A fluorophore's electron-deficiency does matter in designing high-performance near-infrared fluorescent probes

• Published in: Chemical science (Cambridge) Vol. 11; no. 41; pp. 1125 - 11213
• Main Authors: Zhang, Xue-Xiang, Qi, Huan, Liu, Ya-Lan, Yang, Song-Qiu, Li, Peng, Qiao, Yan, Zhang, Pei-Yu, Wen, Shu-Hao, Piao, Hai-long, Han, Ke-Li
• Format: Journal Article
• Language: English
• Published: CAMBRIDGE Royal Soc Chemistry 07.11.2020; Royal Society of Chemistry; The Royal Society of Chemistry
• Subjects: Background noise; Chemistry; Chemistry, Multidisciplinary; Electron transfer; Electrons; Fluorescent indicators; Glutathione; Hydrophilicity; Near infrared radiation; Physical Sciences; Receptors; Science & Technology; Selectivity; Xenotransplantation; CELLS; ACID; SUBSTRATE; CALCIUM; BETA-GALACTOSIDASE; OPTICAL-PROPERTIES; GLUTATHIONE-S-TRANSFERASE; TISSUES; LEVEL; EXPRESSION
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d0sc04411c

The applications of most fluorescent probes available for Glutathione S -Transferases (GSTs), including NI3 which we developed recently based on 1,8-naphthalimide ( NI ), are limited by their short emission wavelengths due to insufficient penetration. To realize imaging at a deeper depth, near-infrared (NIR) fluorescent probes are required. Here we report for the first time the designing of NIR fluorescent probes for GSTs by employing the NIR fluorophore HCy which possesses a higher brightness, hydrophilicity and electron-deficiency relative to NI . Intriguingly, with the same receptor unit, the HCy -based probe is always more reactive towards glutathione than the NI -based one, regardless of the specific chemical structure of the receptor unit. This was proved to result from the higher electron-deficiency of HCy instead of its higher hydrophilicity based on a comprehensive analysis. Further, with caging of the autofluorescence being crucial and more difficult to achieve via photoinduced electron transfer (PET) for a NIR probe, the quenching mechanism of HCy -based probes was proved to be PET for the first time with femtosecond transient absorption and theoretical calculations. Thus, HCy2 and HCy9 , which employ receptor units less reactive than the one adopted in NI3 , turned out to be the most appropriate NIR probes with high-sensitivity and little nonenzymatic background noise. They were then successfully applied to detecting GST in cells, tissues and tumor xenografts in vivo . Additionally, unlike HCy2 with a broad isoenzyme selectivity, HCy9 is specific for GSTA1-1, which is attributed to its lower reactivity and the higher effectiveness of GSTA1-1 in stabilizing the active intermediate via H-bonds based on docking simulations. An abnormal and intriguing phenomenon that the fluorophore's electron-deficiency could affect a probe's performance is now revealed for the first time.