where the connections to be removed are those between atoms n1-n2, n3-n4, n5-n6, etc. See atom numbering. Thus if connections were to be removed between atoms 4 and 5, and between atoms 665 and 670, then CVB(4:-5;665:-670) would be used. If connections are needed, then don't use the minus sign. Thus if connections were needed between atoms 4 and 5, and between atoms 665 and 670, then CVB(4:5,665:670) would be used. CVB is also useful when RESIDUES and PDBOUT are present. A topology sometimes has an unwanted connection between two chains that prevents some of the residues being identified. To correct this fault, remove the unwanted connection using CVB. If there are a several unwanted connections , and these connections involve a specific element, then consider the alternative of using VDWM to change the Van der Waals radius for that element.
Using atom labels
Here Labeln is the label, in PDB or JSmol format, for the atom. Using the PDB format, the label starts with the first non-blank character of the atom name and ends with the last character of the residue number. For example: CVB("2HB SER B 4":-"OD1 ASP B 119","1HH1 ARG B 5":"C TYR B 7"). Spaces are not important, but using spaces makes it easier to read the atom label. With JSmol, the label starts with a "[" and ends at the character before " #". As with PDB format, spaces are not important when JSmol format is used, i.e., CVB("[LYS] 4 : A . CD":"[LYS]4:A.CG"). The advantage of using atom labels is that if atoms are added or removed the atom label would not change. Another advantage is that PDB TER lines, positional disorder, and other things that cause the PDB line number to not match the atom number do not cause problems.
Wildcards are allowed for the residue name and for the chain letter. When a wildcard is used, a letter is replaced by an asterisk, "*", thus: CVB("2HB *** B 4":-"OD1 *** * 119","1HH1 ARG * 5":"C TYR B 7") and CVB("[***] 4 : * . CD":"[***]4:*.CG"). The first occurrence that matches will be used.
A frequent problem when modeling intermediates and transition states in enzyme mechanisms is that the structure cannot be written in Lewis form. An example occurs in the transition state when a cystine sulfur starts to bond to the carbon of a guanidinium group in an Arg, and the hydrogen that was on the sulfur migrates to one of the nitrogen atoms of the guanidinium. If this happens, the resulting -NH3 group would have a positive charge and the carbon would be given a negative charge. (A carbon atom bonded to a cation and having only three valencies used would automatically be assigned a negative charge.) Sulfur having only one bond (to its CH2 group) is, by default, also given a negative charge. The net charge on this assembly is now -1 instead of the expected +1. To correct this error CVB is used to make a bond between S and the H of -NH3. This automatically causes the N-H bond to be deleted giving -NH2, a component of the normal guanidinium group. Now that sulfur is bonded to its hydrogen as will as to its CH2 group, and the guanidinium has the usual +1 charge, the net charge in this region is correct.
A guiding principle would be to use CVB to make the structure as similar to either the reactant or product as possible.
Note that CVB is used only in the generation of a starting Lewis structure or in generating a PDB file. The presence of unrealistic connections in this structure will not normally give rise to an incorrect SCF.
See also SETPI to explicitly assign π bonds, and atom labels to explicitly assign charges.
See also SETPI, LEWIS, CHARGES, METAL, VDWM,
CHARGE, XENO, and MOZYME
See also: Lewis Structures, MOZYME introduction, atom numbers