Lanthanides may substitute calcium in calcium-binding proteins, such as, for instance, EF-hand proteins. Paramagnetic lanthanides are capable of orienting the protein in high magnetic fields to an extent similar to that obtained by using orienting devices, and each lanthanide orients according to its magnetic susceptibility tensor. Here, Ce(3+), Tb(3+), Dy(3+), Ho(3+), Er(3+), Tm(3+), Yb(3+) in the C-terminal site of calbindin D(9k) have been investigated. Such systems provide (1)H-(15)N residual dipolar couplings (rdc) which can be used for solution structure determinations. Within the frame of optimizing the use of residual dipolar couplings for efficient solution structure determination, it is proposed here to use a number of lanthanides (e.g., >2) to obtain the orientations of the internuclear vectors with respect to an arbitrary reference system. This is facilitated by the independent knowledge of the magnetic susceptibility anisotropy tensor of each metal, obtained from the analysis of the pseudocontact shifts. A further module of the program PARAMAGNETIC-DYANA, called RDCDYANA-ANGLES, is developed to efficiently incorporate such rdc-derived orientations, instead of the rdc themselves, as constraints in the solution structure calculation. This strategy is absolutely general and can be extended to any other pair of dipole-dipole coupled nuclei. The effect of mobility is also assessed. In principle, information on the mobility can be obtained with a number of lanthanide ions >5, or by combining a smaller number of lanthanide ions with a few orienting devices.

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