Brilliant Blunders: From Darwin to Einstein - Colossal Mistakes by Great Scientists That Changed Our Understanding of Life and the Universe (17 page)

The paper, entitled
“Polypeptide Chain Configurations in Crystalline Proteins,” was written by an illustrious trio: Lawrence Bragg, Nobel Prize for Physics laureate in 1915, and two molecular biologists who eventually shared the 1962 Nobel Prize for Chemistry—John Kendrew and Max Perutz—all from the Cavendish Laboratory at Cambridge. At the time, this famous lab was the world’s leading center for X-ray crystallography. This method for analyzing crystals was largely Bragg’s baby; he and his father, Sir William Henry Bragg, together worked out the mathematics underlying the physical phenomenon and developed the experimental technique.

The idea behind X-ray crystallography was genius in its simplicity. Physicists had known from the beginning of the nineteenth century that if they shined visible light onto a finely spaced grating, the light that passed through formed a diffraction pattern of bright and dark spots on a screen on the other side. The bright spots marked the locations where light waves from different slits in the grating combined to enhance each other, while the dark spots formed where the different waves underwent destructive interference (as when a crest from one wave was superimposed onto a trough from another). Physicists also knew, however, that for this diffraction pattern to form, the spacings between the different slits needed to be of the same order as the wavelength of the light (the distance between two successive crests in the wave). While it was relatively easy to manufacture such fine gratings for visible light, it was impossible to produce them for X-rays, since a typical wavelength for X-rays is a few thousand times shorter than wavelengths in the visible part of the spectrum. The first person to realize that natural, periodic crystals could serve as gratings for X-ray diffraction experiments was Max von Laue. The German physicist recognized that the inter-atomic distances in crystals were precisely of the order of the presumed wavelengths of X-rays. Following in Laue’s footsteps, Lawrence Bragg formulated the mathematical law describing the diffraction of X-rays on a crystalline structure. Amazingly, he obtained this
important result during his first year as a research student at Cambridge. The father and son team then went on to build the X-ray spectrometer that allowed them to analyze the structure of many crystals. Lawrence Bragg remains, by the way, the youngest person to be awarded a Nobel Prize. (He won it at age twenty-five!)

Given this formidable legacy, we can imagine that when Pauling saw the title of the paper by Bragg, Kendrew, and Perutz, his heart missed a beat. The first two paragraphs of the paper indeed gave the impression that Bragg’s team may have beaten him to the punch:
“Proteins are built of long chains of amino-acid residues . . . In this paper an attempt is made to glean as much information as possible about the nature of the chain from x-ray studies of crystalline proteins, and to survey the possible types of chain which are consistent with such evidence as is available.” Pauling read quickly all thirty-seven pages and was relieved to discover that while the Cavendish researchers described some twenty structures, the alpha-helix wasn’t one of them. Moreover, they concluded that none of the examined structures was acceptable as a model for alpha keratin. Pauling agreed happily with this conclusion, especially since he thought that Bragg’s team did not apply the most important constraint to its configurations but did impose a handicap that he regarded as totally unnecessary. On one hand, none of Bragg’s models assumed the planarity of the peptide group, of whose correctness Pauling was absolutely convinced. On the other, the Cavendish team appeared to be hung up on the notion that in every full turn of its helical structures, there had to be an
integer number
of amino acids. Pauling’s alpha-helix broke with tradition and had about 3.6 amino acids per turn, and he saw nothing wrong with that. Coming from an X-ray crystallography background, Bragg also adhered religiously to the apparent 5.1 angstrom distance between turns suggested by Astbury’s data. Perutz later described that to start the team off,
Bragg hammered nails representing amino acid residues into a broomstick in a helical pattern with an axial distance between successive turns of 5.1 centimeters.

Pauling was always extremely competitive in nature. Even though
he was pleased to see that the Cambridge team had missed a few key points, the appearance of Bragg’s paper prompted him into action, for fear he might be scooped. In October 1950
he and Corey sent a short note describing the alpha-helix and the gamma-helix to the
Journal of the American Chemical Society.
Around the same time, some encouraging results were coming from another British research group at Courtaulds Research Laboratories. There, Clement Bamford, Arthur Elliott, and their collaborators succeeded in producing fibers of synthetic polypeptides. To Pauling’s delight, X-ray diffraction photographs of those fibers showed clearly that the repeat distance along the axis was 5.4 angstroms—consistent with Pauling’s findings—rather than 5.1 angstroms. This raised the suspicion that the latter feature in the X-ray photographs of hair could simply be an artifact produced by overlapping reflections rather than a major clue to the structure. Increasingly convinced of the truth of this interpretation, Pauling submitted a paper by himself, Corey, and Branson
that contained a detailed explanation of the alpha- and gamma-helices. It was only fitting that this important paper was submitted precisely on the day of Pauling’s fiftieth birthday, February 28, 1951.

There is, incidentally, an interesting anecdote concerning the use of the term “helix,” which I heard from chemist Jack Dunitz, who at the time was a postdoctoral fellow with Pauling. Dunitz recalled that in 1950 Pauling kept using the term “spiral” to describe the structure of alpha keratin. Even in Pauling and Corey’s short communication in the
Journal of the American Chemical Society
, they wrote exclusively about spirals. One day, said Dunitz, he remarked to Pauling
that he thought that the word “spiral” referred only to the two-dimensional, planar shape, while the three-dimensional one had to be called a “helix.” Pauling responded that a spiral could be either two-dimensional or three-dimensional, but added that on second thought, he liked the word “helix” better. When the extensive manuscript by Pauling, Corey, and Branson was submitted, it avoided the word “spiral” altogether. Its title read: “The Structure of Proteins: Two Hydrogen-Bonded Helical Configurations of the Polypeptide Chain.” Pauling was by then so confident in his model that he and
Corey followed the alpha-helix paper with a barrage of papers on the folding of polypeptide chains.

That spring in England, Max Perutz went one Saturday morning to the library, and there, in the latest issue of the
Proceedings of the National Academy of Sciences,
he found the series of papers by Pauling. Some thirty-six years later, he described what he had experienced that morning (in a somewhat technical language, but the emotions were crystal clear):

I was thunderstruck by Pauling and Corey’s paper. In contrast to Kendrew’s and my helices, theirs was free of strain; all the amide groups were planar and every carboxyl group formed a perfect hydrogen bond with an amino group four residues further along the chain. The structure looked dead right. How could I have missed it? Why had I not kept the amide groups planar? Why had I stuck blindly to Astbury’s 5.1 angstrom repeat? On the other hand, how could Pauling and Corey’s helix be right, however nice it looked, if it had the wrong repeat? My mind was in a turmoil. I cycled home to lunch and ate it oblivious of my children’s chatter and unresponsive to my wife’s inquiries as to what the matter was with me today.

Thinking a bit more about Pauling’s model, Perutz realized that the alpha-helix resembled a helical staircase, in which the amino acid residues (marked by “R” in
figure 12
) were forming the “steps.” The height of each step was about 1.5 angstroms. Bragg’s X-ray diffraction theory therefore predicted the existence of never-before-reported X-ray reflection signatures, separated by 1.5 angstroms, from planes perpendicular to the fiber axis. None of the models of Bragg’s group would have produced such a mark, while this would have been a distinct “fingerprint” of Pauling’s alpha-helix.

Just as he was about to conclude that the lack of such reflections in Astbury’s data was sufficient to refute Pauling’s model, Perutz suddenly recalled that Astbury’s particular experimental setup—with
the fibers oriented such that their long axes were perpendicular to the beam of X-rays—would not have really allowed for the detection of the 1.5 angstrom signature. Rather, calculations predicted that the optimal conditions to observe the reflection would have required inclining the fibers at an angle of about 31 degrees.

Perutz felt absolutely compelled to make the crucial test right away. He cycled back to the lab, grabbed a horsehair he had in a drawer, inserted it into the apparatus at the angle he calculated to be favorable for detecting the reflection, put a film around it (as opposed to Astbury’s flat-plate camera, which was too narrow and could have missed reflections deflected at large angles), and fired the X-ray beam. The few hours that passed before he could develop the film were sheer agony, but finally Perutz had the answer. The strong reflection predicted by the alpha-helix at a spacing of 1.5 angstroms stuck out unambiguously!

Perutz showed the X-ray photograph to Bragg first thing on Monday morning. Bragg wondered what it was that suddenly gave Perutz the idea to conduct this crucial test. Perutz replied that he was madly furious with himself for not having thought of the alpha-helix. Bragg retorted with what has by now become an immortal phrase: “I wish I had made you angry earlier!”

Not everything that Pauling wrote in that famous series of papers from 1951 was correct. A careful scrutiny of his entire oeuvre for that year reveals several weaknesses. In particular, the gamma-helix eventually had to be abandoned. These minor shortcomings, however, don’t take away anything from Pauling’s groundbreaking achievement: the alpha-helix and its prominent role in the structure of proteins. Pauling’s contributions to our understanding of the nature of life were substantial.
He was one of the first scientists to see that in spite of its inherent complexity, biology is, at its core, molecular science augmented by the theory of evolution. Already back in 1948, he wrote perceptively:
“To understand all these great
biological phenomena we need to understand atoms, and the molecules that they form by bonding together; and we must not be satisfied with an understanding of simple molecules . . . We must also learn about the structure of the giant molecules in living organisms.”

Pauling’s influence on the general theory and methodology of molecular biology was equally impressive. First, in his seminal 1939 book
The Nature of the Chemical Bond and the Structure of Molecules and Crystals: An Introduction to Modern Structural Chemistry,
he remarked prophetically on the importance of the hydrogen bond for biomolecules:
“I believe that as the methods of structural chemistry are further applied to physiological problems it will be found that the significance of the hydrogen bond for physiology is greater than that of any other single structural feature.” Indeed, the structure of many organic molecules, ranging from proteins to nucleic acids, confirmed this prediction fully.

Second, Pauling pioneered model building and turned it into a predictive art form based on strict rules of structural chemistry.
Even the space-filling colored models developed at Caltech became a hot item in the arena of macromolecular research. These models, produced for labs by the Caltech workshop, fetched as much as $1,220 in 1956 for a set that contained about six hundred atom models.

Pauling’s practice of using the X-ray diffraction patterns not as the starting point but as the ultimate arbiter among sophisticated, educated guesses also proved to be enormously effective—Watson and Crick were about to apply the same approach to the structure of DNA.

There was another remarkable observation concerning genetics that Pauling made in a lecture in 1948, but apparently even he did not realize at the time its full implications. In the first part of that lecture, Pauling reminded his audience:

The Gregorian monk Mendel noted that the inheritance of characters by pea plants, such as the character of tallness or of dwarfness, or the character of having purple flowers or white flowers, could be understood on the basis of
hereditary units transmitted from the parent to the offspring. Thomas Hunt Morgan and his collaborators identified these units with genes arranged in a linear array in the chromosomes.

Then, toward the end of the lecture, he added the following comment:

The detailed mechanism by means of which a gene or a virus molecule produces replicas of itself is not yet known. In general the use of a gene or virus as a template would lead to the formation of a molecule not with identical structure but with complementary structure. It might happen, of course, that a molecule could be at the same time identical with and complementary to the template on which it is moulded. However, this case seems to me to be too unlikely to be valid in general, except in the following way.
If the structure that serves as a template (the gene or virus molecule) consists of, say, two parts, which are themselves complementary in structure, then each of these parts can serve as the mould for the production of a replica of the other part, and the complex of two complementary parts thus can serve as the mould for the production of duplicates of itself
[emphasis added].

As we shall soon see, had Pauling remembered his own pronouncement four years later when he was trying to determine the structure of DNA, he might have avoided making a terrible blunder.