A computer program was developed for studying transferred nuclear Overhauser effects in complex spin systems. It permits quantitative analysis of nuclear Overhauser effects observed in biologically important systems, such as ligands interacting with transmembrane receptors in the presence of lipid bilayers. The full generalized relaxation matrix approach takes into account the local mobility, spin equivalence, finite exchange rates, and spectral overlap. The program can be used either to simulate theoretical nuclear Overhauser effect buildup curves or to fit a relaxation matrix of a given model to experimental data. Selected examples illustrate the program's performance.
Changes of the poly(L-lactide-co-glycolide) structure as a function of degradation time in phosphate-buffered saline for 7 weeks were investigated by gel permeation chromatography, differential scanning calorimetry, nuclear magnetic resonance (^1H NMR), and positron annihilation lifetime spectroscopy. Surface properties as wettability by sessile drop and topography by atomic force microscopy were also characterized. Chain-scission of polyester bonds in hydrolysis reaction causes a quite uniform decrease in molecular weight, and finally results in an increase in semicrystallinity. Molecular composition of the copolymer and water contact angle do not change considerably during degradation time. Atomic force microscopy studies suggest that the copolymer degrades by "in bulk" mechanism. The average size of the molecular-level free volume holes declines considerably after one week of degradation and remains constant till the sixth week of degradation. The free volume fraction decreases as a function of degradation time.
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