Sensing of Proteins by ICD Response of Iron(II) Clathrochelates Functionalized by Carboxyalkylsulfide Groups

Recognition of elements of protein tertiary structure is crucial for biotechnological and biomedical tasks; this makes the development of optical sensors for certain protein surface elements important. Herein, we demonstrated the ability of iron(II) clathrochelates (1-3) functionalized with mono-, di- and hexa-carboxyalkylsulfide to induce selective circular dichroism (CD) response upon binding to globular proteins. Thus, inherently CD-silent clathrochelates revealed selective inducing of CD spectra when binding to human serum albumin (HSA) (1, 2), beta-lactoglobuline (2) and bovine serum albumin (BSA) (3). Hence, functionalization of iron(II) clathrochelates with the carboxyalkylsulfide group appears to be a promising tool for the design of CD-probes sensitive to certain surface elements of proteins tertiary structure. Additionally, interaction of 1-3 with proteins was also studied by isothermal titration calorimetry, protein fluorescence quenching, electrospray ionization mass spectrometry (ESI-MS) and computer simulations. Formation of both 1:1 and 1:2 assemblies of HSA with 1-3 was evidenced by ESI-MS. A protein fluorescence quenching study suggests that 3 binds with both BSA and HSA via the sites close to Trp residues. Molecular docking calculations indicate that for both BSA and HSA, binding of 3 to Site I and to an "additional site" is more favorable energetically than binding to Site II.