Inductive Effects in Ruthenium-Modified Amino Acids

Abstract

Emission spectroscopy has been used to probe and quantify electronic interactions mediated by alkyl side chains in a homologous series of amino acids. The ω-amine functions of diaminopropionic acid (DAPA), diaminobutyric acid (DABA), ornithine, and lysine were covalently labeled with a luminescent [Ru(bipy)3]2+ fragment. Protonation of the α-amine function results in a significant dependence of nonradiative excited-state decay rates on the number of methylene groups, n, in the respective side chain. Two analogous series of compounds in which the α-amino acid moiety was replaced by R−NH2 or R−NMe3+ have been studied for comparison. It was found that the emission energies of the chromophore and the pKA values of the remote amine functional groups in our complexes are both affected by inductive effects which can be described by a Hammett−Taft type relationship. We were able to show that the induction decays with 1/n, where n = the number of methylene groups separating the chromophore from the amine and ammonium functions. A qualitative description of the phenomenon is given in terms of acid base theory.