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

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 2(7) 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.

Secondary Structures in Phe-Containing Isolated Dipeptide Chains: Laser Spectroscopy vs Quantum Chemistry

“…The structural changes upon ππ* excitation are also satisfactorily reproduced at the RI-CC2 level, allowing a qualitative interpretation of the excited state experimental data. Finally, one should notice that the observation of the most stable forms of these systems indicates that, in contrast to other aromatic chromophores like tryptophan, the conformation-dependent excited state dynamics of the Phe chromophore, documented in a recent paper, 61 Comparison between the optimized geometries of the f-7 L -7 L (g−) conformer of GFa, obtained in the ground S 0 and excited S 1 states at the RI-CC2/cc-pVDZ level of theory. To illustrate the changes from both the chromophore and environment points of view, either phenyl rings or backbones have been superimposed and shown in the upper and lower panel, respectively.…”

Intrinsic Folding Proclivities in Cyclic β‐Peptide Building Blocks: Configuration and Heteroatom Effects Analyzed by Conformer‐Selective Spectroscopy and Quantum Chemistry

“…[35] In the case of ap henyl-based chromophore, however,n os uch dramatic cases have been evidenced. [74][75][76] The amide AI Rs pectra were recorded by using the IR/UV double resonance spectroscopy,w hich relies on the following principle:t he UV features detected in the R2PI spectra are used to probe the ground-state population of any cold conformer. By sending an IR pulse before the UV pulse, one promotes vibrational transitions from the ground state v = 0l evel to vibrationally excited levels.…”

Nonradiative Relaxation Mechanisms of UV Excited Phenylalanine Residues: A Comparative Computational Study

“…This stabilizes the vibronic state by ∼0.2 eV and brings the calculated values at the RI-CC2/cc-pVDZ level just 0.1 eV above the measured ones. Furthermore, vibronic excitations were measured 530 cm −1 above the UV origin in both NAPA and NAPMA conformers B [ 17 , 19 ]. As the corresponding vibronic transition in toluene is found at 532 cm −1 [ 31 ], we assigned the transitions to the 528 and 521 cm −1 in-plane distortion normal modes, of NAPA and NAPMA, respectively.…”

“…The excitation transfer from the side chain of the chromophoric amino acid to the peptide backbone critically depends on the accessibility of the / CI seam. Therefore, vibrational excitation may lead to faster nonradiative deactivation as has been recently shown for Ac-Gly-Phe-NH 2 [ 19 ].…”

“…More recently, we have studied the photophysics of several conformers of two capped dipeptides containing the Phe residue (see Figure 1 ). N -acetylphenylalanineamide (NAPA) and its amide- N -methylated derivative N -acetylphenylalanylmethylamide (NAPMA) display a range of deactivation mechanisms [ 17 , 18 ] also at play in larger peptides [ 19 ]. This paper reviews these mechanisms and sets the ground for the study of NRD mechanisms in NAPA monohydrates (see Figure 1 ).…”