Chapter 32. Metal cation probes

Metal Cation Probes such as calcium (CA+2) or zinc (ZN+2) receive special treatment. These metal ions tend to interact with the lone pairs of electronegative elements such as oxygen, nitrogen and sulphur. The interactions are anisotropic, and the anisotropy will be considered when the value JTYPE=110 is assigned to the Metal Cation Probe. Thus a carbonyl oxygen of the Target will behave as if it has an atom-centred charge when it interacts with a calcium Probe having JTYPE=0. On the other hand, the orientation of the oxygen's lone pairs will be considered when the calcium Probe has JTYPE=110.

By default this JTYPE value (110) may be assigned to any metal cation.

Chemical "hardness" or "softness" is another property which determines the specificity of metal cation interactions. Programme Grid divides the cations into four sub-classes in order to deal with this aspect. It may then alter the default JTYPE=110 to a new value according to the sub-class of the Probe. Here is a summary of the metal cation Probes whose symbols will be recognised in this way by the current Version of the Programme:

Thus the default value JTYPE=110 would be altered automatically to 111 if you selected the symbol MG+2 to call a magnesium Probe. Moreover, you could use the values JTYPE=111 or 112 or 113 or 114 if you were defining the Energy Variables for a new kind of Probe. For instance, you might enter these Energy Variables for a barium cation Probe:

  1.35  31.  1.50  2.  0.0  0.0  0  0  111

giving it a Van der Waals radius of 1.35 Angstrom and a charge of +2.0 The Probe would interact anisotropically with lone pairs, because any value in the range 110-114 produces this effect. Furthermore, it would discriminate in favour of oxygen ligands because JTYPE=111 instead of the default value 110.

Note that: JTYPE=111 favours oxygen ligands. JTYPE=112 favours either oxygen or nitrogen ligands. JTYPE=113 favours either nitrogen or sulphur ligands. JTYPE=114 favours sulphur ligands.