Lactic acid in solution: investigations of lactic acid self-aggregation and hydrogen bonding interactions with water and methanol using vibrational absorption and vibrational circular dichroism spectroscopies. 2008

Martin Losada, and Ha Tran, and Yunjie Xu
Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada.

The infrared vibrational absorption (VA) and vibrational circular dichroism (VCD) spectral features of L-(+)-lactic acid (LA) in CDCl3 solution are concentration dependent, showing evidence of oligomerization with increasing concentrations. To understand the observed spectra, geometry optimizations, vibrational frequencies, and VA and VCD intensities were evaluated for (LA)n with n=1-4 using density functional theory calculations at the B3LYP6-311++G(d,p), B3LYP/cc-pVTZ, and in some cases, B3LYP/aug-cc-pVTZ levels of theory. Comparisons with the experimental spectra indicate that the lowest energy LA dimer (AA), formed by two C Double Bond O...HO hydrogen bonds, is one of the dominating species in solution at room temperature. Possible contributions from the LA trimer and tetramer are also discussed. To model the VA and VCD spectra of LA in water and in methanol, both implicit polarizable continuum model and explicit hydrogen bonding considerations were used. For explicit hydrogen bonding, geometry optimizations of the AA-(water)n and AA-(methanol)n complexes, with n=2,4,6, were performed, and the corresponding VA and VCD spectra were simulated. Comparisons of the calculated and experimental VA and VCD spectra in the range of 1000-1800 cm(-1) show that AA-(water)n with n=6 best reproduces the experimental spectra in water. On the other hand, AA-(methanol)n with n=2 reproduces well the experimental results taken in methanol solution. In addition, we found evidence of chirality transfer, i.e., some vibrational bands of the achiral water subunits gain VCD strength upon complexation with the chiral LA solute. The study is the first to use VCD spectroscopy to probe the structures of LA aggregates and hydrogen bonding solvation clusters in the solution phase.

UI MeSH Term Description Entries
D007536 Isomerism The phenomenon whereby certain chemical compounds have structures that are different although the compounds possess the same elemental composition. (From McGraw-Hill Dictionary of Scientific and Technical Terms, 5th ed) Isomerisms
D002942 Circular Dichroism A change from planar to elliptic polarization when an initially plane-polarized light wave traverses an optically active medium. (McGraw-Hill Dictionary of Scientific and Technical Terms, 4th ed) Circular Dichroism, Vibrational,Dichroism, Circular,Vibrational Circular Dichroism
D006860 Hydrogen Bonding A low-energy attractive force between hydrogen and another element. It plays a major role in determining the properties of water, proteins, and other compounds. Hydrogen Bonds,Bond, Hydrogen,Hydrogen Bond
D000042 Absorption The physical or physiological processes by which substances, tissue, cells, etc. take up or take in other substances or energy.
D000432 Methanol A colorless, flammable liquid used in the manufacture of FORMALDEHYDE and ACETIC ACID, in chemical synthesis, antifreeze, and as a solvent. Ingestion of methanol is toxic and may cause blindness. Alcohol, Methyl,Carbinol,Sodium Methoxide,Wood Alcohol,Alcohol, Wood,Methoxide, Sodium,Methyl Alcohol
D014867 Water A clear, odorless, tasteless liquid that is essential for most animal and plant life and is an excellent solvent for many substances. The chemical formula is hydrogen oxide (H2O). (McGraw-Hill Dictionary of Scientific and Technical Terms, 4th ed) Hydrogen Oxide
D019344 Lactic Acid A normal intermediate in the fermentation (oxidation, metabolism) of sugar. The concentrated form is used internally to prevent gastrointestinal fermentation. (From Stedman, 26th ed) Lactate,2-Hydroxypropanoic Acid,2-Hydroxypropionic Acid,Ammonium Lactate,D-Lactic Acid,L-Lactic Acid,Propanoic Acid, 2-Hydroxy-, (2R)-,Propanoic Acid, 2-Hydroxy-, (2S)-,Sarcolactic Acid,2 Hydroxypropanoic Acid,2 Hydroxypropionic Acid,D Lactic Acid,L Lactic Acid,Lactate, Ammonium

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