A Tryptophan-Containing Open-Chain Framework for Tuning a High Selectivity for Ca²⁺ and ¹³C NMR Observation of a Ca²⁺−Indole Interaction in Aqueous Solution

Abstract

Two molecular architectures featuring the cation-responsive tryptophan indole were designed and investigated for the development of a novel fluorescent chemosensor for Ca2+. We observed that the Trp-based open-framework chemosensor EW2 exhibits remarkable selectivity for Ca2+ over Mg2+, Ba2+, K+, Na+, and Li+ in water between pH 4.6 and 7.0 on the basis of Ca2+-induced high fluorescence enhancement of the Trp residue. A combined 13C NMR and CD spectroscopic study has demonstrated a dynamic reorientation of the indole ring due to the cation−indole interaction accompanying the Ca2+-induced dramatic fluorescence enhancement. The results suggest that the highly sensitive, metal-ion-dependent Trp indolyl C(3) chemical shifts may serve as a promising indicator for monitoring metal ion−indole noncovalent interaction in solution.