Electron donor-acceptor interactions with flanking purines influence the efficiency of thymine photodimerization.
Quantum yields for thymine photodimerization (Φ(TT)) have been determined for a series of short DNA single-strand and base-paired hairpin structures possessing a single thymine-thymine step with flanking purines. Values of Φ(TT) are strongly dependent upon the oxidation potential of the flanking purine, decreasing in the order: inosine > adenine > guanine > deazaguanine. The dependence of Φ(TT) on the ionization potential of the flanking purine is more pronounced when the purine of lower oxidation potential is located at the 5'- versus 3'-position in either a single strand or a hairpin. Molecular dynamics simulations for hairpin structures indicate that the TT step is π-stacked with both the 5' and 3' purine, but that there is little π-stacking with either purine in single-strand structures. The observation of moderately intense long-wavelength UV absorption features for hairpins having 5'-Z or G flanking purines suggests that excitation of ground state donor-acceptor complexes may account for more extensive quenching of dimerization by 5'- versus 3'-purines. The "purine effect" on Φ(TT) is attributed to a combination of ground state conformation, ground state electron donor-acceptor interactions, and excited state exciplex formation.
Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA.
Zhengzheng
Z
Pan
Mahesh
M
Hariharan
Joshua D
JD
Arkin
Almaz S
AS
Jalilov
Martin
M
McCullagh
George C
GC
Schatz
Frederick D
FD
Lewis
D020029:Base Pairing; D004247:DNA / Q000737:chemistry*; D019281:Dimerization; D004583:Electrons; D008958:Models, Molecular; D009690:Nucleic Acid Conformation; D055668:Photochemical Processes; D011687:Purines / Q000737:chemistry*; D011789:Quantum Theory; D013941:Thymine / Q000737:chemistry*
D011687:Purines; D004247:DNA; D013941:Thymine