Vacuum-Ultraviolet Electronic Circular Dichroism ofl-Alanine in Aqueous Solution Investigated by Time-Dependent Density Functional Theory

Abstract

The electronic circular dichoism (ECD) of L-alanine in the vacuum-ultraviolet region was calculated for various optimized structures using time-dependent density functional theory (TDDFT) to assign the CD spectrum observed experimentally in aqueous solution down to 140 nm [Matsuo, et al. Chem. Lett. 2002, 826]. The structure of L-alanine in vacuo was optimized using density functional theory (DFT) at the B3LYP/6-31G* level. Its hydrated structure was optimized with nine water molecules (six and three around carboxyl and amino groups, respectively) using DFT and a continuum model (Onsager model). The dihedral angles of carboxyl and amino groups in the optimized hydrated structure differed greatly from those in the crystal and in nonhydrated structures optimized using a continuum model only. The ECD spectrum calculated for the hydrated structure had two successive positive peaks with molar ellipticities of about 2000 deg cm2 dmol(-1) at around 205 and 185 nm, which were close to those observed experimentally. These positive peaks were attributable to n pi* transitions of the carboxyl group, with the latter peak also influenced by the pi pi* transition of the carboxyl group that originates below 175 nm. A small negative peak observed at around 252 nm was also predicted from the hydrated structure. These results demonstrate that the hydrated water molecules around the zwitterions play a crucial role in stabilizing the conformation of L-alanine in aqueous solution and that TDDFT is useful for the ab initio assignment of ECD spectra down to the vacuum-ultraviolet region.

On the origin of circular dichroism in trigonal dihedral d6 complexes of bidentate ligands containing only σ‐orbitals. A qualitative model based on a density functional theory study of Λ‐[Co(en)3]3+

“…The formulation of CD in terms of time dependent density functional theory (TD-DFT) [14][15][16] has resulted in a number of TD-DFT applications to moderately sized organic molecules 14,[17][18][19][20] as well as some transition metal complexes. [21][22][23][24] TD-DFT has proved to be an especially attractive approach as it gives reasonable accuracy at a low computational cost.…”

Anisotropy Spectra for Enantiomeric Differentiation of Biomolecular Building Blocks

“…For example, the lowest energy transition (n ! π*) in amino acids lies close to 200 nm [56,57] and carbohydrates are transparent to wavelengths above 200 nm. SRCD is therefore an important tool for the study of small chiral molecules.…”

Construction of a Synchrotron-Radiation Vacuum-Ultraviolet Circular-Dichroism Spectrophotometer and Its Application to the Structural Analysis of Biomolecules

“…We measured the VUVCD spectra of several amino acids 24 and theoretically analyzed the solution structure (hydration effects) using a density functional theory (DFT) and a time-dependent DFT (TDDFT) method. 25 4.1 VUVCD Spectra of Amino Acids. Figure 7 shows the VUVCD spectra of some nonpolar and polar amino acids measured over wavelengths from 260 to 140 nm at high concentrations of 2.010.0 w/v%.…”

“…As a typical example, the VUVCD spectrum of L-alanine was estimated using the DFT and TDDFT. 25 The model structure of L-alanine and the definition of the dihedral angles (º and ¤) are shown in Figure 8. The CD spectra of L-alanine between 260 and 140 nm were calculated for four initial structures with various pairs of (º, ¤) values: (0°, 0°), (0°, 60°), (90°, 0°), and (90°, 60°).…”