Hydrogen Bond Donors Accelerate Vibrational Cooling of Hot Purine Derivatives in Heavy Water

Abstract

Natural nucleobases and many of their derivatives have ultrashort excited state lifetimes that make them excellent model systems for studying intermolecular energy flow from a hot solute molecule to the solvent. UV-pump/broadband-mid-IR-probe transient absorption spectra of canonical purine nucleobases and several xanthine derivatives were acquired in D2O and acetonitrile in the probe frequency range of 1500-1750 cm(-1). The spectra reveal that vibrationally hot ground state molecules created by ultrafast internal conversion return to thermal equilibrium in several picoseconds by dissipating their excess energy to solvent molecules. In acetonitrile solution, where hydrogen bonding is minimal, vibrational cooling (VC) occurs with the same time constant of 10 ± 3 ps for paraxanthine, theophylline, and caffeine within experimental uncertainty. In D2O, VC by these molecules occurs more rapidly and at different rates that are correlated with the number of N-D bonds. Hypoxanthine has a VC time constant of 3 ± 1 ps, while similar lifetimes of 2.3 ± 0.8 ps and 3.1 ± 0.3 ps are seen for 5'-adenosine monophosphate and 5'-guanosine monophosphate, respectively. All three molecules have at least two N-D bonds. Slightly slower VC time constants are measured for paraxanthine (4 ± 1 ps) and theophylline (5.1 ± 0.8 ps), dimethylated xanthines that have only one N-D bond. Caffeine, a trimethylated xanthine with no N-D bonds, has a VC time constant of 7.7 ± 0.9 ps, the longest ever observed for any nucleobase in aqueous solution. Hydrogen bond donation by solute molecules is proposed to enable rapid energy disposal to water via direct coupling of high frequency solute-solvent modes.

Photochemistry of Nucleic Acid Bases and Their Thio- and Aza-Analogues in Solution

“…The mode-specific vibrational energy transfer model proposed by Nielsen and co-workers [155,181] is further supported by the recent studies of Kohler and co-workers [152,154]. These authors investigated the vibrational relaxation dynamics of guanosine 5 0 -monophosphate by femtosecond broadband transient absorption spectroscopy in the 1,400-1,750 cm À1 probe region in deuterated aqueous buffer solution (pD ¼ 7.2) [154].…”

“…The authors rationalized the pronounced solvent isotope effect as caused by a large portion of the excess vibrational energy being transferred to relatively high-frequency solvent librational modes in the 700-1,000 cm À1 range because the vibrational friction spectra of water and deuterated water are nearly identical at frequencies below~700 cm À1 [177][178][179]. Zhang et al [152] recently used femtosecond pump-probe transient absorption spectroscopy in the 1,500-1,725 cm À1 probe region to learn more about the factors that influence the rate of vibrational cooling dynamics in several purine monomers in solution. Figure 2 shows the ground-state recovery signals for six different purine derivatives probed at the specified vibrational modes in deuterated water solution.…”

“…As can be observed in Further structural impacts on vibrational cooling dynamics were investigated by Nielsen and co-workers [155,181], who recently showed that the phosphate group in the adenine, guanine, and cytosine nucleotides does not significantly contribute to the vibrational relaxation process. The authors also used femtosecond broadband Reprinted with permission from [152]. Copyright 2013 American Chemical Society transient absorption spectroscopy to probe the vibrational regions pertinent to the aromatic rings from 1,500 cm À1 to 1,800 cm À1 and the phosphate groups from 1,025 cm À1 to 1,175 cm À1 .…”

“…Note that the hydrogen bond counts do not consider the ribose and phosphate groups of AMP and GMP. Adapted with permission from [152]. Copyright 2013 American Chemical Society populations of a small number of high-frequency modes are non-statistical for times that are long enough to be experimentally measured.…”

Efficient UV-induced charge separation and recombination in an 8-oxoguanine-containing dinucleotide

“…The UV-pump/broadband-mid-IR-probe experimental setup has been described in detail elsewhere (41). Broadband mid-IR probe pulses centered at 6,150 nm were generated with an optical parametric amplifier (OPA) (TOPAS-C+NDFG; Light Conversion).…”