NMR relaxation times have been used to characterize molecular motion and intermolecular complexes in the aqueous phase of bovine chromaffin granules. Partially relaxed '3C and proton spectra have been obtained at 3 and 25°C. T1 measurements of five protonated carbons on epinephrine (C:, Cs, C6, CHOH and NCH3) give a correlation time of 0.15 (10 -9) s at 25°C for the catechol ring and methine carbon, while the effective correlation time for the NCH3 group is somewhat shorter due to its internal degree of rotational freedom. Resonances of protonated carbons on the soluble protein chromogranin give very similar correlation times: 0.20 (10 -9) s for the peptide a-carbon and 0.2 (10 -9) s for the methylene sidechain carbons of glutamic acid. The correlation time (rR) of ATP was not measured directly using '3C T, data due to the weakness of its spectrum, but its reorientation appears to be substantially slower than that of epinephrine or chromogranin. This conclusion is based on three observations: (1) the qualitative temperature dependence of T, for H: and Hs on the adenine ring places rR for ATP to the right of the T, minimum, or ra ~> 1.0 (10 -9) s; (2) '3C resonances of ATP have anomalously low amplitudes compared with epinephrine resonances, a fact that is readily explained only if ATP undergoes substantially slower reorientation; and (3) a comparison of the T, data of Hs in chromaffin granules and in a dilute aqueous solution, where rR for ATP can be measured directly, indicates that ra ~ 1.0 (10 -9) s at 25°C in the granules. The relaxation data are consistent with the concept of a storage complex based on electrostatic interactions between a polyion (chromogranin) and its counterions (ATP and epinephrine), in which ATP cross-links cationic sidechains of the protein.
- chemistry -- research,
- analytic chemistry
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