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Pyranopterin dithiolene distortions relevant to electron transfer in xanthine oxidase/dehydrogenase.

Dong C, Yang J, Leimkühler S, Kirk ML - Inorg Chem (2014)

Bottom Line: The reducing substrates 4-thiolumazine and 2,4-dithiolumazine have been used to form Mo(IV)-product complexes with xanthine oxidase (XO) and xanthine dehydrogenase.The resonance Raman spectra reveal in-plane bending modes of the bound product and low-frequency molybdenum dithiolene and pyranopterin dithiolene vibrational modes.This work provides keen insight into the role of the pyranopterin dithiolene in electron-transfer reactivity.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and Chemical Biology, The University of New Mexico , MSC03 2060, 1 University of New Mexico, Albuquerque, New Mexico 87131-0001, United States.

ABSTRACT
The reducing substrates 4-thiolumazine and 2,4-dithiolumazine have been used to form Mo(IV)-product complexes with xanthine oxidase (XO) and xanthine dehydrogenase. These Mo(IV)-product complexes display an intense metal-to-ligand charge-transfer (MLCT) band in the near-infrared region of the spectrum. Optical pumping into this MLCT band yields resonance Raman spectra of the Mo site that are devoid of contributions from the highly absorbing FAD and 2Fe2S clusters in the protein. The resonance Raman spectra reveal in-plane bending modes of the bound product and low-frequency molybdenum dithiolene and pyranopterin dithiolene vibrational modes. This work provides keen insight into the role of the pyranopterin dithiolene in electron-transfer reactivity.

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Room temperature electronic absorption difference spectra(Eproduct – Ered) for XOred-product complexes in N,N-bis(2-hydroxyethyl)glycine buffer at pH = 8.3.Inset:Structure of the enzyme–product complexes. E = O for 4-TV,and E = S for 2,4-TV.
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fig2: Room temperature electronic absorption difference spectra(Eproduct – Ered) for XOred-product complexes in N,N-bis(2-hydroxyethyl)glycine buffer at pH = 8.3.Inset:Structure of the enzyme–product complexes. E = O for 4-TV,and E = S for 2,4-TV.

Mentions: We anticipated that heavy-atom congeners of lumazinewould resultin bathochromic shifts of the MLCT absorption maximum relative tothe analogous complex formed with lumazine.18,20 The bathochromic MLCT shift would result in high-quality rR databecause it effectively eliminates the dominant higher-energy absorptioncontributions from the 2Fe2S and FAD centers and deleterious contributionsfrom free FAD fluorescence. The electronic absorption spectra forMo-product complexes with 4-TV (758 nm) and 2,4-TV (778 nm) possessNIR absorbance maxima that are red-shifted by ∼3000 and ∼4000cm–1, respectively, relative to the lumazine MLCTcomplex (Figure 2).18,20 Because the MLCT transition derives from a Mo(xy) → product(π*)(HOMO → LUMO) one-electron promotion (Figure S3 in the Supporting Information, SI), optical pumpingof this transition creates an excited state with appreciable MoV-P•– character, and interrogationof this MLCT state by rR spectroscopy (Figure 3 and Table 1) provides important informationregarding the nature of low-frequency Mo-(pyranopterin ditholene)distortions that are coupled to a one-electron oxidation of the MoIV site. These are the same distortions anticipated for theMoIV → MoV ET event in the oxidativehalf-reaction of the enzyme, providing new insight into the extentto which the pyranopterin dithiolene is coupled into ET regenerationprocesses in XO/XDH.


Pyranopterin dithiolene distortions relevant to electron transfer in xanthine oxidase/dehydrogenase.

Dong C, Yang J, Leimkühler S, Kirk ML - Inorg Chem (2014)

Room temperature electronic absorption difference spectra(Eproduct – Ered) for XOred-product complexes in N,N-bis(2-hydroxyethyl)glycine buffer at pH = 8.3.Inset:Structure of the enzyme–product complexes. E = O for 4-TV,and E = S for 2,4-TV.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC4215880&req=5

fig2: Room temperature electronic absorption difference spectra(Eproduct – Ered) for XOred-product complexes in N,N-bis(2-hydroxyethyl)glycine buffer at pH = 8.3.Inset:Structure of the enzyme–product complexes. E = O for 4-TV,and E = S for 2,4-TV.
Mentions: We anticipated that heavy-atom congeners of lumazinewould resultin bathochromic shifts of the MLCT absorption maximum relative tothe analogous complex formed with lumazine.18,20 The bathochromic MLCT shift would result in high-quality rR databecause it effectively eliminates the dominant higher-energy absorptioncontributions from the 2Fe2S and FAD centers and deleterious contributionsfrom free FAD fluorescence. The electronic absorption spectra forMo-product complexes with 4-TV (758 nm) and 2,4-TV (778 nm) possessNIR absorbance maxima that are red-shifted by ∼3000 and ∼4000cm–1, respectively, relative to the lumazine MLCTcomplex (Figure 2).18,20 Because the MLCT transition derives from a Mo(xy) → product(π*)(HOMO → LUMO) one-electron promotion (Figure S3 in the Supporting Information, SI), optical pumpingof this transition creates an excited state with appreciable MoV-P•– character, and interrogationof this MLCT state by rR spectroscopy (Figure 3 and Table 1) provides important informationregarding the nature of low-frequency Mo-(pyranopterin ditholene)distortions that are coupled to a one-electron oxidation of the MoIV site. These are the same distortions anticipated for theMoIV → MoV ET event in the oxidativehalf-reaction of the enzyme, providing new insight into the extentto which the pyranopterin dithiolene is coupled into ET regenerationprocesses in XO/XDH.

Bottom Line: The reducing substrates 4-thiolumazine and 2,4-dithiolumazine have been used to form Mo(IV)-product complexes with xanthine oxidase (XO) and xanthine dehydrogenase.The resonance Raman spectra reveal in-plane bending modes of the bound product and low-frequency molybdenum dithiolene and pyranopterin dithiolene vibrational modes.This work provides keen insight into the role of the pyranopterin dithiolene in electron-transfer reactivity.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and Chemical Biology, The University of New Mexico , MSC03 2060, 1 University of New Mexico, Albuquerque, New Mexico 87131-0001, United States.

ABSTRACT
The reducing substrates 4-thiolumazine and 2,4-dithiolumazine have been used to form Mo(IV)-product complexes with xanthine oxidase (XO) and xanthine dehydrogenase. These Mo(IV)-product complexes display an intense metal-to-ligand charge-transfer (MLCT) band in the near-infrared region of the spectrum. Optical pumping into this MLCT band yields resonance Raman spectra of the Mo site that are devoid of contributions from the highly absorbing FAD and 2Fe2S clusters in the protein. The resonance Raman spectra reveal in-plane bending modes of the bound product and low-frequency molybdenum dithiolene and pyranopterin dithiolene vibrational modes. This work provides keen insight into the role of the pyranopterin dithiolene in electron-transfer reactivity.

Show MeSH