Anfinsen, Christian B.
ANFINSEN, CHRISTIAN B.
(b. Monessen, Pennsylvania, 26 March 1916; d. Pikesville, Maryland, 14 May 1995)
biochemistry, protein structure, molecular biology, evolution.
Anfinsen received the Nobel Prize in Chemistry in 1972 for his biochemical investigations of protein structure. Proteins catalyze essential chemical reactions in living cells as well as performing many other vital regulatory and structural functions. Proteins are made of amino acids in long linear sequences; they are functional only when these chains (called polypeptides) are folded into specific three-dimensional shapes. Anfinsen studied how a protein molecule acquires and maintains this specific shape by exposing one well-characterized protein, the enzyme ribonuclease, to conditions in which it became unfolded and structurally disordered. He and his coworkers demonstrated that the completely unfolded, or denatured, ribonuclease could spontaneously regain its fully native, functional state, including reforming the correct covalent chemical links between non-adjacent amino acids, the disulfide bonds. In conjunction with these findings, he developed the “thermodynamic hypothesis,” the notion that a protein in solution acquires its three-dimensional native structure because it is the most stable, i.e., the Gibbs free energy of that state is lowest. Anfinsen perceived in his thermodynamic theory a biological implication, that all of the information needed for the correct folding of a protein is contained in the linear polypeptide chain, and hence in the genetic sequence specifying the string of amino acids. This notion became a central part of the conceptual framework of molecular biology, and put into relief the “protein folding problem”—understanding how a particular amino acid sequence specifies the three-dimensional configuration—that has occupied protein chemists and structural biologists since the 1960s.
Youth and Early Career. Anfinsen was born in Menossen, a mill town south of Pittsburgh in which half of the 1910 population was foreign-born, and spent part of his childhood in nearby Charleroi. He was the child of Norwegian immigrants, Christian Boehmer Anfinsen Sr., a mechanical engineer, and Sophie Rasmussen Anfinsen, Lutherans who raised their children with the Norwegian language and cultural heritage. The family moved to Philadelphia in the 1920s. In 1933 Anfinsen was admitted to Swarthmore College, where he played on the school’s football team while studying chemistry. After graduating in 1937, he went on to the University of Pennsylvania to pursue a Ph.D. in organic chemistry. During the next two years, however, his interests shifted towards biochemistry. He applied for and was awarded a fellowship from the American Scandinavian Foundation to study enzymes at the Carlsberg Laboratory in Copenhagen, and left the University of Pennsylvania with a master’s degree in 1939. In a memoir he wrote for his fiftieth college reunion, Anfinsen referred to this year in Copenhagen as “perhaps the most formative and exciting year of my life” (p. 2).
The Carlsberg Laboratory had been founded in 1876 through the philanthropy of brewer J. C. Jacobsen to investigate scientific problems associated with brewing. In the early twentieth century it became a leading center for microbiology (in its Department of Physiology) and biochemistry (in the Department of Chemistry). In 1937, Kaj Ulrik Linderstrøm-Lang became head of the Chemistry Department, succeeding S. P. L. Sørenson, known for his work on the importance of pH to enzyme function. Linderstrøm-Lang focused on the ionization of
enzymes, deriving equations for the titration curve of a protein. During his months in Copenhagen Anfinsen became familiar with the problems and techniques of protein chemistry. The growing war—which extended the Nazi occupation into Denmark in April 1940—prompted Anfinsen to return early, with a first-hand sense of the crisis gripping Europe. His immigrant family was directly touched by the horrors of war; according to his second wife, Anfinsen’s Jewish maternal grandmother’s family disappeared after the Germans invaded Bergen, Norway.
Anfinsen applied to Harvard University for graduate work in biological chemistry, having been encouraged to do so by visiting scientists from Harvard at the Carlsberg Laboratory. He began his Ph.D. at Harvard in 1941, investigating the histochemistry of the retina for his dissertation with A. Baird Hastings. That same year he married his first wife, Florence Bernice Kenenger, with whom he had three children (Carol Craft, Margot Britton, and Christian B. Anfinsen III). Anfinsen completed his doctoral degree in 1943, at which point he became an instructor, teaching Harvard Medical School students at a time of accelerated medical training during the war. From 1944 to 1946, Anfinsen also participated as a civilian scientist in war-related research on malaria sponsored by the Office of Scientific Research and Development. Anfinsen’s biochemical research on the metabolism of blood in healthy monkeys and in monkeys infected with Plasmodium knowlesi resulted in four publications.
From 1945 to 1948 Anfinsen was an associate in biological chemistry at Harvard Medical School. There he collaborated with Arthur K. Solomon in applying radioactive isotopes, newly available from the U.S. Atomic Energy Commission’s nuclear reactors, as tracers to investigate metabolism. He spent the 1947–1948 period as an American Cancer Society Visiting Investigator at Hugo Theorell’s laboratory at the Medical Nobel Institute in Sweden. Anfinsen returned to Harvard Medical School in 1948 as an assistant professor of biological chemistry and a Markle Scholar.
Laboratory at the National Institutes of Health. In 1950, James Shannon, associate director of research at the newly created National Heart Institute, approached Anfinsen about becoming chief of the Laboratory of Cellular Physiology. Anfinsen surprised many of his Harvard colleagues by accepting Shannon’s offer. In part, he attributed the decision to the fact that the move doubled his salary overnight. However, he was not the only prominent biochemist of his generation who went to work for the federal government in Bethesda. In the 1950s and 1960s, laboratories at the National Institutes of Health (NIH) were headed by Arthur Kornberg (Nobel Prize, 1959), Bernard Horecker, Earl Stadtman, Leon Heppel, Bruce Ames, Marshall Nirenberg (Nobel Prize, 1968), and Martin Rodbell (Nobel Prize, 1994). Shannon became director of the NIH in 1955, and until 1968 he presided over not only the rapidly expanding extramural grants system but also a prominent program of intramural research. It was during his years at NIH (1950–1981) that Anfinsen made the scientific contributions that defined his career.
Anfinsen did spend sabbaticals during this period away from the NIH, including two years abroad in his first decade there. In 1954, he received a Rockefeller Foundation Fellowship to return to the Carlsberg Laboratory. He went to Copenhagen by way of Cambridge, England, to advance his knowledge of protein sequencing techniques. In Copenhagen during the 1954–1955 period, he worked with Linderstrøm-Lang and members of his group on the structure of ribonuclease. In addition, the award of a Guggenheim Foundation Fellowship enabled Anfinsen to spend a year at the Weizmann Institute of Science in Rehovot, Israel, in 1958–1959. In 1962, Anfinsen returned to Harvard Medical School as a professor in the Department of Biological Chemistry. He was invited to become chair of that department, but instead went back to the NIH in 1963, having been offered the opportunity to head a new laboratory at the National Institute of Arthritis and Metabolic Diseases. Anfinsen named this new unit the Laboratory of Chemical Biology; he was its chief until 1981.
At the beginning of 1950s, Anfinsen employed radioisotopically labeled amino acid precursors to investigate how proteins are synthesized in the cell—whether they are built up from individual amino acids, sequentially, on a template, or whether short peptides serve as intermediates, combined to make complete proteins. Results from radiolabeling chick ovalbumin during biosynthesis inclined him to favor the second mechanism. Other projects in his laboratory concerned the behavior and metabolism of various lipoproteins and cholesterol as well as the purification and characterization of L-glutamic acid dehydrogenase. Whereas some of Anfinsen’s work addressed heart disease, especially biochemical aspects of atherosclerosis, the position at NIH gave him sufficient freedom to pursue basic protein chemistry.
Work on Ribonuclease. During his early years at the NIH, Anfinsen began analyzing the effects of proteolytic enzymes on ribonuclease, an enzyme that digests ribonucleic acid (RNA). The susceptibility of proteins to proteolytic (peptide-cleaving) enzymes, which were first purified in the 1920s and 1930s, provided early evidence that proteins have a polypeptide structure. But these enzymes, such as pepsin, trypsin, and chymotrypsin, could also be used as probes of protein structure. In the late 1940s, Linderstrøm-Lang and his colleagues demonstrated that for B-lactoglobulin, enzymatic digestion with trypsin occurred in two phases, the second of which seemed to result from denaturation of the polypeptide chain, exposing new sites for cleavage. Anfinsen’s analysis of ribonuclease digestion by pepsin offered a similar picture, in which the main initial cleavage product seemed little unchanged in size and shape from intact ribonuclease. His subsequent study indicated that four disulfide bonds bridged different segments of the single polypeptide. Proteolysis produced protein fragments small enough to be sequenced, and in 1954 Anfinsen published the sequence of the four N-terminal residues. This effort was inspired by Frederick Sanger’s pioneering elucidation of the amino acid sequence of insulin, another pancreatic protein.
Anfinsen’s choice of ribonuclease for intensive investigation reflected not only the enzyme’s small size and stability, but also the availability to researchers of large amounts of this protein, which had been purified from bovine pancreas by the Armour Company. Anfinsen took his “precious bottle of ribonuclease” (Richards, 1972, p. 493) with him when he went to the Carlsberg Laboratory in 1954. That year in Linderstrøm-Lang’s laboratory, Anfinsen continued his work on ribonuclease with several other protein chemists there, including Bill Harrington, Aase Hvidt, Martin Otteson, John Schellman, and Fred Richards. In a short communication to Biochimica et Biophysica Acta, six of these scientists reported the surprising result that ribonuclease retained full activity even when in a solution of eight molar urea, which denatures proteins by disrupting hydrogen bonds. In light of this, they claimed that only a small portion of the ribonuclease molecule was responsible for catalysis, and that an ordered secondary structure was not required for the enzyme’s activity. As Anfinsen noted in 1989, he spent the next fifteen years after this paper’s appearance disproving that last conclusion.
In other respects, however, this publication laid the groundwork for Anfinsen’s continuing investigations of ribonuclease structure. When Anfinsen returned to the United States, Stanford Moore and William H. Stein were already making headway on determining the complete 124-amino acid sequence of ribonuclease. Rather than merely entering into a sequencing race, Anfinsen focused his efforts on understanding the relationship between biological function and chemical structure. He used limited proteolysis to show that an aspartic acid residue, the fourth amino acid from the C-terminal end, was essential for catalytic activity. A derivative of ribonuclease missing just those four amino acids was completely inactive. This finding supported the assertion of the 1955 multi-author paper that the active center was a small part of the whole protein, though Anfinsen recognized that amino acids elsewhere on the linear polypeptide chain, brought into proximity in the folded structure, participated in binding and catalysis.
Anfinsen’s laboratory also investigated the role of enzyme’s four disulfide bonds (which covalently link the protein’s eight cysteine residues). Whereas Moore and Stein disrupted these bonds irreversibly, treating the protein with performic acid, Anfinsen cleaved them by simply reducing the disulfide bonds to free (or chemically protected) sulfhydryl groups. This cleavage of disulfide linkages by reduction could only be achieved under conditions—eight molar urea—that also denatured the protein. The activity of ribonuclease was completely lost when all four disulfide bonds were broken, although not all of the disulfide bonds were essential for activity. More intriguingly, in 1957 Anfinsen reported that denatured enzyme with completely reduced sulfydryl residues could recover activity spontaneously, after removal of the denaturant and re-oxidation of some of the disulfide bonds through exposure to air. This provided the core observation underlying Anfinsen’s subsequent experiments.
The Nobel Prize. Over the next five years, members of Anfinsen’s laboratory, particularly Michael Sela, Edgar Haber, and Frederick H. White Jr., characterized the process by which denatured ribonuclease re-acquired its native structure in solution. Under suitable conditions, the eight reduced cysteine sulfhydryls oxidized to produce the four original disulfide bonds, among 105 possible combinations. As these four researchers stated in their 1961 paper, “From chemical and physical studies of the reformed enzyme, it may be concluded that the information for the correct pairing of half-cystine residues in disulfide linkage, and for the assumption of the native secondary and tertiary structures, is contained in the amino acid sequence itself” (p. 1309). The authors obtained evidence that some of the first disulfide linkages re-formed were not the native bonds, but that, over time, rearrangement of the disulfide bridges produced the original linkages. In the course of these continuing studies, ribonuclease became a leading model system for investigating the kinetics of protein folding.
For his work on ribonuclease, Anfinsen was awarded the Nobel Prize for Chemistry in 1972. He received half of the monetary award, the other half being shared by Moore and Stein for their determination of the enzyme’s amino acid sequence and their investigation of its catalytic mechanism. By this time, Anfinsen’s renaturation of ribonuclease emblematized the primacy of the genetic sequence in determining protein folding and function. Like other landmark experiments in molecular biology, Anfinsen’s experimental work gave older genetic concepts new molecular meaning. His thermodynamic hypothesis postulated that the “linear information of the genotype” resulted in the spontaneous formation of phenotypically functional proteins (Anfinsen, 1968, p. 17). But in contrast to a younger generation of molecular biologists, Anfinsen did not want to see protein biochemistry become secondary in significance to work on molecular genetics. The idea that the sequence of amino acids carried the instructions for protein folding helped to include proteins as well as nucleic acids in the new world of informational agents.
From Anfinsen’s first articulation of the thermodynamic hypothesis, he recognized that even if the “folding process is thermodynamically guided” (1961, p. 447),
other factors in the cell may affect the kinetics of the process. In the case of ribonuclease, Anfinsen contended, the process of rearrangement of disulfide bonds was too slow in vitro to account for its rapid, efficient biosynthesis in vivo. Consequently, he embarked on a search for an enzyme in beef liver that catalyzed sulfhydryl-disulfide interchanges. In 1966, members of his laboratory identified and purified this enzyme, protein disulfide isomerase. Since the 1970s, biologists have uncovered a host of other cellular agents that facilitate correct protein folding and assembly in vivo. The scientists who investigate these “molecular chaperones” in the early 2000s continue to cite Anfinsen’s work on ribonuclease as canonical to the study of protein folding, even as they qualify its significance.
Other Scientific Contributions and Honors. Anfinsen published his sole monograph, The Molecular Basis of Evolution, in 1959, the centennial of the appearance of Darwin’s On the Origin of Species. Anfinsen’s book offered an up-to-date survey of discoveries in genetics and protein chemistry, arguing that a convergence of these fields could enable “a greater understanding of the fundamental forces underlying evolutionary process” (p. vii). Anfinsen contended that biological molecules such as proteins and nucleic acids, no less than fossils, are historical records of species variation for evolutionary interpretation. He emphasized the primacy of proteins in evolutionary selection: “The phenotypic picture present by an organism is the summation of the effects, physical and catalytic, produced by the complement of protein molecules characterizing the species in question” (p. 185). In addition to arguing for the salience of molecular evidence to evolutionary biology, Anfinsen sought to preserve a central place for protein chemistry alongside molecular genetics in the emerging field of molecular evolution.
Over the course of the 1960s Anfinsen’s laboratory worked on several proteins, including B-galactosidase and staphylococcal nuclease. Staphylococcal nuclease provided a simpler model for protein folding than ribonuclease because it lacked disulfide bonds. In their work on this enzyme, Anfinsen, Meir Wilchek, and Pedro Cuatrecases applied the technique of affinity chromatography to protein purification. Their method involved coupling a ligand or inhibitor of the desired protein to a Sepharose column, so that the protein was bound—while other cellular proteins passed through—before being selectively eluted. Their joint paper announcing the effectiveness of this method for protein purification remains, after the 1972 Nobel Lecture, Anfinsen’s most-cited publication. Anfinsen also selected staphylococcoal nuclease in his attempt to chemically synthesize a catalytically active enzyme from scratch. In the end, he and his coworkers achieved a semi-synthesis by coupling peptide fragments prepared by solid phase synthesis. In the 1970s, Anfinsen turned his attention to the study of interferon, an antiviral agent of great medical promise that was available only in miniscule quantities. His group used affinity chromatography to purify this protein, and in collaboration with scientists at the California Institute of Technology, they published its amino acid composition and the N-terminal sequence of one of the two components in 1980.
In the 1960s through the 1990s, Anfinsen received numerous honors beyond the 1972 Nobel Prize. His alma mater, Swarthmore College, awarded him an honorary doctorate in 1965, the first of eleven such degrees he received. He was a member of the American Philosophical Society, the National Academy of Sciences, serving on its Council from 1974 to 1977, and the American Society of Biological Chemistry (later the American Society of Biochemistry and Molecular Biology), for which he served as president in 1971–1972. Anfinsen also received recognition beyond the United States, being selected for membership in the Royal Danish Academy and the Vatican’s Pontifical Academy of Science, and receiving medals from the State of Israel and from universities in Naples and Jerusalem.
Political Activities and Other Interests. Anfinsen involved himself in political action from the 1950s, particularly through letter-writing campaigns. He was among more than eleven thousand scientists who signed Linus Pauling’s 1957 petition calling for a ban on the atmospheric testing of nuclear weapons. He opposed the Vietnam War, participating in a vigil held on the NIH Bethesda campus in 1964 after the U.S. Congress passed the Gulf of Tonkin resolution. In 1969 he and his NIH colleague Marshall Nirenberg protested the Brazilian government’s removal of prominent scientists from their university positions. After receiving the Nobel Prize in 1972, Anfinsen made use of his public status to make appeals on behalf of foreign scientists mistreated by their governments and advocate for stronger federal funding of biomedical research.
In 1973, Anfinsen and several other distinguished scientists co-authored a letter to President Nixon, expressing concern about the denial of exit visas to scientists in the Soviet Union and calling for greater scientific exchange between the two countries. That same year, he allied himself with prominent NIH scientists to oppose Nixon’s Conquest of Cancer Agency, on the grounds that it would draw funding away from basic research; the petition they circulated was signed by more than 3,000 biomedical scientists. Along similar lines, in 1983 Anfinsen, Julius Axel-rod, Nirenberg, and D. Carleton Gajdusek, all Nobel prize-winning NIH scientists, coauthored a letter protesting Reagan’s budget cuts to intramural research programs at the NIH. From 1981 to 1989, Anfinsen served as chairperson of the National Academy of Science’s Committee for Human Rights. He was especially concerned about the plight of “Refuseniks” and other dissident scientists in the Soviet Union and Latin America. In the late 1980s, Anfinsen was among the prominent scientists who questioned the scientific value of the Human Genome Project.
In obituaries and reminiscences, his colleagues recall Anfinsen’s down-to-earth nature (everyone in the laboratory called him “Chris”) and emphasize that his dedication to science did not exclude other passions. He played both viola and piano, and when at the Carlsberg Laboratory in 1954–1955, he and his wife formed a chamber music ensemble with other scientists and spouses. The other avocation to which Anfinsen devoted himself was ocean sailing. During the last part of his life, religion also occupied a central place. Anfinsen and his first wife divorced in 1978. The following year he married Libby Esther Shulman Ely, and he converted to Orthodox Judaism. As Anfinsen wrote in 1987, “Although my feelings about religion still very strongly reflect a fifty-year period of orthodox agnosticism, I must say that I do find the history, practice, and intensity of Judaism an extremely interesting, philosophical package” (p. 2).
By the time that Anfinsen converted, he had developed strong connections to the Weizmann Institute in Israel, having served on its Board of Governors since 1962 and chaired its Scientific Advisory Board for many years. Anfinsen’s first point of contact to the Weizmann Institute, where he eventually spent three sabbatical years, was Michael Sela, who came to Anfinsen’s laboratory as a postdoctoral fellow in 1956 and spent two more periods at the NIH working with Anfinsen. Sela was associated with the Weizmann Institute for his entire five-decade career and served as its director from 1975 to 1985. In 1981 Anfinsen retired from the NIH to accept a job at as chief scientist for Taglit, a research company being formed by the research arm of the Weizmann Institute (Yeda) and the investment firm E. F. Hutton. However, just two weeks after he and his wife moved to Israel, E. F. Hutton withdrew its funding, leaving Anfinsen in limbo.
In 1982, Anfinsen returned to Maryland to accept an appointment as a professor of biology at Johns Hopkins University. There he launched his last major research effort, a study of the proteins of thermophilic bacteria— microbes that thrive at temperatures high enough to denature the proteins of other organisms, presenting a puzzle to protein chemists. Anfinsen was working on a project, funded by the National Science Foundation, to develop thermostable enzymes to aid in the remediation of environmental contamination and nuclear waste in the oceans, when he suffered a heart attack and died in May 1995.
The Christian B. Anfinsen Papers are archived in the National Library of Medicine. A finding aid and selected papers are available through the Profiles in Science series at profiles.nlm.nih.gov. The online collection includes copies of short reminiscences by Anfinsen, including the 1987 profile he wrote for the 50th Reunion Yearbook of the Swarthmore College Class of 1937 (quoted above), as well as his curriculum vitae, a full list of his publications, a selection of his correspondence, and a wide range of other primary sources.
WORKS BY ANFINSEN
With Robert R. Redfield, Warren L. Choate, et al. “Studies on the Gross Structure, Cross-Linkages, and Terminal Sequences in Ribonuclease.” Journal of Biological Chemistry 207 (1954): 201–210.
With W. F. Harrington, Aase Hvidt, et al. “Studies on the Structural Basis of Ribonuclease Activity.” Biochimica et Biophysica Acta17 (1955): 141–142.
“The Limited Digestion of Ribonuclease with Pepsin.” Journal of Biological Chemistry221 (1956): 405–412. This publication implicated the C-terminal end, particularly a specific aspartic acid residue, in the activity of ribonuclease.
With Michael Sela and Frederick H. White Jr. “Reductive
Cleavage of Disulfide Bridges in Ribonuclease.” Science 125 (1957): 691–692.
With Frederick H. White Jr. “Some Relationships of Structure to Function in Ribonuclease.” Annals of the New York Academy of Sciences 81 (1959): 515–523.
The Molecular Basis of Evolution. New York: Wiley, 1959.
With Edgar Haber, Michael Sela, and Frederick H. White Jr.“The Kinetics of Formation of Native Ribonuclease During Oxidation of the Reduced Polypeptide Chain.” Proceedings of the National Academy of Sciences, USA 47 (1961): 1309–1314.
With Charles J. Epstein and Robert F. Goldberger. “The Genetic Control of Tertiary Protein Structure: Studies with Model Systems.” Cold Spring Harbor Symposia on Quantitative Biology 28 (1963): 439–449. This contribution introduced the term “thermodynamic hypothesis” to refer to the notion that Gibbs free energy drives protein folding.
With Francesco DeLorenzo, Robert F. Goldberger, et al. “Purification and Properties of an Enzyme from Beef Liver which Catalyzes Sulfhydryl-Disulfide Interchange in Proteins.” Journal of Biological Chemistry 241 (1966): 1562–1567. “Spontaneous Formation of the Three-Dimensional Structure of Proteins.” Developmental Biology Supplement 2 (1968): 1–20.
With Pedro Cuatrecasas and Meir Wilchek. “Selective Enzyme Purification by Affinity Chromatography.” Proceedings of the National Academy of Sciences, USA61 (1968): 636–643.
“Studies on the Principles that Govern the Folding of Protein Chains.” Nobel Lecture, December 11, 1972, available at http://nobelprize.org Also published as “Principles that Govern the Folding of Protein Chains.” Science 181 (1973): 223–230.
“One Hundred Years of Originality, Quality and Style.” Carlsberg Research Communications 41 (1976): 293–298. This
retrospective essay and the one from 1986 (below) recount Linderstrøm-Lang’s charismatic and intellectual influence on protein science, and also give a sense of the fraternity of scientists that emerged from his laboratory.
With Kathryn C. Zoon, Mark E. Smith, et al. “Amino Terminal Sequence of the Major Component of Human Lymphoblastoid Interferon.” Science 207 (1980): 527–528.
“The International Influence of the Carlsberg Laboratory on Protein Chemistry.” Perspectives in Biology and Medicine 29 (1986): S87–S89.
“Commentary on ‘Studies on the Structural Basis of Ribonuclease Activity.’” Biochimica et Biophysica Acta1000 (1989): 197–199. This article comments on the erroneous conclusion of the 1955 paper on ribonuclease published from the Carlsberg Laboratory.
Kresge, Nicole, Robert D. Simoni, and Robert L. Hill, “The Thermodynamic Hypothesis of Protein Folding: The Work of Christian Anfinsen.” Journal of Biological Chemistry281 (2006): e11–e13.
Michaelis, Anthony R. “Obituary: Christian B. Anfinsen.” Interdisciplinary Science Reviews 20 (1995): 96.
Moudrianakis, Evangelos N. “From Protein Coagulation and Reversible Denaturation to the Protein Folding Problem: Chris Anfinsen Defining the Tradition.” The FASEB Journal 10 (1996): 179–183.
Richards, Frederic M. “The 1972 Nobel Prize for Chemistry.” Science 178 (1972): 492–493.
———. “Linderstrøm-Lang and the Carlsberg Laboratory: The View of a Postdoctoral Fellow in 1954.” Protein Science 1 (1992): 1721–1730.
Young, Michael. “Christian B. Anfinsen (1916–1995): Remembering His Life and His Science.” Protein Science 4 (1995): 2237–2239.
Angela N. H. Creager