Manganese(III) porphyrins complexed with P22 virus-like particles as T1-enhanced contrast agents for magnetic resonance imaging

Virus-like particles are powerful platforms for the development of functional hybrid materials. Here, we have grown a cross-linked polymer (cross-linked aminoethyl methacrylate) within the confines of the bacteriophage P22 capsid (P22–xAEMA) and functionalized the polymer with various loadings of paramagnetic manganese(III) protoporphyrin IX (MnPP) complexes for evaluation as a macromolecular magnetic resonance imaging contrast agent. The resulting construct (P22–xAEMA–MnPP) has r1,particle = 7,098 mM−1 s−1 at 298 K and 2.1 T (90 MHz) for a loading of 3,646 MnPP molecules per capsid. The Solomon–Bloembergen–Morgan theory for paramagnetic relaxivity predicts conjugating MnPP to P22, a supramolecular structure, would result in an enhancement in ionic relaxivity; however, all loadings experienced low ionic relaxivities, r1,ionic, ranging from 1.45 to 3.66 mM−1 s−1, similar to the ionic relaxivity of free MnPP. We hypothesize that intermolecular interactions between neighboring MnPP molecules block access of water to the metal site, resulting in low r1,ionic relaxivities. We investigated the effect of MnPP interactions on relaxivity further by either blocking or exposing water binding sites on MnPP. On the basis of these results, future design strategies for enhanced r1,ionic relaxivity are suggested. The measured r2,ionic relaxivities demonstrated an inverse relationship between loading and relaxivity. This results in a loading-dependent r2/r1 behavior of these materials indicating synthetic control over the relaxivity properties, making them interesting alternatives to current magnetic resonance imaging contrast agents.