Quantum Mechanical Insights into Biological Processes at the Electronic Level - Computational Modeling of Biological Systems - page 151

Fig. 13 ( a ) The deprotonated chromophore from the GFP in the anonic and cationic forms. ( b )
Relevant molecular orbitals of deprotonated HBDI (HF/6-311G ). In the ground state of the anion,
both 1 and 2 are doubly occupied, and the bright state is derived by the 1 ! excitation.
Oxidized forms are derived by removing the electrons from 1 .( c ) Energy diagram for the relevant
electronic states of deprotonated HBDI. Adapted with permission from [ 29 ]

doublet radical as 6.07 and 4.22 eV, respectively. Finally, the energy gap between
the excited singlet state of the anion and the two excited states of the radical are 1.29
and 3.14 eV, respectively. The results suggested that the doubly oxidized species (the
deprotonated HBDI cation) may be responsible for the oxidative redding through the
following newly proposed mechanism: the first step involves photoexcitation, and
the blue light is sufficient to generate this transition. The second and third steps are
one-electron oxidation steps. The closed-shell character of the cation is consistent
with the relatively chemically stable nature of the red form of GFP. The absorption
in the cation (product of two-electron oxidation) is red-shifted by almost 0.6 eV with
respect to the anion, and the resulting value of 2.02 eV is in good agreement with
the experimental excitation energy of 2.12 eV. The redshift is consistent with the
electronic excitation in the cation being the 1 ! 2 process rather than 1 !
in the anion. This mechanism of redding is distinctly different from previously
characterized ones in which redding was achieved by extending the -system of
the chromophore.
Excited states in DNA fragments are important for understanding the mechanism
of DNA photodamage, and self-rescue due to internal conversion from electronically
excited states. Calculations of high accuracy are expensive, as was eluded. Hence,
only small fragments, such as one or two nucleic bases can be characterized using
high levels of ab intio theory. Roos and coworkers sophisticatedly explicated the
electronic spectra of nucleic base monomers [ 30 ]. The CASSCF and CASPT2
methods were used. The comparison with the experimental measurements speaks
for the stellar qualities of the chosen methodology. For example, for N(9)H-adenine,
the computed valence ! excitation energies are 5.1, 5.2 (4.9), 6.2 (5.7-6.1),