para-Hydrogenation of unsaturated moieties on poly(lysine) derived substrates for the development of novel hyperpolarized MRI contrast agents

Four alkyne-functionalized poly(lysine) derivatives have been synthesized and characterized by H-1 and C-13 NMR spectroscopy. In the first poly(lysine) derivative, phenylpropiolate moieties are directly bound to the aminic arms, whereas in the other derivatives, propargylamine moieties are bound to the aminic poly(lysine) arms through glucaric, acid and diethylene glycol (DG) chains, respectively. para-Hydrogenation of the alkyne-functionalized poly(lysine) compounds has been investigated and the results have been discussed in terms of spin lattice relaxation properties of the hydrogenated products. It is shown that the longer the mobile chain separating the unsaturation from the poly(lysine) backbone, the more intense the polarized signals when para-hydrogenation is carried out. This is due to (a) the maintenance of short reorientational times on the unsaturated ends, and therefore a sufficiently long T-1 of the protons added during hydrogenation, and (b) the minor effect of steric hindrance by the poly(lysine) backbone that decreases interaction of the unsaturation with the catalyst, allowing higher hydrogenation rates.