Distal conformation of thyroid hormones. Crystal and molecular structure of 3,5,3'-triiodo-L-thyronine methyl ester.

In the crystal structure of 3,5,3'-triiodo-L-thyronine methyl ester, the 3' -iodine is distal, i.e., away from the alanine bearing ring, and the overall conformation is cisoid, that is, the alanine moiety and the outer phenyl ring lie on the same side of the inner phenyl ring plane. This conformation, reported here, for the first time, is in contrast to the transoid conformation previously observed for thyroid hormone structures. The torsional angles between the diphenyl either linkages (theta and theta') are -108 and 33 degrees, respectively, while the C-O-C angle is 117 degrees. The value of chi1, which describes the amino acid backbone conformation, is 308 degrees. The structure crystallizes in the tetragonal space group P41 with a = 8.225 (5) and c = 28.42 (1) A. The final R index is 0.06.