Charge recombination in DNA hairpins controlled by counterions.

The slow charge recombination in DNA hairpins of various lengths is investigated. The distance dependence for the charge recombination rate between stilbene donor (Sd(+)) and stilbene acceptor (Sa(-)) linkers separated by AT bridges of various lengths has the double exponential form. We suggest that this puzzling dependence is associated with two tunneling channels distinguished by the presence or absence of a Cl(-) counterion bound to an Sd(+) linker.