Effect of Nitroxides and Molecular Oxygen on Proton Relaxation Rates in Aqueous Systems and in Lipids
Bennett, Harold Frank
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https://hdl.handle.net/2142/70914
Description
Title
Effect of Nitroxides and Molecular Oxygen on Proton Relaxation Rates in Aqueous Systems and in Lipids
Author(s)
Bennett, Harold Frank
Issue Date
1988
Doctoral Committee Chair(s)
Swartz, Harold M.
Department of Study
Nuclear Engineering
Discipline
Nuclear Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biophysics, Medical
Abstract
A fundamental investigation of the potential of nitroxides and O$\sb2$ for enhancement of magnetic resonance imaging (MRI) contrast in vivo is presented. The effect of these species on proton relaxation in both aqueous systems and in lipids is studied. The basic experimental approach for this study is the measurement of proton longitudinal relaxation rates (1/T$\sb1$) over a broad range of magnetic field strengths (corresponding to proton Larmor frequencies of 0.01 to 50 MHz), as a function of temperature. The resulting data, nuclear magnetic relaxation dispersion (NMRD) profiles, provide insights into mechanisms of relaxation as well as empirical values that relate to a variety of clinical and experimental NMR instruments. Electron spin resonance data are also utilized to characterize sample containing nitroxides.
Analysis of the NMRD profiles for two aqueous nitroxide systems--protein-free nitroxides in water, and aqueous solutions of nitroxides bound to albumin--indicates that protein-bound nitroxides in aqueous solutions enhance water proton relaxation primarily via inner sphere processes (ostensibly due to hydrogen bonding between the nitroxide moiety and a water molecule). The contribution of these inner sphere processes are negligible for rapidly tumbling nitroxides free in solution, which affect solvent proton relaxation almost entirely through outer sphere processes.
Typical water soluble nitroxides do not interact appreciably with blood, plasma, or plasma proteins. Fatty acid nitroxides do interact physically with blood, predominantly by intercalation within red blood cell membranes and binding to albumin. The latter interaction results in significantly enhanced relaxivity for the nitroxide/HSA complex.
In a study of proton relaxation in lipids, triolein is shown to be a reasonable model for lipid tissues. Both O$\sb2$ and nitroxides affect lipid proton relaxation predominantly through outer sphere processes, and both enhance the relaxation of lipid protons to a much greater degree than water protons--nitroxides becuse of their slower diffusion in the more viscous environment of lipids and oxygen primarily because of a greater solubility in lipids. The high relaxivity of nitroxides in lipids, combined with high lipid solubility possible with nitroxides, should lead to significant contrast in vivo in lipid environments.
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