N1'-Methylthiaminium Diiodide. Model Study on the Effect of a Coenzyme Bound Positive Charge on Reaction Mechanisms Requiring Thiamin Pyrophosphate

Frank Jordan, Yitbarek H. Mariam

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76 Scopus citations

Abstract

Thiamin (Ia), 4-aminopyrimidine (IIa), and 4-amino-5-methoxymethyl-2-methylpyrimidine (IIIa) were converted to the corresponding pyrimidinium methiodides Ib, IIb, and IIIb, respectively. The relative rates by Ib over Ia were found to be 2.5:1 for thiazolium ylide formation; ca. 3:1 for pyruvate decarboxylation (CO2 loss), ca. 15:1 for acetoin formation from pyruvate, and about 4:1 for thiazolium ring opening. Thus Ib in all respects is a superior model catalyst to Ia. A plausible source of all the demonstrated rate enhancements is an electrostatic field effect of the positive charge in Ib destabilizing the positively charged ground states and/or stabilizing the carbanion and oxyanion-like transition states. The amine deprotonation pKas of IIb and IIIb were determined to be near 12; hence the positive charge in Ib can be expected to convert the amino group into a weak acid. The possible relevance of these findings to thiamin pyrophosphate requiring enzymatic reactions is discussed.

Original languageEnglish (US)
Pages (from-to)2534-2541
Number of pages8
JournalJournal of the American Chemical Society
Volume100
Issue number8
DOIs
StatePublished - 1978

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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