Spin-labeled dendrimer

The results that we obtained from EPR characterization of spin-labeled PAMAMs indicate that, to have end groups on a PAMAM dendrimer reside permanently in proximity, covalent linkage between the groups is required. Strategies to synthesize such PAMAMs are currently underway. 

Maliakal et aV have studied the proton relaxivity produced by dinitroxide spin labels attached to poly(propylene imine) dendrimers as potential MRI contrast agents. They find that proton relaxivity, which is substantially higher than for mononitroxide spin labels, is not seriously affected by electron selfexchange. 

Paramagnetic centers can be directly attached to the site of interest via covalent bonds. Such methodology is particularly well developed for proteins, where the amino acid residue of interest can be converted to a cysteine residue by site-directed mutagenesis, and a thiol-reactive spin label can be attached under mild conditions. If a peptide or protein is synthesized, the nonnative spin-labeled amino acid TOAC can be incorporated directly into the chain.In other synthetic systems, nitroxides can be conveniently attached via ether, ester, or amide bonds, an approach that is also viable for labeling ceramic and metal oxide surfaces. The complexity of supramolecular systems that can be investigated by this approach is exemplified by a study on the interaction between spin-labeled starburst dendrimers and DNA. ... 

As shown in this chapter, ESR provides a very useful technique for the study of dendrimer properties and their local environments, not only when paramagnetic species are directly involved in the dendrimer original structure, but also when paramagnetic units are added as spin probes or spin labels to the dendrimers. The advantage of the ESR technique with respect to other spectroscopic methods of analysis mainly resides in the ability to select the proper spin probe or spin label able to monitor, like a special camera, a selected area internally or externally to the dendrimer. Of course, the basic request is to know if and how the probes or the labels modily the dendrimer properties. 

It is of interest to describe spin systems that are completely different from nitrox-ide-labeled dendrimers, but that also bear in their organic structure a high-spin multiplicity (with S > V) High-spin polycarbenes with ji-conjugation, which are the first and second generations of the spin-mediated dendrimers based on pheny-lacetylenes, were studied by a two-dimensional electron-spin transient nutation (2D-ESTN) method based on pulsed-ESR to determine the spin mulliphcity. The contour plots of the 2D-ESTN spectra of the dendrimer at 3.5 K allowed to identify the nutation frequencies, which were attributed to 4, 4> 4, 3>, 4, 3> <-> 4, 2>, 4, 2> 4, 1>, and 4, 1> 4, 0>. The ESR allowed the detec-... 

The spin labeling of G6-PAMAM dendirmers with TEMPO radicals was used to follow the redox reaction of TEMPOL and TEMPOL-H. The intensity of the ESR signal of TEMPOL, formed by reoxidation of TEMPOL-H with spin-labeled PAMAM dendrimers, increased as a function of time, after addition of the dendrimer at different labeling degrees, demonstrating that spin-labeled G6 dendrimers are potential candidates as contrast agents in MRI. 

The ESR studies of labeled dendrimers intended to clarify their role as MRI contrast agents have been described in several papers. Spin labeling of several different dendrimers (DAB and PAMAM, at different generations) let the authors calculate the rates of nitroxide reduction due to superoxide, by observing the change of the low-field spectral peaks in the ESR spectra as a function of time (Table 3). Dendrimers labeled with nitronylnitroxides work well as contrast agents in MRI, and therefore their characterization by means of ESR is of interest. ... 

An interesting medical application of the nitronylnitroxide-labeled dendrimers is spin trapping of nitric oxide. Nitronyl nitroxides can spin trap NO, but are unstable... 

The ESR studies performed on the dendrimers have demonstrated the utility of this technique to characterize their structure and properties and to clarify their behavior in specific applications. This chapter presented ESR studies describing the use of radical spin probes, spin labels, paramagnetic ions as spin probes, and met-allo-dendrimers with paramagnetic ions in their structure. 

Coordination causes electron-spin density redistribution in the N-O fragment the contribution of resonance structure II increase. The redistribution of spin density results in changes in the parallel component of the nitrogen hyperfine tensor. TEMPO and anthraquinone (AQ) have been used in this way to probe the Lewis acidity of alumina and Li and Mg doped alumina matrices.176 The differences in the Lewis acidic strength towards TEMPO and anthraquinone are discussed. An interesting study has appeared aimed to study the guest-host interaction between poly(amidoamine) dendrimers labelled with nitroxides and several porous solids including alumina.177... 

A variety of spin probe methods have also been used to study the morphological features of the nano-channels present within MCM 41, as well as dynamical aspects connected to molecular diffusion in the inner pores,186-188 EPR has been used to investigate the adsorption and interactions of nitroxide-labelled de-ndrimers within porous silica.181 This method allows one to investigate the effective porosity of a solid surface (as a host) which is determined by the accessibility of the host surface to an adsorbed guest molecule. Information on the adsorption and interaction of dendrimers with the porous surface arises from computer-aided analysis of the EPR spectra based on of the well-established procedure proposed by Schneider and Freed.189... 

---