Photochemical Chiral Symmetry Breaking in Alanine

We introduce a general theoretical approach for the simulation of photochemical dynamics under the influence of circularly polarized light to explore the possibility of generating enantiomeric enrichment through polarized-light-selective photochemistry. The method is applied to the simulation of the photolysis of alanine, a prototype chiral amino acid. We show that a systematic enantiomeric enrichment can be obtained depending on the helicity of the circularly polarized light that induces the excited-state photochemistry of alanine. By analyzing the patterns of the photoinduced fragmentation of alanine we find an inducible enantiomeric enrichment up to 1.7%, which is also in good correspondence to the experimental findings. Our method is generally applicable to complex systems and might serve to systematically explore the photochemical origin of homochirality

Photodynamics of Free and Solvated Tyrosine

We present a theoretical simulation of the ultrafast nonadiabatic photodynamics of tyrosine in the gas phase and in water. For this purpose, we combine our TDDFT/MM nonadiabatic dynamics (Wohlgemuth et al. <i>J. Chem. Phys.</i> <b>2011</b>, <i>135</i>, 054105) with the field-induced surface hopping method (Mitrić et al. <i>Phys. Rev. A</i> <b>2009</b>, <i>79</i>, 053416) allowing us to explicitly include the nonadiabatic effects as well as femtosecond laser excitation into the simulation. Our results reveal an ultrafast deactivation of the initially excited bright ππ* state by internal conversion to a dark <i>n</i>π* state. We observe deactivation channels along the O–H stretching coordinate as well as involving the N–H bond cleavage of the amino group followed by proton transfer to the phenol ring, which is in agreement with previous static energy path calculations. However, since in the gas phase the canonical form of tyrosine is the most stable one, the proton transfer proceeds in two steps, starting from the carboxyl group that first passes its proton to the amino group, from where it finally moves to the phenol ring. Furthermore, we also investigate the influence of water on the relaxation processes. For the system of tyrosine with three explicit water molecules solvating the amino group, embedded in a classical water sphere, we also observe a relaxation channel involving proton transfer to the phenol ring. However, in aqueous environment, a water molecule near the protonated amino group of tyrosine acts as a mediator for the proton transfer, underlining the importance of the solvent in nonradiative relaxation processes of amino acids