Molecular mobilities in chromaffin granules. Magnetic field dependence of proton T1 relaxation times.

The magnetic field dependence of the NMR spin-lattice relaxation t ime of water protons in intact bovine chromaff in vesicles has been studied over the range 1.00--23.49 kG. The T~ relaxation time shows a dispersion at field values near 20 kG. The observed pro ton resonance arises mainly from solvent protons (~H20), but the relaxation rate, which is a weighted average over all sites with which the solvent protons rapidly exchange (i.e., NH and OH protons), is dominated by exchangeable protons in the most slowly moving soluble component . The field dependence of the T1 dispersion demonstrates the existence of a site of exchangeable protons for which Tr = 1.9 + 0.5 ns at 3°C. This site is assigned to ATP and cationic groups to which its phosphate esters are complexed, since previously measured correlation times of epinephrine and the chromogranin backbone are nearly an order of magnitude too short to explain the T~ dispersion. Quantitative estimates of the relative numbers of exchangeable protons on the different soluble components support this interpretat ion. The temperature dependence of T1 of the peak due to exchangeable protons has also been measured over a temperature range --3 to 25°C. T~ lengthens by about 30% over this range and exhib i t s 'no discontinuous behavior, as would be expected if a gel transition or structural alterations in the storage complex occurred. T~ lengthens by less than 10% in chromaff in granule pastes that have been maintained at 25°C for 24 h, indicating considerable thermal stability in the storage complex. Possible effects on the solvent T~ due to paramagnetic ions have been considered with the conclusion that they are probably negligible or of minor significance.