Distance geometry chiral constraints

All the distance and chiral constraints can be assigned automatically by the distance geometry program directly from connectivity, atom type, bond length, and bond angle data (132). Additional distance and chiral constraints can also be assigned by the user to generate specific conformations or intermolecular interactions (several molecules at a time can also be entered into a distance matrix). 

Struaures generated by distance geometry will not satisfy all distance and chiral constraints perfectly. Minor violations less than 0.5 A generally do not... 

Many enhancements have been made to the basic distance geometry method. Some of the most useful enhancements result from the incorporation of chemical information. For example, if the lower bound for the 1,4 distances is set to a value equivalent to a torsion angle of 60° rather than one of 0° then eclipsed conformations can be avoided. Similarly, amide bonds can be forced to adopt a nearly planar structure by an appropriate choice of distance bounds and chiral constraints. 

While one could require these conformations to satisfy a wide variety of geometric conditions by means of constraints on suitable polynomials in the interatomic squared distances and signed volumes, it turns out that the simplest possible such constraints are also the most widely useful. These are lower and upper bounds on the interatomic squared distances themselves, together with the signs (+1, —1, or 0) of the volumes of selected quadruples of atoms. The latter, called chirality constraints, determine the chirality of the quadruple, or force it to be planar if the sign is zero. The totality of constraints of this form is called a distance geometry description. Experience has shown that most conformation spaces of practical interest in chemical problems can be accurately described by means of these simple constraints alone. 

Thirteen measurements were obtained from the NMR experiments—five H-H distances and eight H-C-C-H torsions—which were compared to those predicted by computing all possible isomers of [Ga(coelichelin)]. Note that the calculations were not constrained apart from fixing the chirality at each stereocenter. This contrasts with the typical use of NMR constraints which are usually explicitly included to restrain the geometry. In our case, only one combination of chiral centers gave uniform, simultaneous agreement with all the experimental data (Fig. 8). 

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