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Версия для печати | Главная > Институт > Научные советы > Научный совет по катализу > ... > 2017 год > № 84

№ 84

 

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  • Премии Правительства Российской Федерации
    2017 года — нашим коллегам-химикам
  • В.Б. Фенелонов
    Академик В.Н. Ипатьев – выдающийся ученый ХХ века,
    один из основателей гетерогенного катализа
  • С.В. Телешов
    Снять покров забвения
    (к 150-летию академика В.Н. Ипатьева)
  • За рубежом
  • Приглашения на конференции



Премии Правительства Российской Федерации
2017 года — нашим коллегам-химикам

Переход к элементу

Свернуть/Развернуть


В.Б. Фенелонов
Академик В.Н. Ипатьев – выдающийся ученый ХХ века

Переход к элементу

Свернуть/Развернуть


С.В. Телешов
Снять покров забвения

Переход к элементу

Свернуть/Развернуть


За рубежом


Single-molecule experiment reveals polymer growth spurts

Researchers get the first detailed look at how catalysts crank out individual polymer chains


By attaching a polymer chain to a glass slide and to a magnetic particle bound to the ruthenium catalyst (linkage shown at right) and pulling the chain out with a pair of magnetic tweezers, researchers are able to watch the wait and jump dynamics of ring-opening polymerization.

When chemists think about polymerization, they typically envision a wormlike polymer growing smoothly and continuously from a catalyst. But the actual view of how polymer growth unfolds has remained murky because of the limitations of analytical techniques.

Using a pair of magnetic tweezers, optical microscopy, and spectroscopic techniques, Cornell University researchers led by  Peng Chen , Geoffrey W. Coates and  Fernando A. Escobedo have achieved the first real-time visualization of single polymer chain growth. What they report is startling: Individual polymer chains don’t increase steadily but instead undergo consecutive wait and jump steps.

With the aid of molecular dynamics computer simulations, the researchers attribute this jerky mechanism to formation of polymer tangles—which they call hair balls—that form around the catalyst as thousands of new monomer units are added to the growing chain. The hair balls sporadically unravel after a couple of minutes, and a new hair ball starts to form.

Besides helping researchers better understand catalyst activity, polymerization rates, and bulk polymer properties, the researchers suggest their discovery of the growth spurts may be relevant to how cells produce biopolymers such as proteins, nucleic acids, and polysaccharides (Science 2017,  DOI: 10.1126/science.aan6837).

These new findings are “very cool,” says Suzanne A. Blum of the University of California, Irvine. Blum’s group has used fluorescently labeled molecules to study single-molecule dynamics and recently used this approach to watch catalytic activity as labeled monomers randomly got added to a growing polymer chain (Angew. Chem. Int. Ed. 2017, DOI: 10.1002/anie.201708284).

were previously obscured by “ensemble averaging,” Blum explains, in which researchers use techniques such as dynamic light scattering to observe all the molecules in a sample at once, and information about size distributions and other polymer parameters are extracted from the data. Single-molecule approaches avoid limitations of ensemble averaging, Blum says, but these measurement techniques are often a double-edged sword because the resulting data are a challenge to interpret without corroborating spectroscopic methods. By including molecular dynamics simulations, the Cornell team was able to get a clearer picture of conformational changes in the growing polymer.

“The ability to see dynamics in an important reaction like polymerization and to understand them through modeling is an exciting technological advance,” Blum says.

In their single-molecule experiment, the Cornell researchers attached the free end of a polymer chain to a glass surface using a silane linkage and attached the ruthenium catalyst at the growing end of the polymer to a magnetic particle held in place by a pair of magnetic tweezers. By tracking the position of the magnetic particle, the team achieved real-time visualization of a single chain’s growth during ring-opening polymerization.

“This is just a superb piece of science,” says Craig J. Hawker of the University of California, Santa Barbara. Hawker’s group recently reported the one-pot synthesis of block copolymers using five different monomers with widely varying properties (Angew. Chem. Int. Ed. 2017,  DOI: 10.1002/anie.201707646). The new polymer-growth monitoring process could help researchers understand how to better control such exotic polymerizations and allow them to tune the macroscopic properties of polymer networks to design new functional materials, he says.

“The Cornell team’s tantalizing view of how polymer chains grow sheds light on many synthetic challenges dating from the earliest days of polymer chemistry,” Hawker adds. “This work will have major implications far beyond single-chain dynamics.”

 

Enzyme beats catalysts at oxidation game

Directed evolution yields enzyme that oxidizes alkenes in a unique way

Oxidizing the end carbon atom of a terminal alkene to form an aldehyde, a process called anti-Markovnikov alkene oxidation, is difficult because the reaction is energetically unfavorable. The preferred process, in terms of energetics, is Markovnikov selectivity—oxidation of the substrate’s next-to-last carbon, forming a ketone instead of an aldehyde.

Chemists have long wanted to find better ways to achieve anti-Markovnikov selectivity in alkene oxidations—for example, to convert the aldehyde products to linear alcohols, which are commonly used in flavorings, perfumes, lubricants, and cosmetics. Catalysts exist that can perform the tricky anti-Markovnikov oxidations, but they are very unproductive and are not enantioselective.

Frances H. Arnold and coworkers at Caltech thought a modified enzyme might succeed where traditional catalysts have struggled. The team now reports using directed evolution, an iterative process of protein mutation and screening for activity, to identify amino acid substitutions that convert an enzyme into a predominantly anti-Markovnikov alkene oxidation catalyst (Science 2017,  DOI:10.1126/science.aao1482).

The evolved enzyme, called aMOx, is fairly productive, catalyzing about 3,800 oxidation reactions, or turnovers, before running out of steam, compared to fewer than 10 for earlier anti-Markovnikov oxidation catalysts. In conjunction with other reagents and catalysts, the enzyme could one day convert terminal-alkene substrates to a variety of aldehyde, alcohol, and amine products for commodity and fine chemical, pharmaceutical, and agrochemical use.

Markovnikov oxidation of terminal alkenes (left) to form ketones (center) is energetically
favored, whereas anti-Markovnikov reactions leading to aldehydes are more difficult.

The Caltech researchers started with an iron-based cytochrome P450 enzyme called P450LA1 because it catalyzes a related reaction, metal-oxo alkene oxidation, in which a metal-linked oxygen attacks the double bond, typically forming an epoxide. An earlier study by another group had found that P450LA1 also made 19% aldehyde side-product by opening the epoxide. In the new study, postdoc Stephan C. Hammer, now at the University of Stuttgart, found that the enzyme actually made 45% aldehyde directly, without going through an epoxide intermediate.

The researchers then used 10 rounds of directed evolution, with styrene as a substrate, to boost anti-Markovnikov selectivity from 45% to 81%, in aMOx. aMOx works well because it stabilizes high-energy radical and carbocation intermediates created when its iron-oxo group attacks styrene’s terminal alkene, allowing anti-Markovnikov aldehyde formation to predominate.

It took 12 mutations distributed throughout the protein to transform P450LA1 into aMOx. “No one could explain how these substitutions confer this reaction selectivity, much less predict them,” Arnold says. “This illustrates the power of directed evolution to create catalysts that have eluded chemists.”

Key advantages of the evolved enzyme include its enantioselectivity and its turnover number, “which probably still isn’t sufficient for commercial viability but pretty respectable,” says John T. Groves of Princeton University, who specializes in biomimetic catalysis.

There’s currently a push in academia and industry to replace conventional catalytic reactions with enzymatic processes that use aqueous solvents at ambient temperatures and pressures, work readily in flow systems, and don’t require catalyst separations, Groves says. “These days, a tie will go to the biocatalyst for those reasons. The new study is an important step in the right direction.”

 

Catalyst closes a gap in hetero Diels-Alder reaction

Powerful acid family enables cycloaddition of unactivated aldehydes and dienes


Chiral IDPi catalysts expand the scope of hetero Diels-Alder
reactions beyond electronically biased partners.

The Diels-Alder reaction is a classic transformation that typically marries a diene (with four π electrons) and an alkene (with two π electrons) to form a six-membered ring—it’s known as a [4+2] cycloaddition. This reaction has been studied exhaustively in organic synthesis and tried in many variations, yet some reacting partner combinations remain out of reach.

Researchers led by Benjamin List at the Max Planck Institute for Kohlenforschung have now closed one of these gaps. The team has reported a method that brings together unactivated and inexpensive dienes and aldehydes for the first time in a hetero Diels-Alder reaction, a variant that trades a carbon atom in the alkene partner for a heteroatom, which is oxygen in the case of the aldehyde (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b08357).

Diels-Alder reactivity is ruled by the energy gap between the two reacting partners. Chemists need to narrow this gap to drive the reaction forward, generally by modulating the reaction partners’ electronic properties through different substituent groups and/or with catalysts. For hetero Diels-Alder reactions between dienes and aldehydes, success has been limited to electronically engineered reacting partners.

By using highly acidic chiral imidodiphosphorimidate (IDPi) catalysts, List and coworkers were able to access electronically unactivated dienes and aldehydes to form dihydropyrans, which are often found in pharmaceutical and agrochemical settings. The researchers propose that the catalysts create a confined environment that enables high stereocontrol and avoids a number of acid-promoted side reactions.

Efforts to measure the new IDPi catalysts’ acidity are ongoing, but the compounds are estimated to be more acidic than an analogous IDPi compound that is reportedly the strongest chiral acid ever synthesized, according to the researchers.

“I really think this catalyst class is the most significant contribution to come from my lab,” List says. He believes the catalysts hold “enormous potential” for asymmetric synthesis because they are so active and selective.

The new class of strong IDPi acids are “extremely effective” at producing dihydropyrans in high enantiomeric excess, says Varinder Aggarwal, a synthetic chemist at the University of Bristol. Aggarwal points out that these dihydropyran products are valuable in the fragrance industry, adding that List’s lab must be “smelling pretty good.”

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