Discovery of Most Stable Structures of Neutral and
Anionic Phenylalanine through Automated Scanning of Tautomeric and
Conformational Spaces
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Abstract
We
have developed a software tool for combinatorial generation
of tautomers and conformers of small molecules. We have demonstrated
it by performing a systematic search for the most stable structures
of neutral and anionic phenylalanine (Phe) using electronic structure
methods. For the neutral canonical tautomer we found out that the
conformers <i>with</i> and <i>without</i> the
intramolecular (O)H···NH<sub>2</sub> hydrogen bond
are similarly stable, within the error bars of our method. A unique
IR signature of the conformer without the hydrogen bond has been identified.
We also considered anions of Phe, both valence type and dipole-bound.
We have found out that tautomers resulting from proton transfer from
the carboxylic OH to the phenyl ring do support valence anions that
are vertically strongly bound, with electron vertical detachment energies
(VDE) in a range of 3.2–3.5 eV. The most stable conformer of
these valence anions remains adiabatically unbound with respect to
the canonical neutral by only 2.17 kcal/mol at the CCSD(T)/aug-cc-pVDZ
level. On the basis of our past experience with valence anions of
nucleic acid bases, we suggest that the valence anions of Phe identified
in this report can be observed experimentally. The most stable conformer
of canonical Phe is characterized by an adiabatic electron affinity
of 53 meV (a dipole-bound state)