Excitation Class I

The excited state analysis method of Plasser and Lischka was applied to the spectra calculations of ThDP intermediates.^[150] Thereby, the systems were divided into four fragments: the pyrimidine ring (P), the CH₂-linkage (L), the thiazolium moiety (T) and the substituent (S). The calculated charge-transfer numbers ΩAB between these fragments were employed to classify their spectral signatures. The first excitation class considers the charge-transfer from the pyrimidine to the thiazolium ring (delocalized to the substituent), being the equivalent excitations related to the AP and IP-ThDP cofactors in the resting states of enzymes. The graphical representation of the charge-transfer matrices Ω for these excited states combined with their calculated excitation energies are collected in Figure 7.5.

Regarding the representation of the charge-transfer matrices for this excitation class, the systems can be further divided into two groups. These reflect two structural motifs of the intermediates. In the first case the substituent is at least partly coplanar towards the thiazolium ring (A,B,C,K,OandP), sharing a conjugated thiazolium-substituent π-system. Besides the prominent Ω_PT element of this excitation class, these intermedi-ates feature a significant contribution of the charge-transfer from the pyrimidine ring to the substituent (Ω_PS). The result is an expansion of the particle space of the excitation, while the hole space remains nearly constant. Therefore, the charge-transfer excitation energy is red shifted according to the unsubstituted cofactors, which were observed in the energy range of 4.0–4.2 eV in the model calculations. Furthermore, a correlation is observed for the size of the red shift of the excitation energy and the contribution ΩPS, being associated with the degree of conjugation between the thiazolium ring and the substituent. Increasing the charge-transfer number from the pyrimidine ring to the substituent (Ω_PS), the excitation energy is lowered. This is well reflected, considering just the first row in Figure 7.5. Moving from acryolyl-ThDP (A) over keto-AcThDP (B) to enol-AcThDP (C) the calculated excitation energies are2.80±0.26, 3.12±0.23and 3.66±0.33 eV, respectively. Another issue is observed for the charge-transfer matrix

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0 0.25 0.5 0.75 1

P

P L T S

L T S

A (S1 2.80 ± 0.26 eV) B (S1 3.12 ± 0.23 eV) C (S1 3.66 ± 0.33 eV)

E (S1 4.21 ± 0.05 eV) F (S2 4.33 ± 0.04 eV) J (S2 4.50 ± 0.06 eV)

K (S1 2.85 ± 0.26 eV) L (S2 4.22 ± 0.06 eV) N (S2 4.52 ± 0.06 eV)

O (S1 3.95 ± 0.22 eV) P (S1 3.03 ± 0.18 eV)

Figure 7.5.: Graphical representation of the charge-transfer matrices for the excited states of ThDP intermediates, showing a charge-transfer character from the pyrimidine to the thiazolium ring. The donor fragments are arranged row-wise, whereby the acceptor fragments are organized column-wise. The notation follows the labeling in Figure 7.4. Furthermore, the calculated excitation energies at the TD-DFT/TDA level of theory (CAM-B3LYP/def2-TZVP), averaged for the solvents toluene and ether, are given.

of intermediate P. Here, a non-negligible contribution of a local excitation within the substituent (Ω_SS) arises. Clearly, this raises the charge-transfer energy. For the simi-lar intermediate from the reaction with CPB (K) the Ω_SS is close to zero, so that its excitation energy is lower by about 0.2 eV. This feature may, however, be an artifact of the employed computational model, including just a continuum representation of the enzyme environment. The crucial point is the correct trend for both systems with a strong red shift according to the unsubstituted cofactors.

Summarizing the results for the first group of this excitation class, an extension of the conjugation of theπ-system of the thiazolium ring through the substituent generally decreases the charge-transfer excitation energy compared to the reference AP and IP-ThDP systems. The size of this red shift correlates with the degree of conjugation (contributionΩ_PS). The shift can be very strong with even more than 1 eV. In conclusion, such intermediates have to be considered for the interpretation of experimental spectra, where absorption bands with maxima <3.5 eV (>354 nm) are observed.

The second group of intermediates within this excitation class reveals a different struc-tural motif (E,F,J,Land N). Here, the carbon atom bonded to the thiazolium ring is tetrahedral coordinated, so that the degree of conjugation of the thiazolium ring is re-tained with respect to the unsubstituted cofactors. In consequence, the charge-transfer from the pyrimidine to the thiazolium ring will not be delocalized towards the sub-stituent. This is well reflected in the charge-transfer matrices, having negligible Ω_PS contributions for all these systems. This results in a different trend for the excitation energies compared to the first group of molecules. For the second group of intermedi-ates, the charge-transfer energy remains constant or is even blue shifted with respect to the references. The size of the blue shift seems now to correlate with a contribution of a local excitation within the pyrimidine ring (ΩPP). Obviously, the larger the ΩPP

contribution the stronger is the blue shift (compare for example intermediates L and N in Figure 7.5). If this effect agrees indeed with the reality or is just an artifact of the employed computational model is unclear as already discussed before. Here, the crucial point is that the charge-transfer energy remains at least constant, comparing to the reference states. Intermediates included in this set are typically substrate-ThDP adducts or related compounds which were frequently assigned to low-energy bands with maxima <3.5 eV (>354 nm).^[24;26] These assignments cannot be rationalized by the charge-transfer excitation from the pyrimidine to thiazolium ring. An alternative would be the occurence of a different excitation character for these systems, leading to the

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second excitation class in the next section.