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Figure 3
Molecular packing of the NCS dimers in the TK crystals (a) in the ac plane and (b) in the ab plane. For each NCS dimer, the Cα skeleton of one molecule is drawn in bold and that of the second molecule is drawn in thin lines. The TK NCS dimers form tightly packed layers within the ac plane. A second layer of dimers is generated by the 21 component along the b axis. Within one layer of TK molecules in the ac plane there are three major contact areas: (i) between the same β-strand βC1 of the N-terminal lobe of two adjacent molecules, forming an antiparallel β-sheet-like arrangement, (ii) two helix/helix packing zones, involving always the packing of helix αC1 from one molecule and helix αC7 from a neighbouring molecule. In contrast, the sparse contacts perpendicular to these molecular layers are exclusively formed by residues in loop regions. The number of strong specific packing interactions (hydrogen bonds, salt bridges) between adjacent molecules has been estimated using CONTACT (Collaborative Computational Project, Number 4, 1994BB12). There are 18 specific contacts per TK molecule within the ac layer, but only three specific interlayer contacts. The ratio of the con­tacts in different orientations demon­strates the dominance of packing interactions in the ac plane.

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