Biography of Mark A. Ratner BIOGRAPHY ark Ratner describes him- theory is a relatively simple mathemati- self as a theoretical materi- cal model of a disordered system that als chemist. Arguably the relates concentration of conductor sites M youngest of the chemical within a theoretical lattice structure to sciences, materials chemistry is con- the flow (or percolation) of charge, par- cerned with how chemical interactions ticularly ionic diffusion. Below a thresh- control and determine the properties of old concentration, or the percolation materials. Throughout his career, Rat- threshold, the substance is an insulator, ner has aimed to develop models to de- and above that concentration it is a con- fine a theoretical language for how the ductor. Over the past 10 years, Ratner molecular structures of a material are and Nitzan have examined the proper- manifested in its physical properties. His ties both of electron transfer within work has focused on several areas in- molecules and electron transport in mo- cluding charge transport (1, 2), ion lecular wire junctions (11, 12). In such transfer (3), nonlinear optical behavior junctions, single molecules or small (4), and quantum dynamics (5, 6). Elec- groups of molecules conduct electrical tron-transfer reactions, so fundamental current between two electrodes. Because to life, underlie biological processes such of the very small size of the structures, as photosynthesis, cytochrome p450 re- both the molecular electronic structure actions, and cellular respiration as well and the connection between the mole- as materials processes such as electro- chemistry and corrosion. ‘‘It’s one of the cule and the electrodes strongly influ- most important reactions in chemistry, Mark A. Ratner ence the current–voltage characteristics which is why I’ve spent 30 years on it of the junction. and will spend the rest of my life on it,’’ and an electron-deficient ‘‘acceptor’’ Survival of the Fittest he said. group, spatially separated by a noncon- Born in Cleveland in 1942, Ratner Ratner’s Inaugural Article (13), pub- jugated bridge, could be placed in a sin- graduated from Harvard University lished in this issue of PNAS, tackles one gle molecule, the electronic asymmetry (Cambridge, MA) in 1964 with an un- of the most daunting challenges in com- dergraduate degree in chemistry. He created would lead to an equivalent putational chemistry: how to determine obtained his Ph.D. in chemistry from asymmetry in the electrical conduction a protein’s folded tertiary structure Northwestern University (Evanston, IL), through the molecule. That is, the mole- based on its primary amino acid se- did postdoctoral work in Aarhus and cule would act as a molecular rectifier. quence. A basic statistical calculation Munich, and taught chemistry at New Importantly, these molecules could tells us that 20 amino acids can produce York University (New York) from 1970 replace or perhaps complement semi- an astronomical number of sequences. until 1974. Later he served as a visiting conductors in electronic applications. Take any 2 of the 20 amino acids and professor with the National Sciences Although not yet at the stage of com- you can produce 400 possible sequences; Research Council at Odense University mercialization, this approach has many put 3 together, and you produce 8,000 (Odense, Denmark). Currently, Ratner advantages, and the trend is clear. The sequences, and so on. The number is Morrison Professor of Chemistry at size of the molecules, between 1 and quickly escalates, especially when you Northwestern University, where he 100 nm, makes them cheaper, more consider a peptide made up of hundreds served as department chair from 1988 efficient, and more precisely reproduc- or thousands of amino acids. Only a few until 1991 and as associate dean of the ible than the smallest possible silicon of the possible structures actually exist College of Arts and Sciences from 1980 circuits (9). in nature, but this calculation under- until 1984. He was nominated to the Israeli Connection scores the complexity of the problem. National Academy of Sciences in 2002. Ratner and colleagues approach this CHEMISTRY Throughout his career, Ratner has spent sequence-to-structure problem by using Molecules as Electronic Devices extended periods of time in Israel and an evolutionary algorithm computa- has worked with many distinguished sci- As Ratner tells it, one of his most im- tional method. Conventional approaches entists, including Joshua Jortner and portant contributions to science came to the computation of protein structures Abraham Nitzan from Tel Aviv Univer- early in his career in 1974, when he and include the solving of Newton’s equa- sity (Tel Aviv) and Raphael Levine, his graduate student Ari Aviram pub- tions using molecular dynamics and the Robert Gerber, and Ronnie Kosloff lished a paper in Chemical Physical Let- random sampling and selection of new from the Hebrew University of Jerusa- BIOPHYSICS ters in which they introduced the idea geometries based on energy criteria us- that molecules could act as electronic lem (Jerusalem). ‘‘Nitzan has influenced ing Monte Carlo methods. However, circuit components (7). The idea re- me more than any other scientist,’’ Rat- both of these approaches can fail to find ceived little attention at the time but ner said. ‘‘He has taught me how formal energy minima, especially for structures has since been recognized as a major theory integrates with the real world; he that are dense and compact. contribution to the field of molecular is a master at that.’’ In collaboration electronics (8). This seminal paper sug- with Nitzan, Ratner has employed dy- gested that single molecules can per- namic percolation theory to investigate This is a Biography of a recently elected member of the form some functions of electronic the dynamics of molecular materials and National Academy of Sciences to accompany the member’s devices. Aviram and Ratner proposed charge transport and relaxation in mate- Inaugural Article on page 7215. that if an electron-rich ‘‘donor’’ group rials systems (3, 10). Dynamic percolation © 2004 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0402757101 PNAS ͉ May 11, 2004 ͉ vol. 101 ͉ no. 19 ͉ 7213–7214 Downloaded by guest on October 2, 2021 With their approach, Ratner and col- Jan Linderberg, for showing him how Ratner and Ratner explain some of the leagues select structures in a manner theory can be useful in understanding many applications of nanotechnology. In analogous to Darwinian natural selec- nature. In addition, he names several one section they describe molecular tion. In the Darwinian process, popula- scientific heroes, including Rudolph motors, organic molecules combined tions compete for resources; the ones Marcus, who won a Nobel Prize in with metal atoms, which are capable of that survive and evolve are the ones that moving molecules many times larger compete most successfully. Likewise, than the device itself. Such devices may Ratner compares several candidate Ratner identifies the one day be able to deliver drugs directly structures based on their energy, on to cells. In their latest book, New analogies to other known structures, and area of artificial Weapons for New Wars: Nanotechnology on a combination of these two criteria. and Homeland Security (15), Ratner Much like Darwin’s ‘‘survival of the photosynthesis as and Ratner describe ways in which fittest,’’ the structures that best fit the nanotechnology-based sensors can be criteria are favored for continuation in crucially important. used to detect food, water, or air con- the next generation of computations. taminated with biological weapons, as This evolutionary method is especially well as nanotechnology-based remedia- efficient for complex problems that are chemistry for his work on chemical ki- tion technologies that could heal difficult to attack with conventional netics, and Josef Michl of the chemistry environmental damage ensuing from methods such as molecular dynamics and biochemistry department at the terrorism. On a more technical level, and Monte Carlo simulations. By using University of Colorado (Boulder), a Ratner and his colleague George Schatz this approach, Ratner’s team was able to friend of 30 years and an unending have written two textbooks on quantum find several previously unknown stable source of ideas and imagination. mechanics in chemistry (16, 17). structures for two peptides: unsolvated Another person whom Ratner credits Ratner says ultimately he would like met-enkephalin (Tyr-Gly-Gly-Phe-Met) with influencing him is scientist and sci- to be able to design nanoscale self- ϩ ϩ assembled structures. ‘‘Right now, we and Ac-(Ala-Gly-Gly)5-Lys H . ence writer Roald Hoffmann, who won a Nobel Prize in chemistry for theories can determine the structure of a mole- Popular Science concerning the course of chemical reac- cule, but we don’t know as much about Ratner, who is married and has two tions. Hoffmann, a professor at Cornell how to design a molecule to have a cer- grown children, credits several people in University (Ithaca, NY), writes poetry tain structure,’’ he said. In the area of addition to Nitzan with serving as role and books explaining chemistry to the electron transfer, Ratner identifies one models. The most important of these, general public. ‘‘It’s his appreciation of major theme as crucially important: the Ratner said, was his father, who came to the role of science in society and the process of artificial photosynthesis, or the United States from Poland ‘‘with way he conveys these concepts to the photovoltaics, with the ultimate goal of nothing’’ and demonstrated how to live public that I find very important,’’ ‘‘trying to find a way to capture energy a life of service to others. ‘‘He spent his Ratner said. that is environmentally friendly and use- time setting up schools to train people Ratner has written some popular ful.’’ According to Ratner, this type of who didn’t have job skills, chairing hos- books of his own, including two recent work is usually done in a wonderfully pital and school boards, things like books he coauthored with his son, intuitive fashion by brilliant people with that,’’ he said.