13C Magnetic Relaxation Study of Hindered Motion of Arginine in Clupeine

13C NMR parameters such as chemical shift, spin–lattice relaxation time (T1), and nuclear Overhauser enhancement (NOE) were measured for the arginine residue of clupeine in aqueous solution. The correlation time for molecular reorientation of the backbone and side-chain carbons were estimated from T1 and NOE. The correlation time for the α-carbon in the backbone was 0.57 ns at 300 K, indicating rapid segmental motion and that the clupeine molecule is thus flexible. The side chain carbons are more mobile due to the presence of internal motion than the backbone carbon. The internal motion of the side chain of arginine was found to be more restricted compared with that of lysine. This is interpreted in terms of the presence of the bulky guanidino group.