Far and away, the a-aminocarboxylate moiety has
become the prevailing molecular architecture of the new
generation of detergent builders. There are a number of reasons why this has happened. First, more effective chelation
is achieved by branched structures than by linear molecules,
due to simple statistical considerations. Once a single che-late-metal ion bond has formed, the formation of additional
bonds is facilitated by the close proximity of the branched
structure. A linear molecule must establish additional chelate bonds sequentially—a statistically less likely process.
The handy trivalent nature of the nitrogen atom provides a
convenient handle for a branch point in a molecule.
Also, it’s easy to design and build molecules which form
five-member chelate rings from the a-aminocarboxylates.
The formation of five-member rings is highly desirable.
Smaller rings are energetically disfavored due to ring strain.
Larger rings are entropically undesirable. Statistically, a metal
atom is as likely to bond to another chelant molecule as it is
to another site on the same molecule when the ring size is
too large. Five-member rings provide the best compromise
between these two competing factors.
A third reason the a-aminocarboxylates enjoy current
popularity is that the granddaddy of all chelants, EDTA, features the a-aminocarboxylate structure and is also a food
additive. It is more difficult to ban a material as a “deadly
toxin” which has already achieved status as a food additive.
EDTA, shown above in the acid form, works very well
as a detergent builder, but has its share of disadvantages.
It’s a relatively large molecule (molecular mass = 292 g mol-
1), yielding relatively few moles of chelating ability for each
pound of material. Its rate of degradation in the environment is slow. While EDTA does a good job complexing calcium and magnesium ions—water hardness ions—it is a
superb chelant for many other metals, some of which are
Some formulators worry that detergents built with EDTA
will be discharged into river beds or lake bottoms containing safely immobilized heavy metals. The EDTA will readily
give up its complexed calcium ion to form a much stronger
chelate with mercury or lead. Solubilizing these metals creates an obvious environmental disaster.
At first glance the development of NTA, shown below,
provided a perfect solution.
NTA (molecular mass = 191 g mol-1) is highly soluble
as the sodium salt and generally easy to incorporate into a
wide array of product groups. It is a smaller molecule than
EDTA, packing more chelating ability into a pound of raw
material. It effectively chelates calcium and magnesium. It
degrades rapidly, minimizing the potential problem of solubilizing heavy metals.
The strongest evidence of its excellent performance is
that it suffered the usual fate of all successful molecules. It
was cast in the starring role of an array of studies as “potential
carcinogen.” Unfortunate conscripts of rats and mice were
fed NTA up to 2% of their diets [ www.hc-sc.ca/ewh-semt/
pubs/water-ean/nitrilotriacetic_acid/ index-eng.php], a level
more than 10,000 times higher than any expected real world
exposure [ www.ncbi.nlm.nih.gov/pubmed/12844192]. Not
surprisingly, it wasn’t long before NTA was “known to the
state of California” to be a possible carcinogen and subject
to the kind of labeling that puts a damper on product sales.
Maybe NTA is sufficiently troublesome that we need to
minimize our exposure to it. On the other hand, consider
the possibility that NTA was a simple victim of bad public
An important tool in our arsenal of identifying carcinogens is the Ames test. Developed by Bruce Ames—for
which he won a Nobel Prize—the test’s advantage is that it
can be done quickly. The Ames test correlates mutagenicity in a bacterium with carcinogenicity potential. Bacteria
grow fast while tumors are notorious for developing slowly.
Ames, world-class biologist that he is, hypothesized that
it made no sense that so many synthetic materials he stud-
ied were identified as possible carcinogens (59% of materials
tested). He subjected a number of natural materials to the
Ames test and found a similar high rate of carcinogenicity
potential (57% of materials tested) [ www.bruceames.org/
The world at large, deeply entrenched in the fad that
“natural is always good,” was not ready for this message. Ed
Bradley of 60 Minutes famously defamed Ames over the “nat-
ural” issue. The public had spoken: “I’ve made up my mind;
don’t confuse me with the facts. Natural is always good.”
We chemists have finally begun to get the hint. Increas-
ingly, we make molecules from naturally occurring bases.
Glutamic acid N,N-diacetic acid is a detergent builder derived
from the naturally occurring glutamic acid. It’s a good chelant
for all the usual reasons. It is a little on the large size (molecu-
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