Thyroactive crystal structures

This apparent dilema was resolved when further crystallographic data on T3 compounds became available (11,12,13,14,15). (See reference 11 for a detailed list of thyroactive crystal structure studies.) These determinations show the 3 -I in the distal conformation and further showed that either the distal or proximal conformer could be isolated by changing the crystallizing media (13,14). These data prompted further NMR and MQ studies which showed a nearly equal distal/proximal ratio in solution (16) and a much smaller barrier to internal rotation (17). Thus these data have shown that the distal and proximal conformers are readily accessible in solution and that the energy barrier to free rotation is small enough so that the stereospecific conformer required by a receptor is easily accessible. 

Figure 6. Plot of the torsion angle (C5-C4-041-C1 ) and (C4-041-C1 -C6) for those thyroactive crystal structures listed in Ref. 11. Diphenylether conformation for all thyroid hormones.

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