The Mesomeric Effect of Thiazolium on non-Kekulé Diradicals in Pichia stipitis Transketolase

Ning Shian Hsu, Yung Lin Wang, Kuan Hung Lin, Chi Fon Chang, Syue Yi Lyu, Li Jen Hsu, Yu Chen Liu, Chin-Yuan Chang, Chang Jer Wu, Tsung Lin Li*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

4 Scopus citations


It is theoretically plausible that thiazolium mesomerizes to congeners other than carbene in a low effective dielectric binding site; especially given the energetics and uneven electronegativity of carbene groups. However, such a phenomenon has never been reported. Nine crystal structures of transketolase obtained from Pichia stipitis (TKps) are reported with subatomic resolution, where thiazolium displays an extraordinary ring-bending effect. The bent thiazolium congeners correlate with non-Kekulé diradicals because there is no gain or loss of electrons. In conjunction with biophysical and biochemical analyses, it is concluded that ring bending is a result of tautomerization of thiazolium with its non- Kekulé diradicals, exclusively in the binding site of TKps. The chemophysical properties of these thiazolium mesomers may account for the great variety of reactivities carried out by thiamine–diphosphate-containing (ThDP) enzymes. The stability of ThDP in living systems can be regulated by the levels of substrates, and hydration and dehydration, as well as diradical-mediated oxidative degradation.

Original languageEnglish
Pages (from-to)1802-1807
Number of pages6
JournalAngewandte Chemie - International Edition
Issue number7
StatePublished - 12 Feb 2018


  • Pichia stipitis
  • non-Kekulé diradicals
  • thiamine diphosphate
  • thiazolium
  • transketolase


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