sup 13 C NMR relaxation times of hepatic glycogen in vitro and in vivo

The field dependence of relaxation times of the C-1 carbon of glycogen was studied in vitro by natural-abundance {sup 13}C NMR. T{sub 1} is strongly field dependent, while T{sub 2} does not change significantly with magnetic field. T{sub 1} and T{sub 2} were also measured for rat hepatic glycogen enriched with (1-{sup 13}C)glucose in vivo at 4.7 T, and similar relaxation times were observed as those obtained in vitro at the same field. The in vitro values of T{sub 1} were 65 {plus minus} 5 ms at 2.1 T, 142 {plus minus} 10 ms at 4.7 T, and 300 {plus minus} 10 ms at 8.4 T, while T{sub 2} values were 6.7 {plus minus} 1 ms at 2.1 T, 9.4 {plus minus} 1 ms at 4.7 T, and 9.5 {plus minus} 1 ms at 8.4 T. Calculations based on the rigid-rotor nearest-neighbor model give qualitatively good agreement with the T{sub 1} field dependence with a best-fit correlation time of 6.4 {times} 10{sup {minus}9} s, which is significantly smaller than {tau}{sub M}, the estimated overall correlation time for the glycogen molecule (ca. 10{sup {minus}5} s). A more accurate fit of T{sub 1} data using a modified Lipari and Szabo approach indicates that internal fast motions dominate the T{sub 1} relaxation in glycogen. On the other hand, the T{sub 2} relaxation is dominated by the overall correlation time {tau}{sub M} while the internal motions are almost but not completely unrestricted.