  Anomalous non-exponential relaxation in hydrated biomolecules is commonly
attributed to the complexity of the free-energy landscapes, similarly to
polymers and glasses. It was found recently that the hydrogen-bond breathing of
terminal DNA base pairs exhibits a slow power-law relaxation attributable to
weak Hamiltonian chaos, with parameters similar to experimental data. Here, the
relationship is studied between this motion and spectroscopic signals measured
in DNA with a small molecular photoprobe inserted into the base-pair stack. To
this end, the earlier computational approach in combination with an analytical
theory is applied to the experimental DNA fragment. It is found that the
intensity of breathing dynamics is strongly increased in the internal base
pairs that flank the photoprobe, with anomalous relaxation quantitatively close
to that in terminal base pairs. A physical mechanism is proposed to explain the
coupling between the relaxation of base-pair breathing and the experimental
response signal. It is concluded that the algebraic relaxation observed
experimentally is very likely a manifestation of weakly chaotic dynamics of
hydrogen-bond breathing in the base pairs stacked to the photoprobe, and that
the weak nanoscale chaos can represent an ubiquitous hidden source of
non-exponential relaxation in ultrafast spectroscopy.
