The Alpha Helix

Space-filling model of the alpha helix.

[The Paulings in England: Part 5 of 5]

It has been said that sometimes blessings come in disguise, and so it may be that we have the damp English spring to thank for the elucidation of the alpha-helix structure of alpha-keratin – a fundamental and ubiquitous secondary structure pattern found in many proteins.

Linus Pauling was plagued by sinusitis for much of his time in England, and for three days in March 1948 it had become severe enough to put him in bed (as he was fond of saying over the years, this was before his vitamin C days). After a day spent devouring mystery novels, Pauling asked Ava Helen if she would bring him some paper and his slide rule, at which point he started trying to figure out how polypeptide chains might fold up into a satisfactory protein structure.

Pauling’s canvas was just an ordinary 8 1/2 by 11 inch sheet of paper. His first step was to draw the correct bond angles and distances onto the sheet, as determined from previous x-ray crystallographic work on polypeptides. Next he folded the sheet along parallel lines into a sort of squared-off tube. Doing so allowed him to add in representations of hydrogen bonds, which the impromptu model suggested would form between amino acid residues and, as a result, hold the turns of the polypeptide together.

The model made sense and pretty quickly it was clear that Pauling had discovered something important.  As he later wrote, his folded creation “turned out to be the structure of hair and horn and fingernail, and also present in myoglobin and hemoglobin and other globular proteins, a structure called the alpha-helix .”

Reconstruction of the alpha-helix paper model. Drawn and folded by Linus Pauling, 1982.

Pauling kept this idea to himself until his return to the United States because something didn’t match up quite right with the current laboratory data. Specifically, the turns of Pauling’s helix didn’t mirror the 5.1 angstrom repeat found in all of William T. Astbury‘s x-ray patterns. Pauling’s structure came close, but made a turn every 5.4 angstroms, or every 3.7 amino acid residues.

After his return home, with the assistance of colleagues Robert Corey and Herman Branson, Pauling continued refining his alpha helix structure and developing others, including the beta sheet. Simultaneously, the Caltech group’s chief British rivals at the Cavendish Laboratory published a paper titled “Polypeptide Chain Configurations in Crystalline Proteins.” The paper promised more than it delivered though, and while it listed many possible structures, Pauling found none of them to be likely. The competition was still on.

Pauling was finally convinced to publish when he received word that a British chemical firm called Courtaulds had created a synthetic polypeptide chain that showed no sign of Astbury’s 5.1 angstrom reflection in x-ray diffraction images. This was enough evidence for Pauling to decide that the 5.1 angstrom repeat was, perhaps, not a vital component of all polypeptide chains.  And so it was that in April 1951 Pauling, Corey and Branson published “The structure of proteins: Two hydrogen-bonded helical configurations of the polypeptide chain,” in the Proceedings of the National Academy of Sciences.

After devouring the Pauling group’s results shortly after their publication, Max Perutz headed to the Cavendish lab at Cambridge to check the data himself. Having confirmed the structure in images of horsehair, porcupine quill, synthetic polypeptides, hemoglobin and, for good measure, some old protein films that had been tucked away, Perutz wrote to Pauling, “The fulfillment of this prediction and, finally, the discovery of this reflection in hemoglobin has been the most thrilling discovery of my life.” He then published an analysis of his own data, concluding, “The spacing at which this reflexion appears excludes all models except the 3.7 residue helix of Pauling, Corey and Branson, with which it is in complete accord.”

Video Link: Pauling Recounts His Discovery of the Alpha Helix

It wasn’t until a year later that the mystery of Astbury’s 5.1 angstrom reflection was finally solved. In 1952, on a visit to the Cavendish, Pauling met Francis Crick, the then-graduate student who would go on to play a huge part in the discovery of the structure of DNA. The two maintained similar interests and during a taxi ride around Cambridge found themselves discussing the matter of the alpha helix. “Have you thought about the possibility,” Crick asked Pauling, “that alpha helixes are coiled around one another?” Whether Pauling had or had not considered this possibility remains a point of contention, but Pauling remembered replying that he had, because he had been considering a number of higher-level schemes for his helixes, including some which wound around each other.

Regardless, Pauling returned to Caltech and both he and Crick set to work on the problem. With help from Corey, Pauling discovered a means by which the alpha helixes could wrap around each other in a coiled-coil to produce the problematic 5.1 angstrom found in Astbury’s pictures of natural keratin.  Crick, in the meantime, was conducting a very similar study.  Pauling and Crick, independent of one another, ultimately submitted the solution to this puzzle for publication within days of each other, and at first there was a bit of grumbling as to whom the credit should be awarded. Though Crick’s note was published first, the Cavendish camp eventually conceded that Pauling’s paper included considerably more detail of consequence, and it was finally settled that both scientists had independently come to the same general conclusion.

Pauling receiving his honorary degree from the University of Paris, 1948.

After Pauling’s two fruitful terms as Eastman Professor at Oxford were up in July, the family split their remaining time between travels in Amsterdam, Switzerland and Paris. Pauling rounded off the trip by receiving yet another honorary degree from the University of Paris, and on August 25, 1948, the Paulings set sail once more on the Queen Mary.

His eight months in Europe had been productive and enlightening, but Pauling was ready to return to Pasadena where he could share the myriad ideas he had generated and gathered during his time away from Caltech. As we have seen, he was especially eager to get back to work on proteins, writing shortly before his departure that “I have continued to work on my theory of metals, and have been doing nothing about proteins. However, I am looking forward to being back home, and to thinking about that subject again.”


An Era of Discovery in Protein Structure

Linus and Ava Helen Pauling, Oxford, 1948.

[The Paulings in England: Part 4 of 5]

Though metals were consuming a good portion of his time during his fellowship at Oxford, Linus Pauling’s other projects never strayed far from his thoughts.  High on the list were the mysteries of proteins, whose structures and functions were slowly starting to be unraveled.

Pauling’s interest in proteins was spurred in the mid-1930s when the Rockefeller Foundation began to look most favorably upon the chemistry of life when deciding where their grant money would go. Early on, Pauling set out to tackle hemoglobin and though his affair with the molecule lasted for the remainder of life, Pauling certainly didn’t limit himself to the study of just one protein.

At a time when most were looking at proteins from the top down, trying to sort out the complicated data produced by an x-ray diffraction photograph of an entire protein, Pauling was working from the bottom up, in the process determining the structures of individual amino acids – the building blocks of proteins.

A specific protein that kept coming back into view over the years was keratin. In the 1930s, the English scientist William Astbury had studied the structure of wool, which along with hair, horn, and fingernail is made up primarily of this enigmatic protein, keratin. Astbury proposed that the structure was akin to a flat, kinked ribbon, but Pauling disagreed. “I knew that what Astbury had said wasn’t right,” Pauling recalled, “because our studies of simple molecules had given us enough knowledge about bond lengths and bond angles and hydrogen-bond formation to show that what he said wasn’t right. But I didn’t know what was right.” Pauling attempted to construct a model at the time, but could not match his structure to the measurements dictated by Astbury’s blurry x-ray diffraction images. Pauling wrote the project off as a failure and continued pursuing other interests.

In 1945 Pauling found himself seated next to Harvard medical Professor William B. Castle on a railroad journey from Denver to Chicago. Castle was a physician working on the nature of sickle cell anemia and the conversation that he shared with Pauling planted a seed in Pauling’s mind about the cause of this debilitating disease.

In the bodies of those suffering from sickle cell anemia, red blood cells assume a sickled shape when they are in the deoxygenated venous system but retain their normal flattened disk shape in the oxygen-rich arterial system. Noting this, Pauling suggested that perhaps the source of the problem could be a defect in the oxygen-carrying protein itself: hemoglobin.

Amidst his travels in Europe, Pauling continued to act on this idea as maestro from afar, directing the scientists in his Caltech laboratory to continue searching for differences in the hemoglobin of normal and sickled cells. In the meantime, he sought out and communicated new ideas gleaned from meetings such as the Barcroft Memorial Conference on Hemoglobin, held at Cambridge in June 1948. Pauling’s research team, in particular Harvey Itano and S. Jonathan Singer, were able to show experimentally that his hunch had been right, and less than a year after his return to Pasadena a paper was published that established sickle cell anemia as the first illness to be revealed as a truly molecular disease.

Linus and Peter Pauling at the model Bourton-on-the-water, England. 1948.

While in England, Pauling had occasion to interact closely with a number of scientific greats.  Among these were his close friend Dorothy Crowfoot Hodgkin, who is credited as a pioneer in the development of protein crystallography and was the winner of the 1964 Nobel Prize for Chemistry.  Likewise, Pauling conversed with Max Perutz, a protege of Sir William Lawrence Bragg‘s at the Cavendish Laboratory at Cambridge, who would go on to discover the structure of hemoglobin and receive the Nobel Prize for Chemistry in 1962.  While fruitful in many respects, these interactions served to increase Pauling’s feelings of urgency as concerned the race to determine the structure of proteins.

Bragg shared the 1915 Nobel Prize in Physics with his father for their early development of X-ray crystallography, and though there existed a long-standing scientific rivalry between Pauling’s and Bragg’s laboratories, it wasn’t until Pauling saw, with his own eyes, the work that was being done that he admitted he was “beginning to feel a bit uncomfortable about the English competition.” As he wrote to his colleague Edward Hughes back at Caltech

It has been a good experience for me to look over the x-ray laboratory at Cambridge. They have about five times as great an outfit as ours, that is, with facilities for taking nearly 30 x-ray pictures at the same time. I think that we should expand our x-ray lab without delay.

This realization prompted Pauling to get researchers in his lab started on work with insulin – an arduous and complicated process that required sample purification and crystallization prior to x-ray investigation. In relaying research findings from English scientists working on insulin to his partners back in Pasadena, Pauling intimated that

It is clear that there is already considerable progress made on the job of a complete structure determination of insulin. However, there is still a very great deal of work that remains to be done, and I do not think that it is assured that the British school will finish the job. I believe that this is the problem that we should begin to work on, with as much vigor as possible, under our insulin project.

Little did Pauling know that, while laying in bed, using little more than a piece of paper, a pen and a slide rule, he would soon make a major breakthrough in protein chemistry on his own.

The Passport Imbroglio

Ava Helen and Linus Pauling's passport photo. 1953.

Ava Helen and Linus Pauling's passport photo. 1953.

A quick glance at the “Today in Linus Pauling” widget found at the top of the left sidebar of the Pauling Blog gives an excellent representation of the span and influence of Linus Pauling’s career. Rarely does a day go by where he didn’t write at least one manuscript or give a speech at a university or some other institution. Most days, readers will also note that he won some sort of award – including, of course, his two Nobel Prizes in chemistry and peace. Basically, Pauling’s career fits very well with the old cliché that anything can be done if the mind is simply set on it.

However, if one looks closely enough, a few failures can still be picked out of Pauling’s illustrious career. One of these failures is undoubtedly his attempt at determining the correct primary structure of DNA. Pauling first started working with DNA in the early 1950s, right around the time when his scientific career was reaching its peak. During this time, Pauling’s pursuits had also taken a controversial political shift – work which caused him to be denied a passport for a short period of time. This passport denial, because it is believed by many to be the reason why Pauling was beaten to the structure of DNA, is the topic of today’s post.

Near the end of 1951, Pauling received an invitation to attend a meeting of the Royal Society in England; a meeting that was specially designed for him to address questions about his protein structures. The meeting was scheduled for May 1, 1952, and promised to give Pauling an opportunity to visit King’s College in London, where he knew Maurice Wilkins had some excellent X-ray patterns of DNA.

However, when Pauling sent in his passport renewal application in January 1952, he was upset but unsurprised to find it denied by Ruth B. Shipley, the head of the State Department’s passport division. Shipley didn’t give Pauling a good reason for the denial, stating only that “the Department is of the opinion that your proposed travel would not be in the best interests of the United States.” Reading between the lines, Pauling’s liberal views had clearly earned him the label of “possible Communist,” and Shipley, who was a fervent anti-Communist, had the authority to deny passports at her discretion.

Video Link: Pauling discusses his reaction to the refusal of his passport.

Fortunately for Pauling, the delay caused by the situation was not a long one. In the summer of 1952, he sent in another passport application. Again, Shipley immediately denied it, but her decision was overruled – after much deliberation – by higher-level employees of the State Department. Eventually, Pauling was notified that he would be granted a limited passport to travel for a short period of time in England and France if he agreed to sign an affidavit stating that he wasn’t a Communist. Surprised and pleased by the news, Pauling immediately agreed and received his new passport within days.

Thus equipped with the necessary papers, Pauling traveled to England, where he stayed for a month. He visited the same places and talked with the same people that he would have earlier in the year, but he did not visit King’s College to view Wilkins’ X-ray data. As it turns out, Pauling wasn’t even thinking about DNA during his time in England.

After England, Pauling traveled to France, where he learned of the results of the Hershey-Chase blender experiment:  DNA was in fact the site of the gene, not proteins, as Pauling had believed. Upon learning of the keen importance of DNA, he decided that he would solve the structure of the molecule.

However, when he returned to Caltech in September of 1952, he continued to work almost exclusively with proteins. It wasn’t until November that Pauling would finally take a serious stab at the structure of DNA. And, as has been well-documented, even with his excellent knowledge of structural chemistry, Pauling’s data – presented in the form of blurry X-ray patterns created by William T. Astbury – was insufficient. He ended up creating a model that was nearly identical to one Watson and Crick had made over a year earlier. Of course, Pauling soon learned that his structure was incorrect, and before he could make another attempt, Watson and Crick had solved DNA.

The importance of Pauling’s passport imbroglio is, as it turns out, counter to the popular mythology of the DNA story. Although the denial of Pauling’s passport caused minor delays in his travels, it surely did not keep him from determining the structure of DNA. Even if he had traveled to England as originally planned, it is unlikely that he would have visited Wilkins to view his X-ray data. Pauling, even after finding out that DNA was extremely important, made no effort to obtain better data, nor did he even work specifically with DNA for quite some time. One is forced to conclude then, that the reason that Linus Pauling was not able to solve DNA is that he never really put his mind to the matter, not because of a pesky passport denial that delayed his travels a mere ten weeks.

For more information on Linus Pauling’s DNA pursuits, please visit the website Linus Pauling and the Race for DNA: A Documentary History. For other information on Pauling, check out the Linus Pauling Online portal.

The X-Ray Crystallography that Propelled the Race for DNA: Astbury’s Pictures vs. Franklin’s Photo 51

Rosalind Franklin, March 1956

Rosalind Franklin, March 1956

During their so-called race to discover the structure of DNA, Linus Pauling and the unlikely pair of James Watson and Francis Crick utilized remarkably similar approaches in attempting to solve the riddle of the genetic material. In fact, one of the main tactics used by Watson and Crick was to approach the problem in the same manner that they assumed Pauling would. Although Pauling and Watson and Crick did, at one point, come up with nearly identical, yet incorrect, structures, it was Watson and Crick who would eventually solve DNA. Why then, if the pair were thinking like Pauling, were they able to beat him to the structure?

Although there were a variety of reasons behind Watson and Crick’s success, a good portion of it can be attributed to the relative superiority of resources available to them. Watson and Crick obviously had each other to keep themselves in check, but they also benefited from other voices of criticism such as Rosalind Franklin, Maurice Wilkins, and later Jerry Donohue. Linus Pauling also shared his ideas with his colleagues, but none of them were very familiar with DNA, and therefore couldn’t offer much feedback. (And they were largely ignored even when they did offer criticisms of Pauling’s structure.)

Another vital resource available to Watson and Crick was an excellent X-ray crystallography pattern, the famous photo 51, taken by Rosalind Franklin. Although, in all likelihood, Pauling could have also viewed Franklin’s photographs had he tried, he settled on using blurry patterns published by William T. Astbury several years before Franklin’s superior images. These X-ray photographs are the main topic of today’s post. In particular, the factors accounting for the difference in quality between Franklin’s and Astbury’s patterns will be discussed. Before delving into this subject, however, a brief overview of X-ray crystallography is necessary.

William T. Astbury, ca. 1950s.

William T. Astbury, ca. 1950s.

X-ray crystallography, also sometimes known as X-ray diffraction, is used to determine the arrangement of atoms within a crystalline molecule. It is a rather complicated procedure, and the photos taken in the process can be interpreted only by a person with significant training. The steps to obtaining these photos are as follows.

First, an adequate crystal must be obtained. This is a very difficult step because the crystal must be large enough to observe and also sufficiently uniform. If it does not meet these specifications, errors – such as blurriness – will occur, often rendering the resulting crystallographic patterns useless, at least for purposes of determining atomic arrangement.

After an adequate crystalline specimen is obtained, a beam of X-rays is shined through it. When the beam strikes the electron clouds of the atoms in the crystal, it is scattered. These scattered beams can then be observed on a screen placed behind the crystal. Based on the angles and intensities of the scattered beams, a crystallographer can create a three dimensional picture of the electron density of the crystal.

Finally, from the electron density information, the mean positions of the atoms within a crystal can be determined, and the structure of the molecule can be considered “solved.” That said, just one image is not nearly enough to determine the structure of an entire crystal. Therefore, the crystal must be rotated stepwise through angles up to and even slightly beyond 180 degrees, depending on the specimen. Patterns are required at each step, and complete data sets may contain hundreds of photos.

Clearly, because the process of X-ray crystallography is so cumbersome, there are many opportunities for mistakes that may have led to the poor quality of Astbury’s photographs. However, Astbury’s techniques seem to have been excellent. He was a very experienced crystallographer, and had achieved great success in his earlier work with X-ray diffraction on substances such as keratin.

As it turns out, Astbury’s photos were of poor quality because of the DNA sample he was using. In the early 1950s, Rosalind Franklin had discovered that DNA came in two forms – a dry condensed form and a wet extended form. Astbury’s DNA sample was well prepared from calf thymus, but it contained a mixture of the two forms. This turned out to be the major reason why Astbury’s photographs were so blurry

Astbury's images, 1947. Plate 2.

Astbury's images, 1947. Plate 2.

It is important to note that, even if Astbury had known he was using a poor crystalline sample of DNA, he probably still wouldn’t have been able to compete with the quality of Franklin’s photos. In 1950, three years after Astbury’s images were published, Maurice Wilkins developed a way to obtain much better X-ray patterns of DNA through the use of a solution of sodium thymonucleate. This solution is highly viscous, and Wilkins found that thin strands could be drawn out by gently dipping a glass stirring rod into a sample and slowly pulling it out. These thin strands were pure DNA, and Wilkins was able to get excellent X-ray patterns from them.

Before long, Wilkins had also acquired better equipment and had also hired Rosalind Franklin to run it. Franklin, essentially working independently, used the same basic technique developed by Wilkins. She did, however, add several of her own smaller experimental refinements, which made the photographs even better. Eventually, she developed photo 51, which would later be shown to Watson and Crick. The rest, as they say, is history.

Rosalind Franklin and William Astbury were both excellent crystallographers, but Franklin’s experience with DNA gave her a clear advantage when working with the molecule. Her brilliant X-ray patterns would later prove to be a major determining factor in the “race for DNA”. For more information on DNA, please visit the Race for DNA website. For much more on Linus Pauling, check out the Linus Pauling Online portal.

The Pauling-Corey Structure of DNA

Today, the structure of DNA series is continued with the model proposed by Linus Pauling and Robert Corey in 1953. As a result of insufficient data and an overloaded research schedule, Pauling’s structure turned out to be incorrect. However, it is interesting to see the ways in which one of the world’s leading scientists went wrong with his approach to the structure of this hugely-important molecule.

Linus Pauling played around with nucleic acids as early as 1933 when he hypothesized a structure for guanine, a base ring. In the summer of 1951, he again became interested in DNA when he heard that Maurice Wilkins at King’s College had developed a few good photographs of nucleic acids. Unfortunately for Pauling, Wilkins was unwilling to share his research. In November of that same year, a structure of nucleic acids was proposed and then published by Edward Ronwin. Pauling could tell almost immediately that Ronwin’s structure wasn’t correct, but it did contain a few good ideas that got him thinking about other possible structures. Pauling hypothesized that DNA was likely helical in shape, with the large base groups facing out and the phosphate groups stacked in the core. At this juncture, however, Pauling was again distracted by other research and let the project drop.

Until 1953 nucleic acids weren’t considered to be very important. At the time, proteins, rather than DNA, were considered by most scientists to be the carriers of genetic material. Partly because of this, Pauling’s attention was focused on proteins, not DNA. In May of 1952, Pauling was scheduled to attend a special meeting of the Royal Society where he would address questions pertaining to his protein structures. This trip would also give him an opportunity to discuss DNA with Rosalind Franklin, who was Maurice Wilkins’ assistant. She had recently developed an especially clear photograph of DNA which likely would have saved Pauling from making some key mistakes when determining the structure of DNA.

As a result of his very-public anti-war and anti-nuclear activities, Pauling’s initial request for a passport was denied, though he was granted a limited passport only ten weeks later. However, when Pauling arrived in England, he did not visit King’s College. He was preoccupied with his protein research and he assumed that Wilkins still wouldn’t be willing to share his data.

Soon after his visit to England, Pauling was granted a full passport and traveled to France. Here he was informed, through an experiment performed by Alfred Hershey and Martha Chase, that DNA was in fact the genetic master molecule. Upon learning this, Pauling decided that he would solve the structure of DNA. However, when he returned to California, he continued to work primarily with proteins. It wasn’t until November 25, 1952 that Linus Pauling would make a serious attempt at the structure of DNA.

Unfortunately, when Pauling did decide to put in some time with DNA, he still had insufficient data to correctly deduce its structure. Using only a few blurry x-ray patterns done by William Astbury in the 1930s and a photograph published by Astbury in 1947, Pauling decided that DNA was indeed a three-chain helix with the bases facing outward and the phosphates in the core.

Astbury's 1947 photographs of DNA.

Astbury's 1947 photographs of DNA.

However, it was immediately clear that making room for so many phosphates in the center of the molecule would be quite a task. Pauling spent a great deal of time manipulating his model, and eventually produced a satisfactory representation. He then asked Robert Corey, his chief assistant at Caltech, to perform detailed calculations on the proposed atomic positions. Corey’s calculations proved that, despite Pauling’s efforts, there still wasn’t enough room for all of the atoms. Pauling, refusing to consider the possibility that his structure was incorrect, resorted to further manipulation. (In fact, Pauling refused to concede even after a colleague pointed out that there was no room for sodium ions in the core of his model, a feature that is essential in the creation of sodium salts of DNA.) Convinced that the finer details would later fall into place, Pauling and Corey spent the last week of the year writing up their structure, and on the last day of 1952, they submitted “A Proposed Structure for the Nucleic Acids” to the Proceedings of the National Academy of Sciences.

Diagram of the Pauling-Corey structure for DNA, as published in PNAS.

Diagram of the Pauling-Corey structure for DNA, as published in PNAS.

The paper was uncharacteristic of Pauling. Instead of his usual confidence, he stated that the structure was “promising” but also “extraordinarily tight.” Pauling likewise noted that the model accounted only “moderately well” for the x-ray data, and that the atomic positions were “probably capable of further refinement.” As it turned out, Pauling wasn’t seeking perfection with his structure. In reality, he wanted to be the first to publish a roughly correct structure of DNA. Rather than having the final say, he wanted the first.

Once the article was published in February of 1953, it became more and more apparent that Pauling’s structure wasn’t even roughly correct. By this time, Pauling had already moved on to other projects, and was surprised at the fact that his paper was received so poorly. Once he caught wind of the talk surrounding his structure, he decided to return to the topic of DNA. Despite the negative reaction, Pauling still believed that his structure was essentially right. However, he soon received better nucleotide samples from Alex Todd, an organic chemist at Cambridge, and began a more rigorous approach to determining the structure of DNA.

Unfortunately, by this time it was too late. Upon the publication of Pauling’s unsatisfactory model, James Watson and Francis Crick were given the green light to pursue their own model of DNA. Before long, Pauling saw that the work they were doing was very promising. A few days after first seeing their structure, Pauling received an advance copy of the Watson and Crick manuscript. At this point, he still retained a fair amount of confidence in his own model, but acknowledged that there was now another possible model. In a letter to Watson and Crick written on March 27, 1953, Pauling noted

I think that it is fine that there are now two proposed structures for nucleic acid, and I am looking forward to finding out what the decision will be as to which is incorrect.

However, he had still not seen Rosalind Franklin’s data; Watson and Crick had. (Interestingly enough, Robert Corey had traveled to England in 1952 and viewed Franklin’s photographs. It is unknown whether or not he purposely failed to provide Pauling with the details of the images.)

This fact would soon change. In April of 1953, Pauling was to attend a conference on proteins in Belgium. On his way, he stopped in England to see the Watson and Crick model of DNA as well as Franklin’s photographs. After examining both, Pauling was finally convinced that his structure was wrong and that Watson and Crick had solved DNA.

Linus Pauling, although disappointed with the results, accepted his defeat graciously. He gave Watson and Crick full credit for their discovery and assisted them in tying up a few loose ends with their model. For Pauling, this event was a single failure in a sea of successes. In fact, the very next year, he would win the Nobel Prize in Chemistry – the first of his two Nobel Prizes. Despite his embarrassing mistakes, Pauling was to remain in good standing with the scientific community.

Please check back on Thursday for the conclusion of the DNA structure series – an examination of the correct structure deduced by Watson and Crick. For more information on DNA, please visit the website Linus Pauling and the Race for DNA. For more information on Linus Pauling, visit the Linus Pauling Online Portal.