Electronic Relaxation Dynamics of UV-Photoexcited 2-Aminopurine–Thymine Base Pairs in Watson-Crick and Hoogsteen Conformations

The fluorescent analogue 2-aminopurine (2AP) of the canonical nucleobase adenine (6-aminopurine) base-pairs with thymine (T) without disrupting the helical structure of DNA. It therefore finds frequent use in molecular biology for probing DNA and RNA structures and conformational dynamics. However, detailed understanding of the processes responsible for fluorescence quenching remains largely elusive on a fundamental level. Although attempts have been made to ascribe decreased excited-state lifetimes to intrastrand charge-transfer and stacking interactions, possible influences from dynamic interstrand H-bonding have been widely ignored. Here, we investigate the electronic relaxation of UV-excited 2AP center dot T in Watson Crick (WC) and Hoogsteen (HS) conformations. Although the WC conformation features slowed-down, monomer-like electronic relaxation in tau similar to 1.6 ns toward ground-state recovery and triplet formation, the dynamics associated with 2AP center dot T in the HS motif exhibit faster deactivation in tau similar to 70 ps. As recent research has revealed abundant transient interstrand H-bonding in the Hoogsteen motif for duplex DNA, the established model for dynamic fluorescence quenching may need to be revised in the light of our results. The underlying supramolecular photophysical mechanisms are discussed in terms of a proposed excited-state double-proton transfer as an efficient deactivation channel for recovery of the HS species in the electronic ground state

Ultrafast Chemical Dynamics

Cytidine and Thymidine Faraday Discussion dat

Efficient intersystem crossing in 2-aminopurine riboside probed by femtosecond time-resolved transient vibrational absorption spectroscopy

The photophysical dynamics of 2-aminopurine riboside (2APr) in CHCl3 have been studied following excitation at (pump) = 310 nm by means of femtosecond transient vibrational absorption spectroscopy (TVAS) aided by quantum chemical density functional theory (DFT) and ab initio calculations. The experiments identified numerous vibrational marker bands in the regions of the NH2 stretch and the 2AP ring vibrations which could be assigned to the bleach of the S-0 electronic ground state (GS) and to transient populations in the (1)* and (3)* excited electronic states. The temporal evolution of the transient vibrational bands shows that the decay of the (1)* population is accompanied by a partial recovery of the GS and a concurrent population of the (3)* state with a time constant of (2) = 740 +/- 15 ps. The ensuing electronic relaxation is concluded to proceed via the (1)n* state as intermediate state. The absence of observable transient vibrational bands of this state hints at an upper limit for its lifetime of < 100 ps. The triplet quantum yield is found to be phi(T) = 0.42 +/- 0.07

Ultrafast Chemical Dynamics: CN radical reactions in solution in acetonitrile.

Ultrafast time resolved spectroscopy data for the study of CN radical reactions with acetonitrile and with solutions of THF or methanol in acetonitrile