Interaction of H^sub 2^@C^sub 60^ and Nitroxide through Conformationally Constrained Peptide Bridges

We synthesized two molecular systems, in which an endofullerene C^sub 60^, incarcerating one hydrogen molecule (H^sub 2^@C^sub 60^) and a nitroxide radical are connected by a folded 310-helical peptide. The difference between the two molecules is the direction of the peptide orientation. The nuclear spin relaxation rates and the para [arrow right] ortho conversion rate of the incarcerated hydrogen molecule were determined by ^sup 1^H NMR spectroscopy. The experimental results were analyzed using DFT-optimized molecular models. The relaxation rates and the conversion rates of the two peptides fall in the expected distance range. One of the two peptides is particularly rigid and thus ideal to keep theH^sub 2^@C^sub 60^/nitroxide separation, r, as large and controlled as possible, which results in particularly low relaxation and conversion rates. Despite the very similar optimized distance, however, the rates measured with the other peptide are considerably higher and thus are compatible with a shorter effective distance. The results strengthen the outcome of previous investigations that while the para [arrow right] ortho conversion rates satisfactorily obey the Wigner's theory, the nuclear spin relaxation rates are in excellent agreement with the Solomon-Bloembergen equation predicting a 1/r^sup 6^ dependence. [PUBLICATION ABSTRACT]