The C(19) position of 25-hydroxyvitamin D(3) faces outward in the vitamin D sterol-binding pocket of vitamin D-binding protein.

The radiolabeled affinity and photoaffinity analogues of 25-hydroxyvitamin D(3) (25-OH-D(3)) with probes at the C-19 position failed to specifically label the 25-OH-D(3)-binding pocket of vitamin D-binding protein (DBP). However, a hybrid analogue, with a bromoacetate affinity probe and a photoaffinity probe at C(3)-OH and C(19) positions, respectively, specifically labeled the ligand-binding pocket, suggesting that C(3)-OH points towards the 'inside' of the binding cavity while the C(19) position faces away from it.