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Nat Commun ; 12(1): 7285, 2021 12 14.
Artigo em Inglês | MEDLINE | ID: mdl-34907186


DNA owes its remarkable photostability to its building blocks-the nucleosides-that efficiently dissipate the energy acquired upon ultraviolet light absorption. The mechanism occurring on a sub-picosecond time scale has been a matter of intense debate. Here we combine sub-30-fs transient absorption spectroscopy experiments with broad spectral coverage and state-of-the-art mixed quantum-classical dynamics with spectral signal simulations to resolve the early steps of the deactivation mechanisms of uridine (Urd) and 5-methyluridine (5mUrd) in aqueous solution. We track the wave packet motion from the Franck-Condon region to the conical intersections (CIs) with the ground state and observe spectral signatures of excited-state vibrational modes. 5mUrd exhibits an order of magnitude longer lifetime with respect to Urd due to the solvent reorganization needed to facilitate bulky methyl group motions leading to the CI. This activates potentially lesion-inducing dynamics such as ring opening. Involvement of the 1nπ* state is found to be negligible.

Nucleosídeos de Pirimidina/química , Processos Fotoquímicos , Nucleosídeos de Pirimidina/efeitos da radiação , Pirimidinas/química , Pirimidinas/efeitos da radiação , Solventes/química , Espectrofotometria Ultravioleta , Raios Ultravioleta , Uridina/análogos & derivados , Uridina/química , Uridina/efeitos da radiação , Vibração
J Phys Chem Lett ; 12(45): 11070-11077, 2021 Nov 18.
Artigo em Inglês | MEDLINE | ID: mdl-34748341


Epigenetic DNA modifications play a fundamental role in modulating gene expression and regulating cellular and developmental biological processes, thereby forming a second layer of information in DNA. The epigenetic 2'-deoxycytidine modification 5-methyl-2'-deoxycytidine, together with its enzymatic oxidation products (5-hydroxymethyl-2'-deoxycytidine, 5-formyl-2'-deoxycytidine, and 5-carboxyl-2'-deoxycytidine), are closely related to deactivation and reactivation of DNA transcription. Here, we combine sub-30-fs transient absorption spectroscopy with high-level correlated multiconfigurational CASPT2/MM computational methods, explicitly including the solvent, to obtain a unified picture of the photophysics of deoxycytidine-derived epigenetic DNA nucleosides. We assign all the observed time constants and identify the excited state relaxation pathways, including the competition of intersystem crossing and internal conversion for 5-formyl-2'-deoxycytidine and ballistic decay to the ground state for 5-carboxy-2'-deoxycytidine. Our work contributes to shed light on the role of epigenetic derivatives in DNA photodamage as well as on their possible therapeutic use.

Angew Chem Int Ed Engl ; 60(18): 10155-10163, 2021 Apr 26.
Artigo em Inglês | MEDLINE | ID: mdl-33595902


Lewis acids have recently been recognized as catalysts enabling enantioselective photochemical transformations. Mechanistic studies on these systems are however rare, either due to their absorption at wavelengths shorter than 260 nm, or due to the limitations of theoretical dynamic studies for larger complexes. In this work, we overcome these challenges and employ sub-30-fs transient absorption in the UV, in combination with a highly accurate theoretical treatment on the XMS-CASPT2 level. We investigate 2-cyclohexenone and its complex to boron trifluoride and analyze the observed dynamics based on trajectory calculations including non-adiabatic coupling and intersystem crossing. This approach explains all ultrafast decay pathways observed in the complex. We show that the Lewis acid remains attached to the substrate in the triplet state, which in turn explains why chiral boron-based Lewis acids induce a high enantioselectivity in photocycloaddition reactions.

Chem Sci ; 11(29): 7599-7608, 2020 Aug 07.
Artigo em Inglês | MEDLINE | ID: mdl-33033610


The development of purely organic materials showing multicolor fluorescent and phosphorescent behaviour represents a formidable challenge in view of practical applications. Herein the rich photophysical behaviour of 3-(pyridin-2-yl)triimidazotriazine (TT-Py) organic molecule, comprising excitation-dependent fluorescence and phosphorescence under ambient conditions in both blended films and the crystalline phase, is investigated by means of steady state, time resolved and ultrafast spectroscopies and interpreted on the basis of X-ray diffraction studies and DFT/TDDFT calculations. In particular, by proper excitation wavelength, dual fluorescence and dual phosphorescence of molecular origin can be observed together with low energy phosphorescences resulting from aggregate species. It is demonstrated that the multiple emission properties originate from the copresence, in the investigated system, of an extended polycyclic nitrogen-rich moiety (TT), strongly rigidified by π-π stacking interactions and short C-H···N hydrogen bonds, and a fragment (Py) having partial conformational freedom.

Opt Express ; 27(8): 11018-11028, 2019 Apr 15.
Artigo em Inglês | MEDLINE | ID: mdl-31052953


Herewith, we describe how intensity and phase of the ultrashort pulse retrieved with second-harmonic frequency-resolved optical gating (SHG FROG) can be utilized for measurement of the nonlinear refractive index (n 2). Through comparison with available literature, we show that our method surpasses Z-scan in terms of precision by a factor of two, and thus, constitutes an interesting alternative. We present results for various materials: fused silica, calcite, YVO 4, BiBO, CaF 2, and YAG at 1030 nm. Unlike the Z-scan, the use of this method is not restricted to free-space geometry, but due to its characteristics, it can be used in integrated waveguides or photonic crystal fibers as well.

Chempluschem ; 84(5): 525-533, 2019 May.
Artigo em Inglês | MEDLINE | ID: mdl-31943903


A composite in which gold nanoparticles (AuNPs) approximately 10 nm in size are embedded in amorphous transparent silica matrix has been produced. The synthetic protocol uses HAuCl4 as the Au ion source, tetraethoxysilane (TEOS) as the SiO2 precursor, and l-ascorbic acid (AA) as the reducing agent. AA is employed before the sol-gel process in an amount sufficient only for reduction of Au3+ ions to Au+ . By using a cationic surfactant, benzylcetyldimethylammonium chloride hydrate (BDAC) and/or cetyltrimethylammonium bromide (CTAB), the Au+ ions are encapsulated within metalomicelles, which prevents them from being reduced to Au0 and enables their homogeneous distribution in the gel. Reduction of Au+ to Au0 and the growth of the AuNPs occurs at room temperature during the gelation, and arises from the release of EtOH during the hydrolysis of TEOS. The composites contain 0.027 wt % of Au. They exhibit nonlinear optical behavior characterized by the third-order nonlinear refraction index, n2 , in the range 3.6-5.7×10-16  cm2 W-1 at λ=1.030 µm. The composites are capable of effective third-harmonic generation of ultrashort near-IR (210 fs, 1.030 µm) laser pulse through a direct third-order mechanism.