Numbat:: an interactive software tool for fitting Δχ-tensors to molecular coordinates using pseudocontact shifts

Pseudocontact shift (PCS) effects induced by a paramagnetic lanthanide bound to a protein have become increasingly popular in NMR spectroscopy as they yield a complementary set of orientational and long-range structural restraints. PCS are a manifestation of the chi-tensor anisotropy, the Delta chi-tensor, which in turn can be determined from the PCS. Once the Delta chi-tensor has been determined, PCS become powerful long-range restraints for the study of protein structure and protein-ligand complexes. Here we present the newly developed package Numbat (New User-friendly Method Built for Automatic Delta chi-Tensor determination). With a Graphical User Interface (GUI) that allows a high degree of interactivity, Numbat is specifically designed for the computation of the complete set of Delta chi-tensor parameters (inclu(d)ing shape, location and orientation with respect to the protein) from a set of experimentally measured PCS and the protein structure coordinates. Use of the program for Linux and Windows operating systems is illustrated by building a model of the complex between the E. coli DNA polymerase III subunits epsilon 186 and theta using PCS.