Efficient Two-Photon Fluorescent Probe for Imaging of Nitric Oxide during Endoplasmic Reticulum Stress

Nitric oxide (NO) is a vital gaseous signal molecule and plays an important role in diverse physiological and pathological processes including regulation of vascular functions. Endoplasmic reticulum (ER) stress is caused by the accumulation of misfolded or unfolded protein in the ER. Besides, ER stress induced by NO can be involved in the pathogenesis of various vascular diseases. Unfortunately, to the best of our knowledge, no ER-targeting probe for NO is reported to study the relationship between ER stress and the level of NO in a biological system. Herein, an ER-targeted fluorescent probe named ER-Nap-NO for imaging of NO is designed and synthesized. ER-Nap-NO consists of three main parts: naphthalimide (two-photon fluorophore), o-phenylenediamino (NO recognition group), and methyl sulfonamide (ER-targetable group). The probe itself is nonfluorescent because a photoinduced electron transfer (PET) process exists. After the addition of NO, the PET process is inhibited and thus strong fluorescence is released. Moreover, the response mechanism is confirmed by ¹H NMR and mass spectra and DFT calculation in detail. In addition, from the experimental results, we can conclude that the probe displays several obvious advantages including high sensitivity, selectivity, and ER-targetable ability. Based on these excellent properties, the probe is used for the two-photon imaging of exogenous and endogenous NO in ER of living cells. Most importantly, the ER-targetable probe has potential capability as a tool for investigating the level of NO during tunicamycin-induced ER stress in cells and tissues, which is beneficial for revealing the role of NO in ER-associated vascular diseases.