Intermolecular Hydrogen Bonds Inducing Paramagnetism for Nondestructive CEST Imaging of Nitric Oxide

Nondestructive monitoring of nitric oxide (NO) generation is crucial for understanding its biological functions and therapeutic effects in vivo, but most existing NO probes interfere with its physiological activity. Here, we propose a novel strategy of intermolecular hydrogen bonds inducing paramagnetism (HIP), which facilitates nondestructive detection of exogenous NO using chemical exchange saturation transfer (CEST) imaging in vivo. Arg-Cu (II) assembles into a copper (II)-carboxylate complex that propagates magnetic dipole-dipole interactions through intermolecular hydrogen bonds, thereby shifting the chemical exchange saturation transfer (CEST) signal of arginine to 34.0 ppm. Upon arginine metabolism and NO release, the intermolecular hydrogen bonds between Arg-Cu (II) units are cleaved. This results in an increase in electron density around Cu²⁺ (from 0.96 to 1.10), shortening of the T₁ relaxation time (from 2684.30 to 1975.70 ms), and a subsequent decrease in proton exchange rate (from 1230.89 to 410.85 s^-1). This approach facilitates nondestructive detection of exogenous NO concentrations as low as 1.02 µM in vitro and 1.24 µM in vivo. A significant correlation between CEST signals and NO-associated Ccnd2 expression further supports its NO-responsive imaging capability. By minimizing interference with physiological processes, this strategy offers a reliable platform for dynamic, nondestructive NO monitoring, advancing precision molecular imaging.