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Identifying the Tautomeric Form of a Deoxyguanosine-Estrogen Quinone Intermediate.

Stack DE - Metabolites (2015)

Bottom Line: This tautomeric form was further verified by use of deuterium labelling of the catechol precursor use to form the estrogen o-quinone.HPLC-MS analysis indicates a reactive intermediate with a m/z of 552.22 consistent with a tautomeric form containing no deuterium.This intermediate is consistent with a reaction mechanism that involves: (1) proton assisted Michael addition; (2) re-aromatization of the estrogen A ring; and (3) glycosidic bond cleavage to form the known estrogen-DNA adduct, 4-OHE₁-1-N7Gua.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, University of Nebraska at Omaha, 6001 Dodge Street, Omaha, NE 68182, USA. dstack@unomaha.edu.

ABSTRACT
Mechanistic insights into the reaction of an estrogen o-quinone with deoxyguanosine has been further investigated using high level density functional calculations in addition to the use of 4-hyroxycatecholestrone (4-OHE₁) regioselectivity labeled with deuterium at the C1-position. Calculations using the M06-2X functional with large basis sets indicate the tautomeric form of an estrogen-DNA adduct present when glycosidic bonds cleavage occurs is comprised of an aromatic A ring structure. This tautomeric form was further verified by use of deuterium labelling of the catechol precursor use to form the estrogen o-quinone. Regioselective deuterium labelling at the C1-position of the estrogen A ring allows discrimination between two tautomeric forms of a reaction intermediate either of which could be present during glycosidic bond cleavage. HPLC-MS analysis indicates a reactive intermediate with a m/z of 552.22 consistent with a tautomeric form containing no deuterium. This intermediate is consistent with a reaction mechanism that involves: (1) proton assisted Michael addition; (2) re-aromatization of the estrogen A ring; and (3) glycosidic bond cleavage to form the known estrogen-DNA adduct, 4-OHE₁-1-N7Gua.

No MeSH data available.


Results of M06-2X/QZVP//M06-2X/6-31 + G(d,p) calculation on model forms of tautomers 1 and 2.
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metabolites-05-00475-f004: Results of M06-2X/QZVP//M06-2X/6-31 + G(d,p) calculation on model forms of tautomers 1 and 2.

Mentions: Figure 4 show the results of the density functional calculations with both the α- and β-forms of tautomers 1 and 2. With either the α-isomer tautomers or the β-isomer tautomers, tautomeric form 2 is significantly lower in energy, −18.42 and −20.80 kcal/mol, respectively. Re-aromatization of the estrogen A ring is no doubt a strong driving force the loss of the C1-proton in addition to the reduction of the overall molecular dipole. These calculations only give the overall thermodynamic preference for the conversion of 1 to 2, and while most proton transfers have small energy barriers, proton transfers to and from carbon atoms are known to occur at slower rates when compared to heteroatom proton transfers [20]. It is also noteworthy that the glycosidic bond is longer (weaker) in tautomeric form 1 when compared to tautomeric form 2. These results still leave the question of when glycosidic bond cleavage occurs unclear. Since previous studies were able to measure the kinetics of the glycosidic bond cleavage (ΔG‡ = 26.8 kcal/mol) [16], we decided to model the key proton transfer step and compare the energy barrier with measured kinetic data.


Identifying the Tautomeric Form of a Deoxyguanosine-Estrogen Quinone Intermediate.

Stack DE - Metabolites (2015)

Results of M06-2X/QZVP//M06-2X/6-31 + G(d,p) calculation on model forms of tautomers 1 and 2.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4588807&req=5

metabolites-05-00475-f004: Results of M06-2X/QZVP//M06-2X/6-31 + G(d,p) calculation on model forms of tautomers 1 and 2.
Mentions: Figure 4 show the results of the density functional calculations with both the α- and β-forms of tautomers 1 and 2. With either the α-isomer tautomers or the β-isomer tautomers, tautomeric form 2 is significantly lower in energy, −18.42 and −20.80 kcal/mol, respectively. Re-aromatization of the estrogen A ring is no doubt a strong driving force the loss of the C1-proton in addition to the reduction of the overall molecular dipole. These calculations only give the overall thermodynamic preference for the conversion of 1 to 2, and while most proton transfers have small energy barriers, proton transfers to and from carbon atoms are known to occur at slower rates when compared to heteroatom proton transfers [20]. It is also noteworthy that the glycosidic bond is longer (weaker) in tautomeric form 1 when compared to tautomeric form 2. These results still leave the question of when glycosidic bond cleavage occurs unclear. Since previous studies were able to measure the kinetics of the glycosidic bond cleavage (ΔG‡ = 26.8 kcal/mol) [16], we decided to model the key proton transfer step and compare the energy barrier with measured kinetic data.

Bottom Line: This tautomeric form was further verified by use of deuterium labelling of the catechol precursor use to form the estrogen o-quinone.HPLC-MS analysis indicates a reactive intermediate with a m/z of 552.22 consistent with a tautomeric form containing no deuterium.This intermediate is consistent with a reaction mechanism that involves: (1) proton assisted Michael addition; (2) re-aromatization of the estrogen A ring; and (3) glycosidic bond cleavage to form the known estrogen-DNA adduct, 4-OHE₁-1-N7Gua.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, University of Nebraska at Omaha, 6001 Dodge Street, Omaha, NE 68182, USA. dstack@unomaha.edu.

ABSTRACT
Mechanistic insights into the reaction of an estrogen o-quinone with deoxyguanosine has been further investigated using high level density functional calculations in addition to the use of 4-hyroxycatecholestrone (4-OHE₁) regioselectivity labeled with deuterium at the C1-position. Calculations using the M06-2X functional with large basis sets indicate the tautomeric form of an estrogen-DNA adduct present when glycosidic bonds cleavage occurs is comprised of an aromatic A ring structure. This tautomeric form was further verified by use of deuterium labelling of the catechol precursor use to form the estrogen o-quinone. Regioselective deuterium labelling at the C1-position of the estrogen A ring allows discrimination between two tautomeric forms of a reaction intermediate either of which could be present during glycosidic bond cleavage. HPLC-MS analysis indicates a reactive intermediate with a m/z of 552.22 consistent with a tautomeric form containing no deuterium. This intermediate is consistent with a reaction mechanism that involves: (1) proton assisted Michael addition; (2) re-aromatization of the estrogen A ring; and (3) glycosidic bond cleavage to form the known estrogen-DNA adduct, 4-OHE₁-1-N7Gua.

No MeSH data available.