Carbon‐13 nuclear magnetic resonance relaxation study of chymotrypsin inhibitor 2 (CI‐2)

Carbon‐13 T 1 and NOE measurements were used to study the internal motions of the single phenylalanine residue in CI‐2 and in a mutant in which an arginine residue adjacent to the Phe had been replaced with an alanine residue. Each protein was specifically enriched with [ 13 C 6 ‐ring]phenylalanine. The arginine to alanine modification resulted in a reduction of the order parameter, S 2 , from 0.39 to 0.32 for the internal motions of the aromatic ring. This is consistent with the aromatic side‐chain of Phe 69 being less restricted, as a consequence of the removal of the neighbouring bulky side chain. Wild‐type CI‐2 was also successfully enriched with [2‐ 13 C]glycine. An average order parameter of 0.60 was obtained for the α‐carbon positions of the three glycine residues. When bound to the serine protease chymotrypsin, S 2 remains unchanged within experimental error. However, the overall tumbling, derived from NMR, slows by a factor of 5, consistent with the increase in molecular weight.