Photodynamics of Free
and Solvated Tyrosine
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Abstract
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