Isolated complexes of the amino acid arginine with polyether and polyamine macrocycles, the role of proton transfer

moiety in cyclen. Natural bond orbital analysis is employed to characterize the intermolecular H-bonds responsible for the stability of both complexes. Protonated arginine interacts with 12-crown-4 through the guanidinium side group, in a conformation that resembles the one expected for crown-Arg binding in peptidic chains. In contrast, the cyclen complex involves the coordination of the carboxylate anionic group with a N-H bond of the protonated amine group cyclen, and plausible but less relevant interactions with the guanidinium group.