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Issue 40, 2014
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On the near UV photophysics of a phenylalanine residue: conformation-dependent ππ* state deactivation revealed by laser spectroscopy of isolated neutral dipeptides

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Abstract

The primary step of the near UV photophysics of a phenylalanine residue is investigated in one- and two-color pump–probe R2PI nanosecond experiments carried out on specific conformers of the Ac-Gly-Phe-NH2 molecule and related neutral compounds isolated in a supersonic expansion. Compared to toluene, whose ππ* state photophysics is dominated by intersystem crossing with a lifetime of ∼80 ns at the origin, the first ππ* state of Phe in the peptide environment is systematically found to be shorter-lived. The lifetime at the origin of transition is found to be significantly shortened in the presence of a primary amide (–CONH2) group (20–60 ns, depending on the conformer considered), demonstrating the existence of an additional non-radiative relaxation channel related to this chemical group. The quenching effect induced by the peptide environment is still more remarkable beyond the origin of the ππ* state, since vibronic bands of one of the 4 conformers observed (the 27-ribbon conformation) become barely detectable in the ns R2PI experiment, suggesting a significant conformer-selective lifetime shortening (below 100 ps). These results on dipeptides, which extend previous investigations on shorter Phe-containing molecules (N-Ac-Phe-NH2 and N-Ac-Phe-NH-Me), confirm the existence of conformer-dependent non-radiative deactivation processes, whose characteristic timescales range from tens of ns down to hundreds of ps or below. This dynamics is assigned to two distinct mechanisms: a first one, consistent with an excitation energy transfer from the optically active ππ* state to low-lying amide nπ* excited states accessed through conical intersections, especially in the presence of a C-terminal primary amide group (–CONH2); a second one, responsible for the short lifetimes in 27 ribbon structures, would be more specifically triggered by phenyl ring vibrational excitations. Implications in terms of spectroscopic probing of Phe in a peptide environment, especially in the presence of a quenching amide group, are discussed.

Graphical abstract: On the near UV photophysics of a phenylalanine residue: conformation-dependent ππ* state deactivation revealed by laser spectroscopy of isolated neutral dipeptides

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Publication details

The article was received on 30 Jul 2014, accepted on 02 Sep 2014 and first published on 02 Sep 2014


Article type: Paper
DOI: 10.1039/C4CP03401E
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Citation: Phys. Chem. Chem. Phys., 2014,16, 22192-22200
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    On the near UV photophysics of a phenylalanine residue: conformation-dependent ππ* state deactivation revealed by laser spectroscopy of isolated neutral dipeptides

    Y. Loquais, E. Gloaguen, M. Alauddin, V. Brenner, B. Tardivel and M. Mons, Phys. Chem. Chem. Phys., 2014, 16, 22192
    DOI: 10.1039/C4CP03401E

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