13C-NMR relaxation in glycogen

This study is the first report on the multiexponential T₂ relaxation of the ¹³C-1 carbon of glycogen. In contrast to T₁ relaxation, which does not display observable multiexponential decay behavior, T₂ relaxation is described by a continuous distribution of T₂ times. Changes in molecular weight and sample viscosity, which affect the overall mobility of the glycogen particle have little influence on T₁ and T₂ relaxation times. This is in contradiction with earlier results that T₂ is dominated by the overall motion of the glycogen particles [L.-H. Zang Biochemistry 29, 6815–6820 (1990)]. T₁ depends strongly on the external field B_o and is almost temperature independent in the range 23–37°C whereas T₂ is field independent and varies appreciably with temperature. The experimental T₁ and T₂ relaxation data are shown to be consistent with existing theoretical models for relaxation, suitably modified to include a distribution of correlation times for the internal motions. The presence of fast decaying components (short T₂) in the FID implies broad line components in the frequency spectrum and the corresponding need to appropriately set the integration limits for the quantification of the glycogen peak.