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Issue No. 3, Article 4/April 13, 2007

Isomers Resolved

Interesting names and phrases abound in the realm of weed management, and frequently they are associated with herbicide trade names and weed common names. Herbicide trade names have invoked perceptions ranging from the Wild West (Roundup, Lasso) to human anatomy (Bicep, Tri-Scept, Strongarm) and herpetology (Cobra, Python). Weed names sometimes convey a relative measure of plant size (giant ragweed, giant hogweed), toxicity (poison hemlock, poison ivy), or color (purple deadnettle, yellow rocket). Now we'll add to the list of interesting monikers one that may not be familiar to weed management practitioners but that is becoming increasingly commonplace: stereoisomer.

At first glance, this name may induce more confusion than understanding and cause more consternation than comfort. I'll employ a brief amount of chemistry (and hopefully a great deal of practicality) and attempt to explain stereoisomers and examine how they are relevant to today's weed management.

A good starting point might be to define the term. Stereoisomers are molecules that have the same atoms bonded to each other but differ in the way the atoms are arranged in space. Figures 1 and 2 will serve as examples for the following discussion. Figure 1 illustrates a 5-carbon ring molecule with two chlorine atoms attached to it; one chlorine atom is positioned above the plane of the ring, while the other is positioned below it. Figure 2 displays the same 5-carbon ring with the same two chlorine atoms, but here both chlorine atoms are positioned above the plane of the ring. Each molecule contains exactly the same number of atoms (5 carbon atoms and 2 chlorine atoms), but the spatial arrangement of the chlorine atoms differs. It is this different orientation of the chlorine atoms that differentiates this pair of stereoisomers.

So how is a differential orientation of atoms or substituent groups (i.e., stereoisomers) relevant to weed management? Even though two molecules may have the same types and numbers of atoms and differ only in the orientation of one or more atoms or groups, a differential orientation can greatly affect the biological activity of the molecule. If, for example, the molecules depicted in Figures 1 and 2 were herbicides, the orientation of the chlorine atoms in Figure 1 might cause this isomer to bind much more effectively at the herbicide target site within the plant, whereas the orientation of the chlorine atoms in Figure 2 might not allow the isomer to bind the target site at all. One might reason that if the molecule depicted in Figure 1 is more herbicidally active than the molecule depicted in Figure 2, it would be better to manufacture or use a product containing the Figure 1 molecule only. While this notion is valid, the process used to manufacture certain herbicides results in a combination of isomers (that is, a mixture of Figures 1 and 2) in the commercially available formulation. An example of stereoisomer chemistry in weed management is the active ingredient metolachlor.

Figure 1. A 5-carbon ring with two chlorine atoms, one positioned above the plane of the ring and the other positioned below it.

Figure 2. The same 5-carbon ring as shown in Figure 1, but here both chlorine atoms are positioned above the plane of the ring.

Metolachlor became commercially available during the 1970s and was sold under the trade name Dual. The process used to formulate Dual resulted in two isomers of metolachlor present in the commercial formulation. One isomer, designated the S-isomer, is much more herbicidally active than the other, designated the R-isomer. Dual and the subsequent product Dual II each contained a 50:50 mixture of the active (S) and inactive (R) isomers of metolachlor. (Dual became Dual II when a safener was added to the original formulation to reduce the potential for adverse crop response.) Application rates for these "nonresolved" formulations were determined based upon this 50:50 mixture of active:inactive isomers.

Fast forward to the 1990s, when improvements in technology allowed manufacturers to increase the amount of active (S) isomer in a formulation, and Dual II became Dual II Magnum. The Magnum formulations (Dual II Magnum, Bicep II Magnum, Bicep Lite II Magnum) still contain the same active ingredient(s) as they always have, but they now contain a higher proportion of the active or resolved (S) isomer compared with the older formulations (i.e., Dual or Dual II, Bicep or Bicep II, Bicep Lite or Bicep Lite II). Specifically, the Magnum formulations contain an 88:12 mixture of the active (S):inactive (R) isomers compared with a 50:50 mixture of the active (S):inactive (R) isomers found in the Dual or Dual II formulations.

So what is a practical implication of having a formulation containing more of the active isomer? Since a higher proportion of the active isomer is present in the Magnum formulations, application rates of those formulations are reduced approximately 35% compared with the application rates of the original formulation.

Perhaps another illustration will help. Say you were able to count out 100 molecules from a container of Dual II and 100 molecules from a container of Dual II Magnum. Assuming the rules of probability hold, the 100 molecules of Dual II would be composed of 50 active (the S or resolved isomer) and 50 inactive (the R or unresolved isomer) molecules. The 100 molecules of Dual II Magnum would be composed of 88 active (the S or resolved isomer) and 12 inactive (the R or unresolved isomer) molecules.

Assuming the unresolved isomer doesn't contribute much to weed control, it takes less Dual II Magnum to obtain the critical number of S-metolachlor molecules needed for weed control than either Dual or Dual II. For example, if 50 molecules of S-metolachlor (the active isomer) are needed to achieve control of a particular weed species, how many total molecules of Dual/Dual II and Dual II Magnum would you need in order to apply at least 50 molecules of S-metolachlor? You would need 100 total molecules of Dual or Dual II (50:50 mixture) to get 50 molecules of S-metolachlor, whereas you would need only 57 total molecules of Dual II Magnum (88:12 mixture) to get 50 molecules of S-metolachlor. Stated another way, if you were to apply the same product rate of Dual and Dual II Magnum, you would apply less active isomer per acre from the Dual formulation.

Figures 3 and 4 illustrate this concept. The circles represent equal volumes of herbicide, with Figure 3 taken from a container of a nonresolved metolachlor-containing herbicide (50:50 mixture of S and R isomers) while Figure 4 was taken from a container of a resolved metolachlor formulation (88:12 mixture of S and R). Each circle contains the same number of total molecules (designated S and R), but a different proportion of the two types.

Figure 3. A droplet from a container of a nonresolved metolachlor-containing herbicide (50:50 mixture of S and R isomers). Note the equal numbers of S's and R's.

Figure 4. A droplet from a container of a resolved metolachlor-containing herbicide (88:12 mixture of S and R isomers). Note the higher proportion of S's relative to R's.

I've provided this description in hopes of helping those who purchase herbicides made up of stereoisomers better understand some of the differences among commercially available products. In some respects this discussion is similar to the one we wrote several years ago highlighting the differences between active ingredient and acid equivalent, especially related to glyphosate-containing products. Nowadays, many metolachlor and S-metolachlor products are commercially available, and there appears to be some confusion with respect to product equivalents among the many formulations. For example, equivalent rates may be defined several ways, including equivalent amounts of active ingredients, equivalent amounts of active isomers, and simply those rates allowed by the respective product label. These are not always synonymous or interchangeable.

Table 1 lists several examples of products containing metolachlor or S-metolachlor. Do not assume that applying the same rate of each product necessarily results in the same amount of either active ingredient or active isomer being applied. In particular, you should note that while applying the same product rate of an S-metolachlor-containing product and a metolachlor-containing product can provide similar amounts of total active ingredient, the amount of the active isomer applied can vary considerably.--Aaron Hager

Author:
Aaron Hager

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