  This paper presents a detailed analysis of the pseudocontact shift (PCS)
field induced by a mobile spin label that is viewed as a probability density
distribution with an associated effective magnetic susceptibility anisotropy.
It is demonstrated that non-spherically-symmetric density can lead to
significant deviations from the commonly used point dipole approximation for
PCS. Analytical and numerical solutions are presented for the general partial
differential equation that describes the non-point case. It is also
demonstrated that it is possible, with some reasonable approximations, to
reconstruct paramagnetic centre probability distributions from the experimental
PCS data.
