Chloroindium phthalocyanine used in optical-limiting device
light through would not protect the wearer from retinal damage. What is needed is a further 10-fold reduction in light transmission—to 0.02%—before the system could be considered for use in eye protection. Perry and Marder are confident that this level of optical limiting is achievable with phthalocyanines, perhaps by better approximating the ideal dye
concentration gradient and by "tweaking the molecules a bit," in Marder's words. Likewise, Larry R. Dalton, a chemistry professor at the University of Southern California who heard Perry's talk in Boston, is "cautiously optimistic" that a further 10-fold improvement in optical limiting will be achieved. The JPL group's demonstration that a nonuniform distribution of dye leads to significantly enhanced optical limiting opens "a new and unanticipated direction" in this field, he tells C&EN. "It's a nice accomplishment." The JPL research—funded by the Advanced Research Projects Agency, the Office of Naval Research, the Air Force Wright Laboratory, and the Air Force Office of Scientific Research—is of particular interest to the military. That is because it promises to protect eyes from damage due to inadvertent exposure to lasers (that can occur while using a laser range-finder) as well as from intentional
Aldol catalysts produced by reactive immunization Researchers at Scripps Research Institute, La Jolla, Calif., have devised a technique called reactive immunization. The technique generates catalytic antibodies that bond covalently to antigens, making the antibodies capable of catalyzing a class of reactions previously inaccessible to antibody catalysis. Catalytic antibodies are immune-system proteins that stabilize the transition state of chemical reactions and thus increase their reaction rates. The Scripps team has demonstrated the practical potential of the technique by producing an antibody that catalyzes the aldol reaction, a fundamental carboncarbon bond-forming reaction for which versatile catalysts have been lacking. The reactive immunization technique was conceived by Scripps President Richard A. Lerner and chemistry professors Kim D. Janda and Carlos F. Barbas III. Lerner and Janda, with Scripps coworkers Peter Wirsching, Jon A. Ashley, and Chih-Hung L. Lo, used the concept to produce covalent antibody-antigen intermediates [Science, 270, 1775 (1995)]. The method was then used by Lerner, Barbas, and coworker Jiirgen Wagner to produce a catalytic antibody for the aldol reaction [Science, 270,1797 (1995)]. In reactive immunization, says Lerner, 'Instead of immunizing with an in-
ert compound, you immunize with a reaction. In essence, one immunizes with a chemical reaction rather than with a chemical. That's a big change. It's the first attempt to immunize with reactive compounds." Antibodies normally bind noncovalently with their substrates. But reactive immunization results in the generation of antibodies that bind covalently, making it possible for them to catalyze reactions that require covalent activation. "What we're trying to do is cause a chemical reaction to take place in the binding pocket of an antibody, because there are elements of that chemical reaction that will be later used in a catalytic event," Lerner explains. "The antibodies go forward and their progeny will make that covalent bond with a similar substrate. That covalent bond then becomes part of the catalytic event." The technique mimics nature, he says, because "a full one-third of natural enzymes work by covalent mechanisms." The researchers have demonstrated the technique by using it to generate catalytic antibodies that catalyze the aldol reaction. "The aldol reaction is arguably the most important carbon-carbon bondforming reaction in biology and chemistry," says Lerner, "but the natural catalysts are too highly restricted in the sub-
exposure (such as from laser weapons designed to blind soldiers). Beyond the military applications, Perry also holds out the possibility of adapting this organic-based optical-limiting system for use in protective goggles or visors for labs and other settings where lasers are used. The most sophisticated goggles in use today rely on heavy interference filters to block a specific wavelength—but they also interfere with normal vision. Accidents can occur when the wearer momentarily lifts the goggles to get a good look at what he or she is doing and gets a reflected laser beam in the eyes. A system with an optical limiter might be more practical because it would protect the eyes without encumbering normal vision. Even if such applications come to pass, don't expect to see phthalocyanines in commercial sunglasses anytime soon. Sunlight, the researchers point out, just doesn't have the intensity to trigger the optical-limiting effect. Ron Dagani strates they use to be of general use to chemists." The goal was to generate antibodies that use the reaction mechanism that gives aldolases their efficiency but that are capable of catalyzing reactions of a greater range of substrates. The mechanism of the catalytic antibodies they developed mimics that of natural class I aldolase enzymes, which catalyze biological aldol reactions. In such reactions, a nitrogen atom on a lysine residue in the aldolase active site can react with a substrate to form a Schiff base (iminium ion). The Schiff base tautomerizes to form an activated enamine intermediate. The intermediate then reacts with an aldehyde to form a new carboncarbon bond. After hydrolysis of the Schiff base, the enzyme and product separate. In the immunization used by Lerner and coworkers, lysine residues in antibody binding sites formed Schiff-base linkages with a hapten (an antigen used to induce antibody formation). Some antibodies induced by the immunization therefore form a similar Schiff-base linkage with substrates that resemble the hapten structurally. The antibodies are then able to catalyze the aldol reaction, using a mechanism similar to that of the aldolases. The researchers were able to use one of these antibodies to successfully catalyze the reaction of aldehydes with acetone and other ketones to form aldol adJANUARY 1,1996 C&EN
25
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JANUARY 1,1996 C&EN
Antibody-catalyzed aldol reaction has mechanism...
... similar to that of aldolase enzymes Lysine side chain
\
NH9
-NH2
Aldolase Reaction with ketone substrate to form Schiff base
Acetone
Schiff-base formation
ΟΡΟ,2-
3-
Ο
HO"
H Iminium ion
HO Iminium ion
Aldolase Tautomerization
ihydroxyacetone phosphate
Tautomerization
% Enamine intermediate
λ
Aldol reaction
Aldehyde
Aldolase Aldol reaction
HO Enamine intermediate
o=>
OH Glyceraldehyde . 0 p o 22-_ 3-phosphate
H O — < _
Aldolase Schiff-base hydrolysis
Schiff-base hydrolysis
NH 9
Aldolase
R' Product
dition products. The rate of the anti body-catalyzed reaction is considerably slower than the rate of reaction cata lyzed by the most well-known aldolase (fructose-1,6-bisphosphate aldolase). However, the rate is comparable to that of aldolase-catalyzed reactions of nonnatural substrates—the type most likely to be used in organic synthesis. According to the Scripps scientists, the work shows that "catalysts that proceed by defined reaction mechanisms can be induced by immunization with reactive compounds. This approach is not limit ed to Schiff-base or enamine mecha nisms and may be used whenever the
Fructose-1, 6-bisphosphate
chemistry to be accomplished is beyond that easily achieved by even a concert of noncovalent interactions." Another potential application is to de velop tight-binding antibodies. For ex ample, says Janda, "people are trying to get antibodies to bind very tightly to the gpl20 glycoprotein on HIV [the AIDS virus] to block it from binding to host cells. An antibody that afforded covalent binding would theoretically provide the tightest binding. So reactive immuniza tion could eventually be useful medical ly to generate antibodies that would bind permanently to a target." Stu Borman
