Relaxivity Studies on Dinitroxide and Polynitroxyl Functionalized Dendrimers: Effect of Electron Exchange and Structure on Paramagnetic Relaxation Enhancement Ashok J. Maliakal, ² Nicholas J. Turro,* ,² Anton W. Bosman, ‡ Jeroen Cornel, ‡ and E. W. Meijer ‡ Department of Chemistry, Columbia UniVersity, 3000 Broadway, New York, New York 10027, and Laboratory of Macromolecular and Organic Chemistry, EindhoVen UniVersity of Technology, PO Box 513, NL-5600 MB, EindhoVen, The Netherlands ReceiVed: April 21, 2003; In Final Form: August 5, 2003 The 1 H NMR relaxivity of o- and p-dinitroxide-substituted phthalate esters and a series of nitroxyl-functionalized poly(propylene imine) dendrimers has been measured in acetonitrile and methanol. Studies of dinitroxide relaxivity indicate that the electron exchange rate has only a small effect on relaxivity. Outer-sphere relaxivity has been measured using benzene as a probe molecule. In studies on dendritic polynitroxides, the per-nitroxide- based outer-sphere relaxivity nearly doubles for the generation 5 nitroxyl-functionalized dendrimer as compared to a mononitroxide model. This relaxivity enhancement may be due to crowding of dendrimer surface groups in higher generation dendrimers. Water relaxivity has been measured for these polynitroxides as well, and a significant inner-sphere contribution to relaxivity is found. Dendritic polynitroxides exhibit higher per-nitroxide- based water relaxivity as compared to a mononitroxide model. This relaxivity enhancement is attributed to an increase in rotational correlation time (τ c ) for the dendritic polynitroxides. Introduction Paramagnetic relaxation enhancement is of fundamental importance to the field of magnetic resonance imaging (MRI) 1-4 andNMR spectroscopy. 5 Typically, the magnitude of paramag- netic relaxation enhancement is proportional to the concentra- tion of paramagnetic species. The proportionality constant has been defined as the relaxivity of the species and is reported in the literature in units of M -1 s -1 or mM -1 s -1 . 1-4 The most commonly used paramagnetic relaxation enhancers (PREs) employed for MRI applications have been gadolinium chelates which exhibit high relaxivities due to the high spin ( 7 / 2 ) of this metal center. 1,2 For years, the use of stable nitroxide free radicals has been evaluated as a potential alternative to gadolinium. 6,7 Stable nitroxides have been shown to be more amenable to in vivo use than gadolinium complexes 7 and may not require the same extensive chelation protocols required to minimize the toxicity of free gadolinium. However, the intrinsically low paramagnetic relaxivity of nitroxides (in part due to their low spin of 1 / 2 ) has prevented their widespread application as MRI contrast agents. 8 Attempts have been made to enhance the relaxivity of nitroxides by using nitroxides which have functional groups capable of binding proteins. 8 Such protein binding could enhance the inner-sphere contribution to relaxivity and thus improve the profile of nitroxides as relaxation enhancers. Dendrimer scaffolds have been used to enhance paramagnetic relaxation both in the case of gadolinium-based PREs 9,10 and nitroxide-labeled systems. 11-13 In the case of gadolinium-based dendritic relaxation enhancers, the dendrimer is used to modify the rotational correlation time of the system and thereby enhance inner-sphere relaxivities. 9,10 In the case of nitroxide-based dendrimers, relaxivity enhancements in aqueous solution are not observed. 13 However, aggregation effects 14 in aqueous solution could be complicating the intrepretation of relaxivity measure- ments made in aqueous solution. The dendrimer framework has been shown to be advantageous in decreasing the bioreduction rate of nitroxide in vivo, an extremely rapid process which destroys the relaxation enhancer. 12 The potential of dendritic PREs in targeting specific organs has been investigated. 10,13,15 Our lab has studied stable nitroxide radicals for several years as ESR probes for surfaces and supramolecular assemblies. 16 In the course of our studies we chose to investigate dinitroxides 1a and 1b illustrated in Scheme 1. 17,18 ESR studies 18 demon- strated that 1b exhibits fast intramolecular electron exchange on the ESR time scale. 19 Furthermore, polarization transfer studies indicated that interactions between polynitroxides and photoexcited triplets occurred in the strong exchange limit [by strong exchange, it is meant that on average the exchange integral (J) is much greater than the hyperfine interaction (a)]. 18,20,21 Thus, the polarization was transferred from the photoexcited triplet to the entire polynitroxide array, rather than to one particular nitroxide unit. These results suggested that the polynitroxides might be functioning as high-spin species. 20,22-24 Since the theory for paramagnetic relaxation 25 predicts relaxivity to be proportional to S(S + 1) (see eqs 1-4 and 6-8 below), where S is the spin on the paramagnetic species, we decided to investigate the relaxivity of a series of small molecule (see Scheme 1) and dendrimeric polynitroxides (see Scheme 2) as a function of the number of nitroxides per molecule, structure, and the rate of electronic exchange in these species. We used benzene as a probe molecule for outer-sphere relax- ivity, since it does not form complexes with nitroxides. MRI applications depend on relaxation enhancers which exhibit high water relaxivity. 1-4 However, poly(propylene imine) dendrimers modified with nonpolar organic groups such as nitroxide ² Columbia University. ‡ Eindhoven University of Technology. 8467 J. Phys. Chem. A 2003, 107, 8467-8475 10.1021/jp0350666 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/12/2003