Interaction of a porphyrinic cage with ethionamide: a spectroscopic and computational study

The interaction of a biologically active molecule, ethionamide (ETH), with a covalent porphyrinic cage in its open conformation, was studied by means of steady-state and time-resolved optical spectroscopies. The cage is composed of two free-base porphyrins, linked four times by ethylene glycol-based pillars through triazole units. The latter are able to bind to Ag(I) ions, resulting in the opening of the cage cavity. Both absorption and emission properties of the open cage are modified upon addition of ETH, indicating an interaction between the porphyrin array and the added organic molecule, which significantly affects the photophysical properties of the former. The nature of this interaction has been investigated by means of ultrafast transient absorption spectroscopy, NMR studies, quantum mechanical calculations and molecular dynamics simulations. The results indicate that two ETH molecules interact with two Ag(I) ions coordinated to the cage, resulting in their removal. Consequently, the cage retains only two Ag(I) ions, inducing a conformational change to a semi-closed V-shaped form, with a shorter distance between the porphyrins. Complexation of the cage with Na+ is also performed to support the proposed rationale.