Overview

The method described in the preceeding section was applied to the model compounds of the proposed intermediates in ThDP-catalyzed reactions. Additionally, further protona-tion states of the substituents as well as possible intermediates from side reacprotona-tions were considered. The structures are schematically represented in Figure 7.4. Furthermore, the calculated excitation energies for their first excited states are given and compared to the experimentally assigned band maxima.

N Figure 7.4.: Overview of the investigated ThDP intermediates. The calculated excitation energies at the

TD-DFT/TDA level of theory (CAM-B3LYP/def2-TZVP), and the corresponding wave-lengths, are given below each intermediate. The excitation energies are averaged for the solvents toluene and ether, employing the COSMO solvation model for the representation of the environment. The error bar is purely provided for solvent effects. The experimentally assigned values are shown in parentheses.^[24–26]

Results and Discussion 115

In the reaction of ThDP with glyceraldehyde-3-phosphate (G3P) a band at 2.86 eV (433 nm) was observed and connected to the formation of acryolyl-AcThDP (A).^[25]The calculated excitation energy of2.80±0.26eV (433±42nm) almost perfectly matches this value. The excitation is found to be of charge-transfer character whereby the electron is transferred from the pyrimidine to the thiazolium ring delocalized to the substituent (similar to the unsubstituted cofactors). This was controversially discussed in the work of Townsend et al.^[25] The authors deemed a shift of about 1 eV related to the unsub-stituted cofactors to be too large to arise from charge-transfer excitations. However, the calculations clearly prove the opposite. The extension of the degree of conjugation of the thiazolium ring towards the substituent strongly influences this charge-transfer excitation, resulting in shifts as large as those observed for acryolyl-ThDP.

The reaction with fluoropyruvate (FP) with ThDP in the E1 component of theE. coli pyruvate dehydrogenase complex revealed an absorption band at 3.18 eV (390 nm).^[24]

This band was associated with the formation of acetyl-ThDP (AcThDP), which was already extensively studied in the reaction mechanism of the phosphoketolase enzyme within this work (see Chapter 5). In the latter the excitation energy was found to be slightly red shifted to 2.95 eV (420 nm), most likely due to a different polarization of the cofactors in the enzyme environments. However, in the work of Patelet al. only the keto and enol states of AcThDP (B andC) were explicitly considered in the assignment, building no distinction between them.^[24] The results show that the keto-AcThDP inter-mediate absorbs in the spectral region of interest, whereas the excitation of enol-AcThDP requires significantly larger energies. Both tautomers identify a similar charge-transfer character compared to the unsubstituted cofactors for these excited states. Further-more, the enolate form of AcThDP (D) was not considered in the interpretation of the experimental spectra. A different charge-transfer character (from the substituent to the thiazolium ring) causes a low-energy band in the range of2.88±0.09eV (414±12nm), somewhat close to the band maximum.

A related issue is observed for the intermediates arising from the reaction of acetyl pyruvate (ACP) with ThDP. The measured absorption band at 3.10 eV (400 nm) was assigned to the ACP-ThDP adducts in their keto or enol form (E and F).^[24] The eno-late state (G) of the substituent was not considered. The first excited state of the keto-ACP-ThDP adduct shows the same charge-transfer character as the IP-ThDP co-factor, resulting in comparable excitation energies (4.21±0.05 eV; 295±3 nm). This is expected since the substituent with its tetrahedral coordinated carbon atom (bonded

to the C2 atom of ThDP) almost retains the electronic structure within the thiazolium ring. Therefore, the intermediate can be excluded for the assignment of the absorption band at 3.10 eV (400 nm). Moving to the enol species, the character of the first excited state changes. Here, a charge-transfer from the enol moiety to the thiazolium ring is observed. However, the excitation energy of 3.92±0.14 eV (316±11 nm) is still too high to explain the origin of the band. The enolate-ACP-ThDP adduct also has to be considered. The introduction of a negative charge causes the calculated excitation en-ergy to drop down to2.44±0.22eV (507±46nm). The effect is certainly overestimated in the employed model, where specific hydrogen bonds are missing in the continuum representation of the enzyme environment. The latter would raise the excitation energy towards the observed band maximum at 3.10 eV (400 nm). Moreover, a dynamic equi-librium between the enol and enolate-ACP-ThDP adducts would be conceivable, so that the absorption could even arise in between the two limiting cases. However, the crucial point for the interpretation of the spectra is the inclusion of the enolate species in the reaction mechanism.

The decarboxylation of the ACP-ThDP adduct results in the enamine intermedi-ate (H), exhibiting a different electronic structure with a delocalized and neutral π-system of the thiazolium-substituent portion. The band maximum was experimentally estimated to 3.81 eV (325 nm).^[24;152] The agreement with the calculated excitation en-ergy of3.82±0.01eV (325±1nm) is striking. A localπ →π^∗ character is, as expected, determined for this excitation. A similar behavior is observed for the postdecarboxyla-tion intermediate for the reacpostdecarboxyla-tion of (E)-4-(4-chlorophenyl)-2-oxo-buteonic acid (CPB) with ThDP (I). The extension of the π-system causes the local π → π^∗ excitation to decrease to 2.86 eV (434 nm). The deviation of about 0.2 eV to the calculated excitation energy is somewhat larger, but still acceptable given the intrinsic error of the employed method.

Besides the assigned absorption band for the enamine intermediate in the reaction of CPB with ThDP, another band was observed with maximum at 2.54 eV (488 nm).^[24]

This low-energy band was connected to the formation of the second postdecarboxylation intermediate (J). This one arises from the protonation of the carbon atom bonded to the thiazolium ring in the enamine state, thereby removing the conjugation between the thiazolium ring and the substituent. The electronic structure is now comparable to the enol-ACP-ThDP adduct (F). As a result, the first excited state of the inter-mediate J shows charge-transfer character from the π-system of the substituent to the

Results and Discussion 117

thiazolium ring, but the excitation energy of3.95±0.30eV (314±24nm) is also much too high in order to explain the measured band. Missing the possibility of the formation of an enolate species, an intermediate of a side reaction was considered. The oxida-tion of enamine-CPB-ThDP (I) would lead to an intermediate with a fully conjugated thiazolium-substituent π-system (K). Consequently, the charge-transfer energy (from the pyrimidine ring to the thiazolium ring delocalized to the substituent) decreases signif-icantly compared to the unsubstituted cofactors, as already seen for acryolyl-ThDP (A) and keto-AcThDP (B). The calculated excitation energy of2.85±0.26eV (435±40nm) is much closer to the observed band maximum at 2.54 eV (488 nm). This is the best candidate for the assignment to the low-energy absorption band. The occurence of the side reaction is not unlikely, since the equivalent side products were detected in the similar reaction of ThDP with (E)-2-oxo-4-(pyridin-3-yl)but-3-eonic acid (PKB).^[26]

The ThDP derivatives arising from the reaction with PKB (L–P) completes the set of studied intermediates. Thereby, many similarities are found in comparison to the previously discussed systems. The PKB-ThDP adduct (L) was experimentally assigned to the low-energy band with maximum at 2.60 eV (477 nm).^[26] The structural motif of this intermediate is comparable to the enol-ACP-ThDP adduct (F) and the second postdecarboxylation intermediate (J) in the reaction with CPB. Consequently, the lowest calculated excitation energy of 4.10±0.31 eV (303±23nm) is much too high in order to explain the band. The same holds for the intermediateN, where the carboxyl group is just replaced by a hydrogen atom. However, ThDP derivatives from side reactions with the enamine intermediate (M) as central point would constitute alternatives. A protonation of the Cγ atom of the substituent results in a partly conjugated thiazolium-substituent π-system (O), but this is not sufficient to shift the charge-transfer state related to the unsubstituted cofactors as much as experimentally observed. On the other hand, the oxidation of enamine-PKB-ThDP (M) reveals an intermediate with a fully conjugated thiazolium-substituent π-system (P), resulting in a low-energy charge-transfer excited state.

The results discussed so far suggest that the spectral signatures of ThDP deriva-tives can be divided into three classes. Firstly, the charge-transfer excitations from the pyrimidine ring to the thiazolium ring (delocalized to the substituent) which are strongly dependent on the degree of conjugation between the thiazolium ring and the substituent.

Secondly, charge-transfer states where the electron is transferred from the substituent to the thiazolium ring and at last local π → π^∗ transitions for the enamine

interme-diates. These excitation classes can clearly be distiguished by the analysis method of Plasser and Lischka,^[150] described in the preceeding section. This method was applied to the excited states of the intermediates. The results are presented for each class in the following sections.