Macrocyclic Gd3+ Complexes with Pendant Crown Ethers Designed for Binding Zwitterionic Neurotransmitters

A series of Gd³⁺ complexes exhibiting a relaxometric response to zwitterionic amino acid neurotransmitters was synthesized. The design concept involves ditopic interactions 1) between a positively charged and coordinatively unsaturated Gd³⁺ chelate and the carboxylate group of the neurotransmitters and 2) between an azacrown ether appended to the chelate and the amino group of the neurotransmitters. The chelates differ in the nature and length of the linker connecting the cyclen‐type macrocycle that binds the Ln³⁺ ion and the crown ether. The complexes are monohydrated, but they exhibit high proton relaxivities (up to 7.7 mM ^?1 s^?1 at 60 MHz, 310 K) due to slow molecular tumbling. The formation of ternary complexes with neurotransmitters was monitored by ¹H relaxometric titrations of the Gd³⁺ complexes and by luminescence measurements on the Eu³⁺ and Tb³⁺ analogues at pH 7.4. The remarkable relaxivity decrease (≈80 %) observed on neurotransmitter binding is related to the decrease in the hydration number, as evidenced by luminescence lifetime measurements on the Eu³⁺ complexes. These complexes show affinity for amino acid neurotransmitters in the millimolar range, which can be suited to imaging concentrations of synaptically released neurotransmitters. They display good selectivity over non‐amino acid neurotransmitters (acetylcholine, serotonin, and noradrenaline) and hydrogenphosphate, but selectivity over hydrogencarbonate was not achieved.