title: Charge-transfer and spin dynamics in DNA hairpin conjugates with perylenediimide as a base-pair surrogate.
abstract: A perylenediimide chromophore (P) was incorporated into DNA hairpins as a base-pair surrogate to prevent the self-aggregation of P that is typical when it is used as the hairpin linker. The photoinduced charge-transfer and spin dynamics of these hairpins were studied using femtosecond transient absorption spectroscopy and time-resolved EPR spectroscopy (TREPR). P is a photooxidant that is sufficiently powerful to quantitatively inject holes into adjacent adenine (A) and guanine (G) nucleobases. The charge-transfer dynamics observed following hole injection from P into the A-tract of the DNA hairpins is consistent with formation of a polaron involving an estimated 3-4 A bases. Trapping of the (A 3-4) (+*) polaron by a G base at the opposite end of the A-tract from P is competitive with charge recombination of the polaron and P (-*) only at short P-G distances. In a hairpin having 3 A-T base pairs between P and G ( 4G), the radical ion pair that results from trapping of the hole by G is spin-correlated and displays TREPR spectra at 295 and 85 K that are consistent with its formation from (1*)P by the radical-pair intersystem crossing mechanism. Charge recombination is spin-selective and produces (3*)P, which at 85 K exhibits a spin-polarized TREPR spectrum that is diagnostic of its origin from the spin-correlated radical ion pair. Interestingly, in a hairpin having no G bases ( 0G), TREPR spectra at 85 K revealed a spin-correlated radical pair with a dipolar interaction identical to that of 4G, implying that the A-base in the fourth A-T base pair away from the P chromophore serves as a hole trap. Our data suggest that hole injection and transport in these hairpins is completely dominated by polaron generation and movement to a trap site rather than by superexchange. On the other hand, the barrier for charge injection from G (+*) back onto the A-T base pairs is strongly activated, so charge recombination from G (or even A trap sites at 85 K) most likely proceeds by a superexchange mechanism.
authors.affiliation: Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, USA.
authors.forename: Tarek A
authors.initials: TA
authors.lastname: Zeidan
authors.forename: Raanan
authors.initials: R
authors.lastname: Carmieli
authors.forename: Richard F
authors.initials: RF
authors.lastname: Kelley
authors.forename: Thea M
authors.initials: TM
authors.lastname: Wilson
authors.forename: Frederick D
authors.initials: FD
authors.lastname: Lewis
authors.forename: Michael R
authors.initials: MR
authors.lastname: Wasielewski
mesh_terms: D020029:Base Pairing; D001665:Binding Sites; D003198:Computer Simulation; D004247:DNA / Q000138:chemical synthesis / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D007094:Imides / Q000737:chemistry*; D008956:Models, Chemical; D009690:Nucleic Acid Conformation; D009841:Oligonucleotides / Q000138:chemical synthesis / Q000737:chemistry; D010569:Perylene / Q000031:analogs & derivatives* / Q000737:chemistry; D011789:Quantum Theory; D013056:Spectrophotometry, Ultraviolet / Q000379:methods
chemical_list: D007094:Imides; D009841:Oligonucleotides; C521332:perylenediimide; D010569:Perylene; D004247:DNA