A Grignard-mediated route to access 4-substituted-3-E-styryl-2H-chromenes: alternate extrinsic FRET probes to explore the IIA and IIIA subdomains of serum proteins

Abstract

A facile and efficient strategy to construct 2H-chromene scaffolds, employing variably substituted spiro-4-oxochroman-3,1′-cyclopropane precursors under mild conditions, is proposed in this study. The transformation proceeds via a ring-opening reaction of cyclopropane mediated by Grignard reagents and furnishes a series of C-3 functionalized 2H-chromenes in good to excellent yields (88–96%) with a broad degree of functional group tolerance. The potential of these diversified 2H-chromenes as effective fluorescence probes is demonstrated using steady-state, time-resolved emission measurements. In-depth investigations of the interactions of this new class of molecular scaffolds with model proteins through structure-optical signalling relationship studies led to the development of them as new extrinsic fluorescence resonance energy transfer (FRET) probes that are able quantify the IIA binding site of human serum albumin (HSA) and the IIIA binding site of bovine serum albumin (BSA). Steady-state and time-resolved fluorescence data appended with circular dichroism (CD) studies reveal swift inclusion without altering the conformation of the secondary structure of the protein. Molecular docking calculations confirm the hydrophobic cavities of subdomain IIA of HSA and IIIA of BSA to be the binding sites of 2H-chromenes. The calculated average distance between the protein donor and 2H-chromenes was found to be in good agreement with the experimental outcomes.