Abstract
| - Since the recent availability of high sensitivity field-cycling relaxometers, it has become possibleto measure the protein proton relaxation in millimolar protein solutions as a function of magnetic field. Inprinciple, this provides direct access to the so-called spectral density function of protein protons and, hence,to a full set of dynamic parameters. Understanding the dynamic behavior of biological molecules isincreasingly appreciated as crucial to understanding their function. However, theoretical tools to analyzethe collective relaxation behavior of protons in solute macromolecules over a wide range of magnetic fieldsare lacking. A complete relaxation matrix analysis of such behavior is described here. This analysis providesexcellent predictions of the experimental proton magnetization decays/recoveriesmeasured to anunprecedented level of accuracy by a last-generation fast field-cycling relaxometerof two different globularproteins, hen egg white lysozyme and human serum albumin. The new experimentally validated theoreticalmodel is then used to extract dynamic information on these systems. A “collective” order parameter SC2,different from, but complementary to, that commonly extracted from heteronuclear relaxation measurementsat high field, is defined and measured. An accurate estimate of the rotational correlation time is obtained: in the case of lysozyme it agrees very well with theoretical predictions; in the case of serum albumin itprovides evidence for aggregation at millimolar concentration.
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