Abstract
Magnesium ions (Mg2+) play an important role in mammalian cell function; however, relatively little is known about the mechanisms of Mg2+ regulation in disease states. An advance in this field would come from the development of selective, reversible fluorescent chemosensors, capable of repeated measurements. To this end, the rational design and fluorescence-based photophysical characterisation of two spiropyran-based chemosensors for Mg2+ are presented. The most promising analogue, chemosensor 1, exhibits 2-fold fluorescence enhancement factor and 3-fold higher binding affinity for Mg2+ (Kd 6.0 µM) over Ca2+ (Kd 18.7 µM). Incorporation of spiropyran-based sensors into optical fibre sensing platforms has been shown to yield significant signal-to-background changes with minimal sample volumes, a real advance in biological sensing that enables measurement on subcellular-scale samples. In order to demonstrate chemosensor compatibility within the light intense microenvironment of an optical fibre, photoswitching and photostability of 1 within a suspended core optical fibre (SCF) was subsequently explored, revealing reversible Mg2+ binding with improved photostability compared to the non-photoswitchable Rhodamine B fluorophore. The spiropyran-based chemosensors reported here highlight untapped opportunities for a new class of photoswitchable Mg2+ probe and present a first step in the development of a light-controlled, reversible dip-sensor for Mg2+.
Highlights
Magnesium ions (Mg2+ ) play an important role in mammalian cell function [1,2,3,4], as an enzymatic cofactor [5], regulator of cellular ion channels [6,7,8] and energy metabolism [9]
Structures of chemosensors 1 and 2, bound with both Mg2+ and Ca2+, were calculated by density functional theory based on a 2:1 binding of ring opened merocyanine isomer to metal ion
Two water molecules were included in the initial calculations to provide additional coordinating sites, based on the metal-bound crystal structure reported for a similar spiropyran [47]
Summary
Magnesium ions (Mg2+ ) play an important role in mammalian cell function [1,2,3,4], as an enzymatic cofactor [5], regulator of cellular ion channels [6,7,8] and energy metabolism [9]. Mag-fura-2 (FURAPTRA) [20,21] is one such commercially available chemosensor for Mg2+ , based on a benzofuran fluorophore scaffold and functionalised with the O-aminophenol-N,N,O-triacetic. The KMG series of chemosensors possess a bidentate, charged beta-diketone binding domain, which gives rise to excellent Mg2+ selectivity over Ca2+ (Figure 1B) [29]
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