the two enantiomers. The origin of enhanced enantio-sensitivity in intense laser fields is attributed to two mechanisms that rely on the recollision dynamics in a chiral system: (1) the excited ionic state dynamics mediated either by the laser field or by the recollision process, and (2) non-dipole effects that alter the electron's trajectories. Our results can serve as a benchmark for testing and developing theoretical tools involving non-dipole effects in strong-field ionization of molecules.

Revisiting absorption and electronic circular dichroism spectra of cholesterol in solution: a joint experimental and theoretical study

Cholesterol is doubtless one of the most studied bio-molecules, which unfortunately features low emitting properties, precluding its in vivo study by fluorescence experiments. The design of fluorescent analogues of cholesterol is thus an appealing challenge in biochemistry, which simultaneously requires minor changes in its chemical structure (to retain main biological properties) and considerable enhancement of light emission. To this aim, the photochemical behaviour of the native molecule has to be deeply understood. In this work, we focused our attention on the electronic absorption of cholesterol in several common organic solutions, combining experimental (through ultraviolet-visible and electronic circular dichroism spectroscopy) and theoretical approaches (at the time-dependent density functional theory level) in order to solve the important discrepancies previously reported in the literature on the maximum absorption wavelengths and on the nature (Rydberg and/or π → π*) of the associated electronic transition.

Electronic circular dichroism of chiral alkenes: B3LYP and CAM-B3LYP calculations

Time-dependent density functional theory (TDDFT) calculations of electronic circular dichroism (ECD) are widely used to determine absolute configurations (ACs) of chiral molecules. Two very popular DFT exchange-correlation functionals, one hybrid (B3LYP) and one long-range corrected (CAM-B3LYP), along with a hierarchical sequence of basis sets were investigated, and the ECD spectra predicted for eight alkenes and compared to gas-phase experimental spectra. Little variation in predicted ECD spectra was found with the basis set size enlargement, but the sensitivity to the functional is greater. Good agreement was obtained only with the CAM-B3LYP functional, leading to the conclusion that TDDFT calculations of ECD spectra can routinely provide reliable ACs if and only if an appropriate functional is used. For camphene, twistene, syn-(E)-bisfenchylidene, and phyllocladene, solvent effects were estimated.

Temperature Dependence of the Optical Rotation in Six Bicyclic Organic Molecules Calculated by Vibrational Averaging

The vibrational corrections and the temperature dependence of the specific rotation of six rigid organic molecules (alpha-pinene, beta-pinene, cis-pinane, camphene, camphor, and fenchone) were calculated at three wavelengths using hybrid time-dependent density functional theory (TDDFT). A technique for calculating the temperature dependence of the vibrational average of a molecular property has been applied to obtain the specific rotation of the molecules as a function of temperature. For cases in which accurate equilibrium optical rotations can be obtained as a "base value," and for which there is little effect from solvation, accurate predictions of the trends in the temperature-dependence of the specific rotations can be calculated. For other cases, the method can be used to extract purely vibrational contributions to the overall temperature dependence of optical rotation.

Reassessment of the electronic circular dichroism criteria for random coil conformations of poly(L-lysine) and the implications for protein folding and denaturation studies

The circular dichroism (CD) spectra of poly(L-lysine) in water and ethanediol/water (2:1) solutions in the temperature range -110 to 85 degrees C are presented. The results combined with vibrational CD data are interpreted in terms of a two-state conformational equilibrium with a left-handed trans polyproline II conformation being preferred at low temperatures. The relevance of these studies to the CD criteria for random-coil conformations, the study of helix-coil transitions and protein/peptide folding is pointed out.