Structure and Dynamics of Electron Injection and Charge Recombination in i-Motif DNA Conjugates

Exciton Absorption and Luminescence in i-Motif DNA

We have studied the excited-state dynamics for the i-motif form of cytosine chains (dC)10, using the ultrafast fluorescence up-conversion technique. We have also calculated vertical electronic transition energies and determined the nature of the corresponding excited states in a model tetramer i-motif structure. Quantum chemical calculations of the excitation spectrum of a tetramer i-motif structure predict a significant (0.3 eV) red shift of the lowest-energy transition in the i-motif form relative to its absorption maximum, which agrees with the experimental absorption spectrum. The lowest excitonic state in i-(dC)10 is responsible for a 2 ps red-shifted emission at 370 nm observed in the decay-associated spectra obtained on the femtosecond time-scale. This delocalized (excitonic) excited state is likely a precursor to a long-lived excimer state observed in previous studies. Another fast 310 fs component at 330 nm is assigned to a monomer-like locally excited state. Both emissive states form within less than the available time resolution of the instrument (100 fs). This work contributes to the understanding of excited-state dynamics of DNA within the first few picoseconds, which is the most interesting time range with respect to unraveling the photodamage mechanism, including the formation of the most dangerous DNA lesions such as cyclobutane pyrimidine dimers.

Tracking Photoinduced Charge Separation in DNA: from Start to Finish

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.