Ligands packing

Besides the packings mentioned above, a range of other stationary phases exists as well, bnt they are rarely used in pharmaceutical analysis. Even with this more limited selection of ligands, packings with quite different selectivities can be created, as in shown in Section III. F. The diversity stems from the choices in the properties of the silica, such as pore volume, pore size, surface area and purity as well as differences in ligand density. Nevertheless, general statements about different phases can be derived, and these are covered in the next section. 

Despite the experimental results and theoretical calculations reported so far, a conclusive description of the ligand packing for homoligand nanoparticles cannot be drawn. The present idea is that the real structure of the ligand shell results from the interplay between van der Waals interactions and the tendency of the thiolates to reach the optimal tilt angle on each facet, as discussed by Stellacci and coworkers on the basis of STM analyses.214,215... 

This formula volume additivity is simply a corollary of the observation, first documented extensively by Zachariason ), that fluoride molecular volumes are primarily determined by the fluorine ligand packing volume, which is usually 18 1 A per atom. 

Alpha helices D and E from the L and M subunits (Figure 12.14) form the core of the membrane-spanning part of the complex. These four helices are tightly packed against each other in a way quite similar to the four-helix bundle motif in water-soluble proteins. Each of these four helices provides a histidine side chain as ligand to the Ee atom, which is located between the helices close to the cytoplasm. The role of the Ee atom is probably to... 

Water [579] is present in the structure of true crystalline hydrates [580] either as ligands co-ordinated with the cation (e.g. [Cu(OH2)4]2+ in CuS04 5 H20) or accommodated outside this co-ordination sphere within voids left in anion packing, further stabilized by hydrogen bonding (e.g. the remaining water molecule in CuS04 5 H20). 

Fig. 13.—Packing arrangement of extended, 6-fold, KOH-amylose (11) helices, (a) Stereo view of two unit cells approximately normal to the ftc-plane. The helix (filled bonds) at the center is antiparallel to the two helices (open bonds) at the comers in the back. Potassium ions (crossed circles) have water molecules (open circles) and hydroxyl groups from amylose helices as ligands.

---