Long-lived excited states in i-motif DNA studied by picosecond time-resolved IR spectroscopy

Abstract

The transient IR absorption spectrum for UV-excited i-motif DNA is reported for the first time and found to possess complex dynamics pointing to multiple decay processes, including possible charge transfer between packed hemi-protonated C bases.

Long‐Lived Excited‐State Dynamics of i‐Motif Structures Probed by Time‐Resolved Infrared Spectroscopy

“…At pH 5 bands characteristic of the i‐motif structure are observed with a strong positive Cotton effect at 286 nm and a negative Cotton effect at 265 nm, see Figure b. Similar behavior in the UV and CD spectra was observed for dC 30 and the C 3 T n family of sequences (Figure S1). However, in the case of the short sequences (C 3 T and TA 2 C 3 ) the CD did not reveal bands characteristic of i‐motif formation at the same concentration of nucleotide (Figure S2).…”

“…Next we considered the dynamics of the dC 30 sequence whose excited state we previously observed did not recover by 2 ns . This sequence presents a slightly more complicated situation as it may potentially form a number of structures in solution.…”

“…To do this a C 3 T n family, namely, (C 3 T 7 ) 3 C 3 , (C 3 T 8 ) 3 C 3 and (C 3 T 8 C 3 T 3 C 3 T 8 C 3 ), together with a C 5 sequence, (C 5 T 3 ) 3 C 5 , was chosen. These sequences allow us to probe the influence of different T‐rich loop sections on the deactivation dynamics and also allow us to establish whether the long‐lived species previously observed for d(CCCTAA) 4 and dC 30 are present . We then report the dynamics of a number of these systems on the extended ps‐to‐ns timescale, which for the first time allows the full decay process to be characterized in the same experiment by TRIR.…”

“…While TRIR studies of thymine derivatives by the Zinth group revealed CPD formation from the singlet state, the presence of long‐lived excited states in pyrimidine‐rich sequences may be expected to be an important source of these photoproducts. A striking example of a DNA structure that is found to stabilize pyrimidine excited states is the tetraplex i‐motif system . This structure is formed by interdigitating base‐pairs of hemi‐protonated dC‐rich sequences at weakly acidic pH .…”

“…In our preliminary study we probed the excited state dynamics of hemi‐protonated dC 30 and the human telomeric sequence d(CCCTAA) 4 using ps‐TRIR (i.e. <2 ns) following UV laser excitation.…”