We present infrared-UV double resonance spectra and quantum chemical calculations of the natural di-peptide H-Trp-Ser-OH. Two conformers are present in the supersonic expansions. They have a compact folded structure with two hydrogen bonds and with the serin residue stacked above the indole ring and an unusual NH(Trp)...O=C interaction in the lowest energy conformer. Conformational assignments are based on comparison with calculated (B97-D/TZV2P) structures and vibrational frequencies. Inclusion of dispersion in the quantum chemical calculations is mandatory for an accurate description of the conformer energies. We provide comparisons between methods often used in biochemistry and the dispersion-corrected double hybrid functional (B2PLYP-D) as a reference. Only concerted experimental and theoretical studies can unravel the conformational complexity already present in a dipeptide.
Vibronic spectra of 9H-adenine, 9-acetyladenine and several alkyladenines have been recorded by resonant two-photon ionization spectroscopy of the laser-desorbed molecules, entrained in a molecular beam. While adenine and the alkyladenines exhibit similar electronic spectra, 9-acetyladenine behaves considerably different. Theoretical absorption spectra of 9H-adenine and 9-acetyladenine were calculated using the combined density functional theory/multi-reference configuration interaction approach and using second order coupled cluster theory, in order to explain striking differences in the experimental spectra. The major differences between the 9H-adenine and the 9-acetyladenine absorption spectra can be traced back to the different configurations, which contribute to the excitations, both of the lowest ππ* and the nπ* states. While the excitations in 9H-adenine are localized in the chromophore, they show considerable charge transfer character from the chromophore to the acetyl group in the case of 9-acetyladenine.
Dark state: The photophysics of 9H-2AP (see figure) depends strongly on the solvent and the temperature. In polar, aprotic solvents the fluorescence quantum yield is lower compared to protic ones. Analysis of the diffuse fluorescence spectra point to the existence of a dark singlet n,pi* state which is long-lived (1 microsecond).
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