# A Bright Spiropyran-Based Zinc Sensor for Live-Cell Imaging

## TL;DR

This paper introduces a new, brighter zinc sensor called SpiroZin2-COOH for studying zinc ions in living cells, which could help understand neurodegenerative diseases.

## Contribution

The paper presents SpiroZin2-COOH, a novel spiropyran-based zinc sensor with significantly improved brightness and a red-shifted emission.

## Key findings

- SpiroZin2-COOH has a quantum yield seven times higher than its parent sensor.
- The sensor shows a 6-fold higher brightness and a 30 nm red-shift in emission.
- It achieves the highest turn-on ratio (14.6) in live-cell studies among the SpiroZin family.

## Abstract

Pools of labile bound
zinc ions are essential for signal
transduction
in the human body. At the cellular level, such pools occur in the
cytosol, discrete organelles, and secretory vesicles. These zinc-containing
vesicles are found in distinct regions of the central nervous system,
modulating calcium ion channels that play an essential role in olfaction,
audition, and somatosensory perception. Dysregulation of these receptors
is associated with a number of neurodegenerative diseases. To understand
the underlying mechanisms at the molecular level, zinc fluorescence
sensors are versatile tools. In this report, a new member of the spiropyran-based
sensor family SpiroZin, which has proven useful for the investigation
of zinc in living cells, is presented: SpiroZin2-COOH. This sensor
can be synthesized in a 5-step synthesis and shows superior zinc-sensing
properties in cuvette as well as live cell studies. The quantum yield
is approximately seven times higher than that of the parent zinc sensor,
which also results in an approximately 6-fold higher brightness and
a turn-on of 30 at pH 7 in cuvette studies. Another advantage is a
significant red-shift of 30 nm in comparison to the parent sensor
SpiroZin2. Other basic properties of the SpiroZin family are retained,
as revealed by a similar binding constant and negligible pH dependence
in zinc sensing. Similar to other members of the SpiroZin family,
SpiroZin2-COOH images intracellular zinc pools in living cells. Lysotracker
costaining reveals lysosomal localization of SpiroZin2-COOH. The turn-on
is determined to be 14.6, which is the highest turn-on within the
SpiroZin family reported so far in live-cell studies.