Absorption, emission, and chiroptical spectra of neurokinin 1 tachykinin receptor antagonists: the role of charge-transfer states on the biological activity.

A spectroscopic investigation, based on both electronic absorption and emission spectra as well as on chiroptical data, was performed on novel neurokinin 1 (NK1) tachykinin receptor antagonists, exhibiting interesting biological activity. These pseudopeptides have two fluorophores, i.e. indole (I) and naphthalene (N), and a central scaffold with different conformational mobility. Absorption spectra in methanol show the presence of a new band with respect to the sum spectrum of the isolated chromophores at around 285 nm, the intensity of which linearly increases as the bioactivity increases. This absorption disappears by using dioxane as solvent. It is ascribed to an intramolecular I-N charge-transfer (CT) complex that forms to different extent, depending on the flexibility of the scaffold. Under this condition, the molecules fold and apparently attain the correct conformation for competing substance P binding to the NK1 receptor, lending plausibility to the role of dipolar charged, spatially close aromatic moieties as topochemical elements in the mechanism of action of substance P antagonists. The excited-behavior parallels that in the ground state, as the quenching of the singlet state at 340 nm is found to be linearly dependent on the biological activity, too. Upon decreasing solvent polarity (methanol vs dioxane) the emissions of the dipolar state at around 370 nm disappears, while exciplex emission in the range of 400-500 nm occurs. This transition from charge-separated to exciplex-like states by lowering the dielectric constant of the medium very likely reflects a change in the structural features of the intramolecular I-N stacked complex, from a twisted or an asymmetrically overlapped conformation of the indolyl and naphthyl rings to a face-to-face geometry. Implications of the rigidity of the molecules, arising from the formation of the intramolecular CT complex, on the ellipticity are briefly discussed.