Chiroptical Study of α-Aliphatic Amino Acid Films in the Vacuum Ultraviolet Region

Abstract

A series of natural circular dichroism (CD) and absorption spectra for films of α-aliphatic amino acids--such as alanine, aminobutyric acid, norvaline, norleucine, valine, leucine, and isoleucine--in the vacuum ultraviolet (VUV) region were observed with the absolute values of optical constants at the undulator-based CD beamline TERAS BL5. Preliminary predictions of some CD spectra were also performed, based on quantum-chemical calculations using the crystal structure. Although the absorption spectra show similar features to each other, significant differences between the CD spectra were found, especially in the 7-8 eV region. The CD spectra of aliphatic amino acids with branched alkyl groups in the side-chain--such as valine, leucine, and isoleucine--exhibit strong negative CD peaks in this energy region. In contrast, the corresponding CD peaks were weak or absent in the spectra of amino acids with straight alkyl groups. Our simple calculation, and the absorption spectra of alkanes, suggest that this difference partly originates from the contribution of the alkyl group. Clear discrepancies between the CD spectra of these amino acids in solutions and those in the solid state were also observed; this is probably caused by the different molecular structures in each state. Our results clearly indicated that CD spectra in the VUV region were very sensitive to the conformations of chiral molecules.

Understanding Photochirogenesis: Solvent Effects on Circular Dichroism and Anisotropy Spectroscopy

“…The origin of the main peak in the CD spectrum of amorphous solids at around 185 nm, resulting in a smaller signal at around 190 nm in the anisotropy spectrum, remains unclear. It has, however, been suggested that this represents the π → π* transition …”

Electron asymmetries in the photoionization of chiral molecules: possible astrophysical implications

“…Within a bottom-up approach of biomolecular complexity, the study of gas-phase, isolated, elementary bricks of life [108] such as amino acids aims at shedding light on their molecular and electronic structures, chemical reactivity, and chiroptical properties. In the next section, we will focus onto alanine (Ala), the simplest chiral proteogenic amino acid, for which not much is known in terms of VUV chiroptics, except for a gas-phase study that reported on the total ion yield providing CD upper values [109], as well as another study on thin films [110]. We will show how PECD of alanine may be of interest in terms of structure (conformational analysis) and, most of all, we will stress a possible link between PECD and the origin of life's homochirality.…”

The Astrophysical Formation of Asymmetric Molecules and the Emergence of a Chiral Bias

“…These measurements required the preparation of amino acids as amorphous thin solid films to avoid the strong VUV absorption of commonly used solvents. Moreover, the isotropic solid form of chiral biomolecules is proposed to better reflect their natural environment in interstellar ices [18,19,20]. Thanks to this progress, CD spectra of meteoritic α-H amino acids and α-methyl amino acids have been recorded [17,18,19,20].…”