Fd. Lewis et al., Dynamics of photoinduced charge separation and charge recombination in synthetic DNA hairpins with stilbenedicarboxamide linkers, J AM CHEM S, 122(12), 2000, pp. 2889-2902
The dynamics of photoinduced charge separation and charge recombination in
synthetic DNA hairpins have been investigated by means of femtosecond and n
anosecond transient spectroscopy. The hairpins consist of a stilbene linker
connecting two complementary B-mer or 7-mer oligonucleotide strands. Base
pairing between these strands results in formation of hairpins in which the
stilbene is approximately parallel to the adjacent base pair. The singlet
stilbene is selectively quenched by guanine, but not by the other nucleobas
es, via an electron-transfer mechanism in which the stilbene singlet state
is the electron acceptor and guanine is the electron donor. In a hairpin co
ntaining only A:T base pairs, no quenching occurs and the restricted geomet
ry results in a long stilbene lifetime and high fluorescence quantum yield.
In families of hairpins which contain a single G:C base pair at varying lo
cations in the hairpin stem, the stilbene fluorescence lifetime and quantum
yield decrease as the stilbene-guanine distance decreases. Transient absor
ption spectroscopy is used to monitor the disappearance of the stilbene sin
glet and the formation and decay of the stilbene anion radical. Analysis of
these data provides the rate constants for charge separation and charge re
combination. Both processes show an exponential decrease in rate constant w
ith increasing stilbene-guanine distance. Thus, electron transfer is conclu
ded to occur via a single-step superexchange mechanism with a distance depe
ndence beta = 0.7 Angstrom(-1) for charge separation and 0.9 Angstrom(-1) f
or charge recombination. The rate constants for charge separation and charg
e recombination via polyA vs polyT strands are remarkably similar, slightly
larger values being observed for polyA strands. The dynamics of electron t
ransfer in hairpins containing two adjacent G:C base pairs have also been i
nvestigated. When the guanines are in different strands, the second guanine
has little effect on the efficiency or dynamics of electron transfer. Howe
ver, when the guanines are in the same strand, somewhat faster charge separ
ation and slower charge recombination are observed than in the case of hair
pins with a single G:C base pair. Thus, the cc step functions as a shallow
hole trap. The relationship of these results to other theoretical and exper
imental studies of electron transfer in DNA is discussed.