Probing for chiral discrimination of alanine adsorption and oxidation on TiO2 particles in connection to homochirality

Abstract: The adsorption of both chiral forms of alanine on TiO2 was studied and compared to see if TiO2 has a chiral preference. Both the L- and D-form of alanine was photo-oxidized on TiO2 by UVlight to evaluate if there is a difference in the oxidation rate. Raman spectroscopy was used as analytical technique for this study because it can be used for in situ measurements and for aqueous solutions. This type of study has not been executed before and therefore a significant part of the project consisted of method development. A setup was build, evaluated and optimised for both experiments. The Raman spectrometer was tuned and solutions with different alanine and TiO2 concentrations were tested. Adsorption was tested by measuring the difference in alanine signal between samples with and without TiO2. The oxidation was monitored by continuously measuring alanine solution containing TiO2 that was exposed to UV-light. The method developed was capable to detect adsorption in almost all measured samples and it was shown that the alanine became photo-oxidized only in the solutions containing TiO2. In one of two measured concentrations D-alanine adsorbed more efficiently than L-alanine, which may indicate a chiral preference of TiO2 surface. However, more experimental data is needed to confirm this, the initial results are encouraging. The oxidation of alanine was different for the L- and the D-form, but the oxidation rates cannot be compared fairly because the different solutions did not contain water of the same purity. Further investigations need to be done were the conditions are identical.