Tracking Photoinduced Charge Separation in DNA: from Start to Finish

Abstract

The initial studies of the dynamics of photoinduced charge separation conducted in our laboratories 20 years ago found strongly distance-dependent rate constants over short distances but failed to detect intermediates in the transport of positive charge (holes). These observations were consistent with the single-step superexchange or tunneling mechanism that had been observed for numerous donor-bridge-acceptor systems at that time. Subsequent studies found weak distance dependence for hole transport over longer distances in DNA, characteristic of incoherent hopping of either localized or delocalized holes. The introduction of synthetic DNA capped hairpin constructs possessing hole donor and acceptor chromophores (or purine bases) at opposite ends of a base-pair domain made it possible to determine the time required for transit of charge from one chromophore to the other and, in some cases, to distinguish between the transit time and the much faster initial charge injection time. These studies eliminated conventional tunneling as a viable mechanism for charge transport in DNA except at very short donor-acceptor separations; however, they did not establish the presence or nature of intermediates in the charge separation process. Recent studies in our laboratories have succeeded in identifying key intermediates as well as untangling the dynamics and efficiency of the charge separation process from start to finish. The dynamics of the initial charge injection process is dependent upon both its free energy and the stacking of the hole donor chromophore and adjacent purine base. The transport of positive charge (holes) over multiple base pairs in duplex DNA occurs most efficiently via repeating adenine bases, known as A-tracts. The transit time across an A-tract is strongly dependent upon the free energy for hole injection, whereas the efficiency of charge separation depends on the competition between charge delocalization and charge recombination in the contact radical ion pair. The guanine cation radical has been detected both by femtosecond transient absorption and by stimulated Raman spectroscopies when the guanine is located near the chromophore employed for hole injection into an A-tract. Replacement of guanine by its derivative 8-phenylethynylguanine (EG), permits tracking of hole transport across longer poly(purine) sequences as a consequence of the stronger transient absorption and stimulated Raman scattering for EG vs G. We have recently obtained evidence based on femtosecond transient absorption spectroscopy for the formation of delocalized A-polarons in A-tracts possessing four or more A-T base pairs. Similar methods have been used to track hole transport across less-common DNA structures including diblock and triblock poly(purines), locked nucleic acids, three-way junctions, and G-quadruplexes. Similar methods are have been applied to the study of photoinduced electron transport in DNA.

Why Are DNA and Protein Electron Transfer So Different?

“…Here, σ V is the standard deviation of the nearest-neighbor couplings, ΔR is the spacing between bases, and σ E is the energy width λk B T ; the result of DNA simulations allowed us to determine σ V and σ E to estimate Φ = 0.6Å −1 , consistent with the experimental rate decays found by Lewis and coworkers (30,70,75). The FR mechanism is accessible in these systems because base-to-base interactions (|V|), energy fluctuations (σ E ), and energy gaps among purines (ΔE) are all on the scale of hundreds of millielectron volts.…”

“…With an experimental β value of 0.6Å −1 from the Lewis experiments (30,70) and ΔE inj of ~400 meV (corresponding to the gap in the oxidation potentials of G and A bases), a tunneling/hopping crossover would be calculated at approximately eight base pairs, not far from the observed transition. The interplay of barrier heights and transport mechanisms is highlighted in the sidebar titled Small β Values Are Incompatible with Tunneling.…”

Guanine‐Stabilized Formamidinium Lead Iodide Perovskites

“…Herein, we demonstrate the utility of guaninium (G)‐derived low‐dimensional perovskites in stabilizing the α‐FAPbI 3 perovskite phase, which is assessed by solid‐state NMR spectroscopy, X‐ray crystallography, molecular dynamics simulations, and DFT calculations. Guanine is a biologically relevant organic molecule that plays an important role in stabilizing the DNA structure (Figure a), while displaying the propensity for the formation of conductive supramolecular assemblies and functional materials . We have therefore utilized its functional units to form a novel hybrid perovskite structure based on the low dimensional G 2 PbI 4 composition, which was used to increase the performance and stability of α‐FAPbI 3 perovskite solar cells.…”

Synthetic strategies tailoring colours in multichromophoric organic nanostructures

“…In addition to the high programmability of the DNA duplex, its rigid helical structure featuring well-defined distance between base pairs allows the incorporation of dyes in a very predictable spatial manner. [32][33][34] For instance, DNA-templated chromophoric assemblies can be achieved through complementary base pairing. Wagenknecht and co-workers used ethynyl pyrene (Py-dU) and ethynyl nile red (Nr-dU) containing 2 0 -deoxyuridine to selectively assemble the dyes along oligo-2 0 -deoxyadenosine ( Fig.…”