Dataset Information

Proton-coupled hole hopping in nucleosomal and free DNA initiated by site-specific hole injection.

ABSTRACT: Nucleosomes were reconstituted from recombinant histones and a 147-mer DNA sequence containing the damage reporter sequence 5'-…d([2AP]T[GGG](1)TT[GGG](2)TTT[GGG](3)TAT)… with 2-aminopurine (2AP) at position 27 from the dyad axis. Footprinting studies with ?OH radicals reflect the usual effects of "in" and "out" rotational settings, while, interestingly, the guanine oxidizing one-electron oxidant CO(3)(?-) radical does not. Site-specific hole injection was achieved by 308 nm excimer laser pulses to produce 2AP(?+) cations, and superoxide via the trapping of hydrated electrons. Rapid deprotonation (~100 ns) and proton coupled electron transfer generates neutral guanine radicals, G(-H)? and hole hopping between the three groups of [GGG] on micro- to millisecond time scales. Hole transfer competes with hole trapping that involves the combination of O(2)(?-) with G(-H)? radicals to yield predominantly 2,5-diamino-4H-imidazolone (Iz) and minor 8-oxo-7,8-dihydroguanine (8-oxoG) end-products in free DNA (Misiaszek et al., J. Biol. Chem. 2004, 279, 32106). Hole migration is less efficient in nucleosomal than in the identical protein-free DNA by a factor of 1.2-1.5. The Fpg/piperidine strand cleavage ratio is ~1.0 in free DNA at all three GGG sequences and at the "in" rotational settings [GGG](1,3) facing the histone core, and ~2.3 at the "out" setting at [GGG](2) facing away from the histone core. These results are interpreted in terms of competitive reaction pathways of O(2)(?-) with G(-H)? radicals at the C5 (yielding Iz) and C8 (yielding 8-oxoG) positions. These differences in product distributions are attributed to variations in the local nucleosomal B-DNA base pair structural parameters that are a function of surrounding sequence context and rotational setting.

SUBMITTER: Liu Y

PROVIDER: S-EPMC3674890 | biostudies-literature | 2012 May

REPOSITORIES: biostudies-literature

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Proton-coupled hole hopping in nucleosomal and free DNA initiated by site-specific hole injection.

Liu Yang Y Liu Zhi Z Geacintov Nicholas E NE Shafirovich Vladimir V

Physical chemistry chemical physics : PCCP 20120424 20

Nucleosomes were reconstituted from recombinant histones and a 147-mer DNA sequence containing the damage reporter sequence 5'-…d([2AP]T[GGG](1)TT[GGG](2)TTT[GGG](3)TAT)… with 2-aminopurine (2AP) at position 27 from the dyad axis. Footprinting studies with ˙OH radicals reflect the usual effects of "in" and "out" rotational settings, while, interestingly, the guanine oxidizing one-electron oxidant CO(3)(˙-) radical does not. Site-specific hole injection was achieved by 308 nm excimer laser pulses ...[more]

PMID: 22526555

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Project description:We have investigated photoinduced hole hopping in a Pseudomonas aeruginosa azurin mutant Re126WWCuI, where two adjacent tryptophan residues (W124 and W122) are inserted between the CuI center and a Re photosensitizer coordinated to a H126 imidazole (Re = ReI(H126)(CO)3(dmp)+, dmp = 4,7-dimethyl-1,10-phenanthroline). Optical excitation of this mutant in aqueous media (≤40 μM) triggers 70 ns electron transport over 23 Å, yielding a long-lived (120 μs) ReI(H126)(CO)3(dmp•-)WWCuII product. The Re126FWCuI mutant (F124, W122) is not redox-active under these conditions. Upon increasing the concentration to 0.2-2 mM, {Re126WWCuI}2 and {Re126FWCuI}2 are formed with the dmp ligand of the Re photooxidant of one molecule in close contact (3.8 Å) with the W122' indole on the neighboring chain. In addition, {Re126WWCuI}2 contains an interfacial tryptophan quadruplex of four indoles (3.3-3.7 Å apart). In both mutants, dimerization opens an intermolecular W122' → //*Re ET channel (// denotes the protein interface, *Re is the optically excited sensitizer). Excited-state relaxation and ET occur together in two steps (time constants of ∼600 ps and ∼8 ns) that lead to a charge-separated state containing a Re(H126)(CO)3(dmp•-)//(W122•+)' unit; then (CuI)' is oxidized intramolecularly (60-90 ns) by (W122•+)', forming ReI(H126)(CO)3(dmp•-)WWCuI//(CuII)'. The photocycle is closed by ∼1.6 μs ReI(H126)(CO)3(dmp•-) → //(CuII)' back ET that occurs over 12 Å, in contrast to the 23 Å, 120 μs step in Re126WWCuI. Importantly, dimerization makes Re126FWCuI photoreactive and, as in the case of {Re126WWCuI}2, channels the photoproduced "hole" to the molecule that was not initially photoexcited, thereby shortening the lifetime of ReI(H126)(CO)3(dmp•-)//CuII. Although two adjacent W124 and W122 indoles dramatically enhance CuI → *Re intramolecular multistep ET, the tryptophan quadruplex in {Re126WWCuI}2 does not accelerate intermolecular electron transport; instead, it acts as a hole storage and crossover unit between inter- and intramolecular ET pathways. Irradiation of {Re126WWCuII}2 or {Re126FWCuII}2 also triggers intermolecular W122' → //*Re ET, and the Re(H126)(CO)3(dmp•-)//(W122•+)' charge-separated state decays to the ground state by ∼50 ns ReI(H126)(CO)3(dmp•-)+ → //(W122•+)' intermolecular charge recombination. Our findings shed light on the factors that control interfacial hole/electron hopping in protein complexes and on the role of aromatic amino acids in accelerating long-range electron transport.

Dataset Information

Proton-coupled hole hopping in nucleosomal and free DNA initiated by site-specific hole injection.

Publications

Proton-coupled hole hopping in nucleosomal and free DNA initiated by site-specific hole injection.

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