Maria Angela Trachsel

Excited-State Structure and Dynamics of Keto–Amino Cytosine: The 1ππ* State Is Nonplanar and Its Radiationless Decay Is Not Ultrafast

We have measured the mass- and tautomer-specific S0 → S1 vibronic spectra and S1 state lifetimes of the keto–amino tautomer of cytosine cooled in supersonic jets, using two-color resonant two-photon ionization (R2PI) spectroscopy at 0.05 cm(–1) resolution. The rotational contours of the 0(0)(0) band and nine vibronic bands up to +437 cm(–1) are polarized in the pyrimidinone plane, proving that the electronic excitation is to a 1ππ* state. All vibronic excitations up to +437 cm(–1) are overtone and combination bands of the low-frequency out-of-plane ν1′ (butterfly), ν2′ (boat), and ν3′ (H–N–C6–H twist) vibrations. UV vibronic spectrum simulations based on approximate second-order coupled-cluster (CC2) calculations of the ground and 1ππ* states are in good agreement with the experimental R2PI spectrum, but only if the calculated ν1′ and ν2′ frequencies are reduced by a factor of 4 and anharmonicity is included. Together with the high intensity of the ν1′ and ν2′ overtone vibronic excitations, this implies that the 1ππ* potential energy surface is much softer and much more anharmonic in the out-of-plane directions than predicted by the CC2 method. The 1ππ* state lifetime is determined from the Lorentzian line broadening necessary to reproduce the rotational band contours: at the 0(0)(0) band it is τ = 44 ps, remains at τ = 35–45 ps up to +205 cm(–1), and decreases to 25–30 ps up to +437 cm(–1). These lifetimes are 20–40 times longer than the 0.5–1.5 ps lifetimes previously measured with femtosecond pump–probe techniques at higher vibrational energies (1500–3800 cm(–1)). Thus, the nonradiative relaxation rate of keto–amino cytosine close to the 1ππ* state minimum is k(nr) 2.5 × 10(10) s(–1), much smaller than at higher energies. An additional nonradiative decay channel opens at +500 cm(–1) excess energy. Since high overtone bands of ν1′ and ν2′ are observed in the R2PI spectrum but only a single weak 2ν3′ band, we propose that ν3′ is a promoting mode for nonradiative decay, consistent with the observation that the ν3′ normal-mode eigenvector points toward the “C6-puckered” conical intersection geometry.

Gas-Phase Cytosine and Cytosine-N1-Derivatives Have 0.1-1 ns Lifetimes Near the S1 State Minimum.

Ultraviolet radiative damage to DNA is inefficient because of the ultrafast S1 ⇝ S0 internal conversion of its nucleobases. Using picosecond pump-ionization delay measurements, we find that the S1((1)ππ*) state vibrationless lifetime of gas-phase keto-amino cytosine (Cyt) is τ = 730 ps or ∼ 700 times longer than that measured by femtosecond pump-probe ionization at higher vibrational excess energy, Eexc. N1-Alkylation increases the S1 lifetime up to τ = 1030 ps for N1-ethyl-Cyt but decreases it to 100 ps for N1-isopropyl-Cyt. Increasing the vibrational energy to Eexc = 300-550 cm(-1) decreases the lifetimes to 20-30 ps. The nonradiative dynamics of S1 cytosine is not solely a property of the amino-pyrimidinone chromophore but is strongly influenced by the N1-substituent. Correlated excited-state calculations predict that the gap between the S2((1)nOπ*) and S1((1)ππ*) states decreases along the series of N1-derivatives, thereby influencing the S1 state lifetime.

Modeling the Histidine–Phenylalanine Interaction: The NH···π Hydrogen Bond of Imidazole·Benzene

NH···π hydrogen bonds occur frequently between the amino acid side groups in proteins and peptides. Data-mining studies of protein crystals find that ∼80% of the T-shaped histidine···aromatic contacts are CH···π, and only ∼20% are NH···π interactions. We investigated the infrared (IR) and ultraviolet (UV) spectra of the supersonic-jet-cooled imidazole·benzene (Im·Bz) complex as a model for the NH···π interaction between histidine and phenylalanine. Ground- and excited-state dispersion-corrected density functional calculations and correlated methods (SCS-MP2 and SCS-CC2) predict that Im·Bz has a Cs-symmetric T-shaped minimum-energy structure with an NH···π hydrogen bond to the Bz ring; the NH bond is tilted 12° away from the Bz C6 axis. IR depletion spectra support the T-shaped geometry: The NH stretch vibrational fundamental is red shifted by -73 cm(-1) relative to that of bare imidazole at 3518 cm(-1), indicating a moderately strong NH···π interaction. While the S0(A1g) → S1(B2u) origin of benzene at 38 086 cm(–1) is forbidden in the gas phase, Im·Bz exhibits a moderately intense S0 → S1 origin, which appears via the D(6h) → Cs symmetry lowering of Bz by its interaction with imidazole. The NH···π ground-state hydrogen bond is strong, De=22.7 kJ/mol (1899 cm–1). The combination of gas-phase UV and IR spectra confirms the theoretical predictions that the optimum Im·Bz geometry is T shaped and NH···π hydrogen bonded. We find no experimental evidence for a CH···π hydrogen-bonded ground-state isomer of Im·Bz. The optimum NH···π geometry of the Im·Bz complex is very different from the majority of the histidine·aromatic contact geometries found in protein database analyses, implying that the CH···π contacts observed in these searches do not arise from favorable binding interactions but merely from protein side-chain folding and crystal-packing constraints. The UV and IR spectra of the imidazole·(benzene)2 cluster are observed via fragmentation into the Im·Bz+ mass channel. The spectra of Im·Bz and Im·Bz2 are cleanly separable by IR hole burning. The UV spectrum of Im·Bz2 exhibits two 000 bands corresponding to the S0 → S1 excitations of the two inequivalent benzenes, which are symmetrically shifted by -86/+88 cm(-1) relative to the 000 band of benzene

Excited-State Structure, Vibrations, and Nonradiative Relaxation of Jet-Cooled 5-Fluorocytosine

The S0 → S1 vibronic spectrum and S1 state nonradiative relaxation of jet-cooled keto-amino 5-fluorocytosine (5FCyt) are investigated by two-color resonant two-photon ionization spectroscopy at 0.3 and 0.05 cm(–1) resolution. The 0(0)(0) rotational band contour is polarized in-plane, implying that the electronic transition is (1)ππ*. The electronic transition dipole moment orientation and the changes of rotational constants agree closely with the SCS-CC2 calculated values for the (1)ππ* (S1) transition of 5FCyt. The spectral region from 0 to 300 cm(–1) is dominated by overtone and combination bands of the out-of-plane ν1′ (boat), ν2′ (butterfly), and ν3′ (HN–C6H twist) vibrations, implying that the pyrimidinone frame is distorted out-of-plane by the (1)ππ* excitation, in agreement with SCS-CC2 calculations. The number of vibronic bands rises strongly around +350 cm(–1); this is attributed to the (1)ππ* state barrier to planarity that corresponds to the central maximum of the double-minimum out-of-plane vibrational potentials along the ν1′, ν2′, and ν3′ coordinates, which gives rise to a high density of vibronic excitations. At +1200 cm(–1), rapid nonradiative relaxation (k(nr) ≥ 10(12) s(–1)) sets in, which we interpret as the height of the (1)ππ* state barrier in front of the lowest S1/S0 conical intersection. This barrier in 5FCyt is 3 times higher than that in cytosine. The lifetimes of the ν′ = 0, 2ν1′, 2ν2′, 2ν1′ + 2ν2′, 4ν2′, and 2ν1′ + 4ν2′ levels are determined from Lorentzian widths fitted to the rotational band contours and are τ ≥ 75 ps for ν′ = 0, decreasing to τ ≥ 55 ps at the 2ν1′ + 4ν2′ level at +234 cm(–1). These gas-phase lifetimes are twice those of S1 state cytosine and 10–100 times those of the other canonical nucleobases in the gas phase. On the other hand, the 5FCyt gas-phase lifetime is close to the 73 ps lifetime in room-temperature solvents. This lack of dependence on temperature and on the surrounding medium implies that the 5FCyt nonradiative relaxation from its S1 ((1)ππ*) state is essentially controlled by the same ~1200 cm(–1) barrier and conical intersection both in the gas phase and in solution.