Solvent-Dependent Pyranopterin Cyclization in Molybdenum Cofactor Model Complexes.
The conserved pterin dithiolene ligand that coordinates molybdenum (Mo) in the cofactor (Moco) of mononuclear Mo enzymes can exist in both a tricyclic pyranopterin dithiolene form and as a bicyclic pterin-dithiolene form as observed in protein crystal structures of several bacterial molybdoenzymes. Interconversion between the tricyclic and bicyclic forms via pyran scission and cyclization has been hypothesized to play a role in the catalytic mechanism of Moco. Therefore, understanding the interconversion between the tricyclic and bicyclic forms, a type of ring-chain tautomerism, is an important aspect of study to understand its role in catalysis. In this study, equilibrium constants (K(eq)) as well as enthalpy, entropy, and free energy values are obtained for pyran ring tautomerism exhibited by two Moco model complexes, namely, (Et4N)[Tp*Mo(O)(S2BMOPP)] (1) and (Et4N)[Tp*Mo(O)(S2PEOPP)] (2), as a solvent-dependent equilibrium process. Keq values obtained from (1)H NMR data in seven deuterated solvents show a correlation between solvent polarity and tautomer form, where solvents with higher polarity parameters favor the pyran form.
Department of Chemistry, Bryn Mawr College , Bryn Mawr, Pennsylvania 19010, United States.
Benjamin R
BR
Williams
Department of Chemistry, Bryn Mawr College , Bryn Mawr, Pennsylvania 19010, United States.
Douglas
D
Gisewhite
Department of Chemistry, Bryn Mawr College , Bryn Mawr, Pennsylvania 19010, United States.
Anna
A
Kalinsky
Department of Chemistry, Bryn Mawr College , Bryn Mawr, Pennsylvania 19010, United States.
Alisha
A
Esmail
Department of Chemistry, Bryn Mawr College , Bryn Mawr, Pennsylvania 19010, United States.
Sharon J Nieter
SJ
Burgmayer
D003067:Coenzymes / Q000737:chemistry*; D003500:Cyclization; D008958:Models, Molecular; D015394:Molecular Structure; D008982:Molybdenum / Q000737:chemistry*; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D011622:Pterins / Q000737:chemistry*; D011789:Quantum Theory; D012997:Solvents / Q000737:chemistry*; D013816:Thermodynamics
D003067:Coenzymes; D009942:Organometallic Compounds; D011622:Pterins; D012997:Solvents; C490076:pyranopterin; D008982:Molybdenum