No abstract

Magnetic relaxation has been used extensively to study and characterize biological tissues. In particular, spin-lattice relaxation in the rotating frame (T 1) of water in protein solutions has been demonstrated to be sensitive to macromolecular weight and composition. However, the nature of the contribution from low frequency processes to water relaxation remains unclear. We have examined this problem by studying the water T1 dispersion in peptide solutions ( 14 N-and 15 N-labeled), glycosaminoglycan solutions, and samples of bovine articular cartilage before and after proteoglycan degradation. We find in model systems and tissue that hydrogen exchange from NH and OH groups to water dominates the low frequency water T 1 dispersion, in the context of the model used to interpret the relaxation data. Further, low frequency dispersion changes are correlated with loss of proteoglycan from the extra-cellular matrix of articular cartilage. This finding has significance for the noninvasive detection of matrix degradation.rotating frame ͉ T1 ͉ extra-cellular matrix ͉ osteoarthritis P rotein degradation with a loss of proteoglycan (PG) from the extra-cellular matrix is thought to be an initiating event of early osteoarthritis (1). A noninvasive imaging method that can monitor the progression of the disease would be highly desirable for the longitudinal evaluation of disease progression and the utility of therapeutic interventions. Because of the excellent soft tissue contrast and its noninvasive nature, MRI is an attractive modality for imaging cartilage. Unfortunately, currently available conventional MRI methods are unable to detect the earliest stages of the disease when biochemical changes occur without gross tissue damage (2). Recently, several MRI methods have been proposed to detect PG loss from cartilage (3, 4). In particular, spin-lattice relaxation in the rotating frame (T 1 ) has been demonstrated to be elevated in PG-depleted cartilage (5).T 1 relaxation is sensitive to molecular motions that have correlation times () such that SL ϳ1, where SL ϭ ␥B SL is the strength of the spin-lock field (6). T 1 increases with the strength of the spin-lock field, a phenomenon termed dispersion. T 1 measurements can therefore provide information about the biophysical mechanisms underlying magnetic relaxation. It has been demonstrated that water T 1 relaxation and dispersion (in the 0.1-10 kHz regime) are sensitive to macromolecule-water interactions in protein solutions and possibly also in biological tissues (7-9). Low frequency (0.1-3 kHz) T 1 dispersion has been observed in several systems such as protein solutions (7), bovine articular cartilage (5), human patellar cartilage (10), rodent brain (11), and murine tumor tissue (9). However, the exact nature of T 1 dispersion in biological tissues remains unclear. The range of spin-lock strengths that can be used for in vivo measurements is 0.1-3 kHz (depending on the duration of the spin-lock pulse), without exceeding power deposition limits. Therefore, we have focused our i...

Native-state hydrogen exchange experiments under EX1 conditions can distinguish partially unfolded intermediates by their formation rates and identify the amide hydrogens exposed and protected in each. Results obtained define a cytochrome c intermediate seen only poorly before and place it early on the major unfolding pathway. Four distinct unfolding steps are found to be kinetically ordered in the same pathway sequence inferred before.