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Long-range hydrogen-bond relay catalyses the excited-state proton transfer reaction.

ABSTRACT: Solvent (e.g., water)-catalyzed proton transfer (SCPT) via the relay of hydrogen (H)-bonds plays a key role in proton migration. In this study, a new class of 1H-pyrrolo[3,2-g]quinolines (PyrQs) and their derivatives were synthesized, with sufficient separation of the pyrrolic proton donating and pyridinic proton accepting sites to probe excited-state SCPT. There was dual fluorescence for all PyrQs in methanol, i.e., normal (PyrQ) and tautomer 8H-pyrrolo[3,2-g]quinoline (8H-PyrQ) emissions. The fluorescence dynamics unveiled a precursor (PyrQ) and successor (8H-PyrQ) relationship and the correlation of an increasing overall excited-state SCPT rate (k_SCPT) upon increasing the N(8)-site basicity. k_SCPT can be expressed by the coupling reaction k_SCPT = K_eq × k_PT, where k_PT denotes the intrinsic proton tunneling rate in the relay and K_eq denotes the pre-equilibrium between randomly and cyclically H-bonded solvated PyrQs. Molecular dynamics (MD) simulation defined the cyclic PyrQs and analyzed the H-bond and molecular arrangement over time, which showed the cyclic PyrQs incorporating ≧3 methanol molecules. These cyclic H-bonded PyrQs are endowed with a relay-like proton transfer rate, k_PT. MD simulation estimated an upper-limited K_eq value of 0.02-0.03 for all studied PyrQs. When there was little change in K_eq, the distinct k_SCPT values for PyrQs were at different k_PT values, which increased as the N(8) basicity increased, which was induced by the C(3)-substituent. k_SCPT was subject to a deuterium isotope effect, where the k_SCPT of 1.35 × 10¹⁰ s^-1 for PyrQ-D in CH₃OD was 1.68 times slower than that (2.27 × 10¹⁰ s^-1) of PyrQ in CH₃OH. MD simulation provided a similar K_eq for PyrQ and PyrQ-D, leading to different proton tunneling rates (k_PT) between PyrQ and PyrQ-D.

SUBMITTER: Chang KH

PROVIDER: S-EPMC10321479 | biostudies-literature | 2023 Jul

REPOSITORIES: biostudies-literature

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Long-range hydrogen-bond relay catalyses the excited-state proton transfer reaction.

Chang Kai-Hsin KH Peng Yu-Chiang YC Su Kuan-Hsuan KH Lin Yi-Hsien YH Liu Jiun-Chi JC Liu Ying-Hsuan YH Hsu Chao-Hsien CH Yang Hsiao-Ching HC Chou Pi-Tai PT

Chemical science 20230530 26

Solvent (e.g., water)-catalyzed proton transfer (SCPT) via the relay of hydrogen (H)-bonds plays a key role in proton migration. In this study, a new class of 1H-pyrrolo[3,2-g]quinolines (PyrQs) and their derivatives were synthesized, with sufficient separation of the pyrrolic proton donating and pyridinic proton accepting sites to probe excited-state SCPT. There was dual fluorescence for all PyrQs in methanol, i.e., normal (PyrQ) and tautomer 8H-pyrrolo ...[more]

PMID: 37416704

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Project description:Fluorescent probes that exhibit solvatochromic or excited-state proton-transfer (ESPT) properties are essential tools for the study of complex biological or chemical systems. Herein, the synthesis and characterization of a novel fluorophore that reveals both features, 5-isocyanonaphthalene-1-ol (ICOL), are reported. Various solvatochromic methods, such as Lippert-Mataga and Bilot-Kawski, together with time-dependent density functional theory (TD-DFT) and time-resolved emission spectroscopy (TRES), were applied to gain insights into its excited-state behavior. To make comparisons, the octyloxy derivative of ICOL, 5-isocyano-1-(octyloxy)naphthalene (ICON), was also prepared. We found that internal charge transfer (ICT) takes place between the isocyano and -OH groups of ICOL, and we determined the values of the dipole moments for the ground and excited states of both ICOL and ICON. Furthermore, in the emission spectra of ICOL, a second band at higher wavelengths (green emission) in solvents of higher polarities (dual emission), in addition to the band present at lower wavelengths (blue emission), were observed. The extent of this dual emission increases in the order of 2-propanol < methanol < N,N-dimethylformamide (DMF) < dimethyl sulfoxide (DMSO). The presence of the dual fluorescence of ICOL in these solvents can be ascribed to ESPT. For ICOL, we also determined ground- and excited-state pKa values of 8.4 ± 0.3 and 0.9 ± 0.7, respectively, which indicates a considerable increase in acidity upon excitation. The TRES experiments showed that the excited-state lifetimes of the ICOL and ICON spanned from 10.1 ns to 5.0 ns and from 5.7 ns to 3.8 ns, respectively. In addition, we demonstrated that ICOL can be used as an effective indicator of not only the critical micelle concentration (cmc) of ionic (sodium lauryl sulfate (SLS)) and nonionic surfactants (Tween 80), but also other micellar parameters, such as partition coefficients, as well as to map the microenvironments in the cavities of biomacromolecules (e.g., BSA). It is also pointed out that fluorescence quenching by pyridine can effectively be utilized for the determination of the fractions of ICOL molecules that reside at the water-micelle interface and in the interior spaces of micelles.

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Long-range hydrogen-bond relay catalyses the excited-state proton transfer reaction.

Publications

Long-range hydrogen-bond relay catalyses the excited-state proton transfer reaction.

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