Interaction of H2 @C60 and nitroxide through conformationally constrained peptide bridges.

We synthesized two molecular systems, in which an endofullerene C-60, incarcerating one hydrogen molecule (H-2@C-60) and a nitroxide radical are connected by a folded 3(10)-helical peptide. The difference between the two molecules is the direction of the peptide orientation. The nuclear spin relaxation rates and the para -> ortho conversion rate of the incarcerated hydrogen molecule were determined by H-1 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 the H-2@C-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 -> 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(6) dependence.