Solution structure of a nucleic acid photoproduct of deoxyfluorouridylyl-(3'-5')-thymidine monophosphate (d-FpT) determined by NMR and restrained molecular dynamics: structural comparison of two sequence isomer photoadducts (d-U5p5T and d-T5p5U).
Acetone-sensitized irradiation using UV-B (sun lamp, lambda max = 313 nm) of deoxyfluorouridylyl-(3'-5')-thymidine monophosphate (d-FpT, F = fluorouracil), produces two major photoproducts, the cis-syn cyclobutane-type photodimer and a defluorinated (5-5) photoadduct, d-U5p5T. Product distribution is dependent on the pH of the irradiation solution, as was the case of irradiated d-TpF. At high pH (8-10) the (5-5) photoadduct is the major photoproduct. Irradiation of d-FpT shows a much faster photodegradation rate than the sequence isomer d-TpF. Multinuclear NMR experiments establish the formation of (5-5) covalent bonding between the C5 (d-U5p-, where the fluorine had been) and the C5 (-p5T) and the C6 (-p5T) acquires an OH group. NOE interproton distances and dihedral angles derived from J coupling analysis are constrained to refine model structures of d-U5p5T in restrained molecular dynamics calculations. The resultant structures obtained show 5S-6S as the most chiralities of the C5 and C6 atoms of the thymine, which is the opposite chirality to the corresponding atoms in the sequence isomer d-T5p5U. The orientation of the C5 substituents (-p5T fragment), the CH3 and the uracil are pseudo-axial and pseudo-equatorial respectively. Glycosidic angles are in the anti regions for both the d-U5p- and -p5T residues. Averaged backbone conformations of the two photoadducts, d-U5p5T and d-T5p5U, are similar, although the overall structure of d-U5p5T appears much more flexible than that of d-T5p5U. In particular, the sugar conformations of the 5'-end residues show a remarkable difference in flexibility.
Kim
J K
JK
Biophysics Department, Roswell Park Cancer Institute, Buffalo, NY 14263, USA.
Soni
S D
SD
Arakali
A V
AV
Wallace
J C
JC
Alderfer
J L
JL
D004249:DNA Damage; D015226:Dinucleoside Phosphates / Q000737:chemistry*; D004307:Dose-Response Relationship, Radiation; D006027:Glycosides; D009682:Magnetic Resonance Spectroscopy; D009690:Nucleic Acid Conformation / Q000528:radiation effects; D012996:Solutions; D014466:Ultraviolet Rays
D015226:Dinucleoside Phosphates; D006027:Glycosides; D012996:Solutions; C094169:deoxy-m5HO6-uridylyl(5-5)-(3'-5')deoxyuridine monophosphate; C094168:deoxyfluorouridylyl(3'-5')thymidine monophosphate