Femtosecond photoisomerization of azobenzene-derivative binding to DNA

Abstract

Ultrafast photoisomerization and relaxation dynamics of trans (T) 4′-methylthioazobenzene (AzD) binding to double-strand DNA (T-AzD-dsDNA) as well as single-strand DNA (T-AzD-ssDNA) and T-AzD were investigated by the measurement of femtosecond absorbance changes on S 2 T excitation (400 nm) with the rate equation analysis. All the solutions showed the fast ( τ 1, A 1) and slow ( τ 2, A 2) decay components and the offset component ( A 3). The greatly different negative or positive absorbance change behaviors by the probe wavelength of 400 or 420 nm for the T-AzD solution were attributed to the remarkable dependence of the absorption cross-section difference between the T-isomer excited and ground states on the probe wavelength and of that between the cis (C)- and T-isomer ground states. The significantly shorter S 2 T state lifetimes τ 1 for T-AzD-dsDNA and T-AzD-ssDNA were observed to be 30 and 60 fs, respectively, compared with that (220 fs) of T-AzD. This is presumably attributed to the intramolecular electron transfer from DNA bases to T-AzD in T-AzD-DNAs, suggesting the first observation of electron transfer in an ultrafast photoisomerization system interacting with DNA. While, the kinetic rate k 2,1 T , I from the S 2 T state to the bottleneck intermediate state I 1 T , C of the initial process in the T to C photoisomerization and the I 1 T , C state lifetime τ 2 hardly changed like 1.3 × 10 11, 1.4 × 10 11 and 1.6 × 10 11s −1 and like 6.7, 6.2 and 6.0 ps, respectively. The latter implies that the birth time of the C-isomer is almost the same for all the solutions. Furthermore, the T-to-C photoisomerization rate η T,C by single-shot excitation was determined from A 3 to be 1.2%, 0.36% and 0.22% at the 100-nJ pulse energy level, indicating that the T-AzD molecule is one of the most efficient T-to-C photoisomerization molecules. The decrease of η T,C in T-AzD-DNAs is due to the dramatic shortening of the excited-state lifetime τ 1.