4-Amino-1,8-naphthalimide based fluorescent photoinduced electron transfer (PET) pH sensors as liposomal cellular imaging agents: The effect of substituent patterns on PET directional quenching

doi:10.1007/s11705-019-1862-8

Front. Chem. Sci. Eng.

2020, Vol. 14

Issue (1) : 61-75 https://doi.org/10.1007/s11705-019-1862-8

RESEARCH ARTICLE

4-Amino-1,8-naphthalimide based fluorescent photoinduced electron transfer (PET) pH sensors as liposomal cellular imaging agents: The effect of substituent patterns on PET directional quenching

Miguel Martínez-Calvo^1,³(

), Sandra A. Bright^1,², Emma B. Veale¹, Adam F. Henwood¹, D. Clive Williams², Thorfinnur Gunnlaugsson¹(

)

¹. School of Chemistry and Trinity Biomedical Sciences Institute (TBSI), Trinity College Dublin, The University of Dublin, Dublin 2, Ireland
². School of Biochemistry and Immunology and Trinity Biomedical Sciences Institute (TBSI), Trinity College Dublin, The University of Dublin, Dublin 2, Ireland
³. Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CIQUS) and Departamento de Química Orgánica Universidade de Santiago de Compostela, Rúa Jenaro de la Fuente s/n, 15782, Santiago de Compostela, Spain

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Abstract

Four new fluorescent sensors (1-4) based on the 4-amino-1,8-naphthalimide fluorophores (Naps) have been synthesized based on the classical fluorophore-spacer-receptor model. These four compounds all gave rise to emission bands centred at ca. 535 nm, which were found to be highly pH dependent, the emission being ‘switched on’ in acidic media, while being quenched due to PET from the amino moieties to the excited state of the Nap at more alkaline pH. The luminescent pH dependence for these probes was found to be highly dependent on the substitution on the imide site, as well as the polyamine chain attached to the position 4-amino moiety. In the case of sensor 2 the presence of the 4-amino-aniline dominated the pH dependent quenching. Nevertheless, at higher pH, PET quenching was also found to occur from the polyamine site. Hence, 2 is better described as a receptor₁-spacer₁-fluorophore-spacer₂-receptor₂ system, where the dominant PET process is due to (normally less favourable) ‘directional’ PET quenching from the 4-amino-aniline unit to the Nap site. Similar trends and pH fluorescence dependences were also seen for 3 and 4. These compounds were also tested for their imaging potential and toxicity against HeLa cells (using DRAQ5 as nuclear stain which does now show pH dependent changes in acidic and neutral pH) and the results demonstrated that these compounds have reduced cellular viability at moderately high concentrations (with IC₅₀ values between ca. 8–30 µmol∙L⁻¹), but were found to be suitable for intracellular pH determination at 1 µmol∙L⁻¹concentrations, where no real toxicity was observed. This allowed us to employ these as lysosomal probes at sub-toxic concentrations, where the Nap based emission was found to be pH depended, mirroring that seen in aqueous solution for 1-4, with the main fluorescence changes occurring within acidic to neutral pH.

Keywords sensors pH photoinduced electron transfer cellular imaging confocal microscopy

Corresponding Author(s): Miguel Martínez-Calvo,Thorfinnur Gunnlaugsson

Just Accepted Date: 15 October 2019 Online First Date: 11 December 2019 Issue Date: 20 January 2020

Cite this article:

Miguel Martínez-Calvo,Sandra A. Bright,Emma B. Veale, et al. 4-Amino-1,8-naphthalimide based fluorescent photoinduced electron transfer (PET) pH sensors as liposomal cellular imaging agents: The effect of substituent patterns on PET directional quenching[J]. Front. Chem. Sci. Eng., 2020, 14(1): 61-75.

URL:

https://academic.hep.com.cn/fcse/EN/10.1007/s11705-019-1862-8
https://academic.hep.com.cn/fcse/EN/Y2020/V14/I1/61

Fig.1 The photoiniduced electron transfer (PET) Nap sensors 1-4 developed in this study.

Fig.2 Scheme 1 The synthesis of NAP sensors 1-4 developed in this study.

Fig.3 Absorption spectra of (a) 1 recorded over the titration range from pH 2 to pH 10, demonstrating that a red shift was observed upon moving into alkaline media; (b) The same titration for 2 commencing in acidic solution and titrating with a base.

Fig.4 (a) The overall changes in the fluorescence emission spectra of sensor 1 from pH 2 to 10. Inset: The changes occurring at 535 nm between pH 2 and pH 10; (b) The same changes observed for sensor 2.

Fig.5 (a) The overall changes in the emission spectra of sensor 3 from pH 2 to 10 (Inset: The changes occurring at 535 nm between pH 2 and pH 10); (b) The same changes observed for sensor 4.

Tab.1 Cellular viability studies for compounds 1-4 in HeLa cells. A. HeLa cells B. and IC₅₀ values for each compound (Values represent the mean of three independent experiments performed in triplicate)

Fig.6 Confocal fluorescence imaging for the pH PET sensors 1 and 2 within HeLa cells that were treated for 24 h with the indicated compounds, upon excitation by a 405 nm argon laser, collecting the emission between 500?550 nm; Nap emission being observed as green fluorescence emission. After the required incubation period, cells were also stained with the nuclear stain DRAQ5, which was subsequently excited by a 633 nm red helium-neon laser, emission>650 nm using live confocal microscopy, and shown as red emission. Images are representative of three independent experiments.

Fig.7 Confocal fluorescence imaging for the pH PET sensors (a) 1 and (b) 2 at 1 mmol?L^?¹, within fixed HeLa cells, which were treated for 24 h with the indicated compounds. After the required incubation period, cells were fixed in 3% paraformaldehyde and stained with the nuclear stain DRAQ5. Samples were rinsed and imaged with buffers at the indicated pH. 1 and 2 were excited by a 405 nm argon laser, emission 500?550 nm, while DRAQ5, was excited by a 633 nm red helium-neon laser, emission>650 nm.

Fig.8 Confocal fluorescence imaging for the pH PET sensors (a) 3 and (b) 4 at 1 mmol?L^?¹, within fixed HeLa cells, which were treated for 24 h with the indicated compounds. After the required incubation period, cells were fixed in 3% paraformaldehyde and stained with the nuclear stain DRAQ5. Samples were rinsed and imaged with buffers at the indicated pH. 4 and 5 were excited by a 405 nm argon laser, emission 500?550 nm, while DRAQ5, was excited by a 633 nm red helium-neon laser, emission>650 nm.

Fig.9 Confocal fluorescence imaging of (a) 1 and (b) 2 at 1 mmol?L^?¹ in HeLa cells with fluorescently labelled lysosomes and mitochondria stains. These were treated for 24 h with the indicated compounds, washed twice with fresh media and analysed by live confocal microscopy. For images (a), (b) cells were counterstained with DAPI. DAPI and the compounds were excited by a 405 nm argon laser, with emission 410?450 nm (DAPI) and 500?550 nm (compounds), DsRed and RFP tags were excited by a 633 nm red helium-neon laser, emission>650 nm. For emission spectra, shown in (c) and (d) for 1 and 2 in the two media, were activated by a 405 nm laser and any emission from 420 to 685 nm was recorded using a Leica SP8 confocal microscope.

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