Detail of a large paper chart constructed of several pages taped together, handwritten in several colors of ink.

Preserving Nirenberg’s Genetic Code Chart

By Kristi Wright and Holly Herro

The National Library of Medicine is home to a series of very important documents in scientific history—Marshall Nirenberg’s Genetic Code Charts. The charts contain original, handwritten data from experiments that determined how protein sequence was dictated by the sequence of precursor ribonucleic acids (RNAs). Conservators at NLM have been studying the charts to determine the best methods of preserving them for the future.

The famous 1953 discovery of DNA’s double-helix structure, which incorporated the work of James D. Watson, Francis Crick, Maurice Wilkins, and Rosalind Franklin, was a major breakthrough in genetic research, but it was just the start of our understanding of DNA. What was not known then was nature’s genetic code; how a DNA sequence is translated into an RNA sequence that is in turn translated into a protein sequence. By 1965, Marshall Nirenberg, with help from his colleagues at the National Institutes of Health (NIH), had become the first to complete the sequencing of the code. The language of protein synthesis was understood, painstakingly worked out and recorded in these charts. As a result Nirenberg was awarded the 1968 Nobel Prize in Physiology or Medicine.

A large paper chart constructed of serveral pages taped together, handwritten in several colors of ink.
Nirenberg’s handwritten genetic code chart, 1965.
Profiles in Science

The Nirenberg charts consist of multiple sheets of 1950s Addison Wesley lined paper which has been joined with pressure sensitive tape. The tape is in poor condition, but it is an important part of the charts because Nirenberg and his lab technicians wrote on top of it. The writing was done in pencil, India ink, and ballpoint pen ink. The need to address the deteriorating state of the ballpoint pen ink on these unique documents became a driving force behind the most recent phase of our ongoing ballpoint pen ink research in the NLM conservation lab, in this case focusing on ideal rehousing methods.

In order to determine the correct housing for the Nirenberg charts, we first determined what type of pens were used. Norma Heaton, one of Dr. Nirenberg’s original lab technicians, told us that the genetic code charts were written with Skilcraft ballpoint pens. Next, we did some testing and researched the history of the Skilcraft pen ink and found the ink formula had not changed in forty some years. Then, we used a technique we’ve recently developed in-house called Photoshop Assisted Spectroscopy for the examination of different lighting and storage environments on this type of ballpoint pen ink. We set up a series of mock documents in a variety of enclosure types and exposed them to different lighting conditions for a period of three months. Before, during, and after the exposure period, we took scans of the documents and performed Photoshop Assisted Spectroscopy to see if and how the color of the ink had shifted.

Photoshop Assisted Spectroscopy is a technique to track color shift, which is a good indicator of deterioration in ink. Color shift is a change in the ratio of red, green, and blue (RGB) values in the color of a substance and occurs when the chemistry of a material, in this case of the ballpoint ink, changes over time and through reaction with it’s environment. To perform the technique, the document must first be digitized using equipment calibrated for color accuracy. The technique uses the ‘info’ window in Photoshop to retrieve the RGB values of individual pixels. By recording a series of these values, we are able to note the color of a document section at the start of the experiment. Later, a new scan is taken and the values for that section recorded again. The values from both scans are plotted on a 3D graph with the X, Y, and Z axes corresponding to each RGB value. Graphing in this manner allows the direction of any color shift across the collected data points to easily be seen.

A graph demonstrating a shift in value along the Z, or blue axis.
Values from before and after a test period are plotted on a 3D graph with X, Y, and Z axes corresponding to each red, green, and blue  (RGB) value respectively. This allows the direction of any color shift across the collected data points to easily be seen.

As a result of this study, we’ve determined that anoxic (oxygen-free) environments are ideal for documents written with blue and black Skilcraft brand ballpoint pens. We have therefore chosen to store the charts in an anoxic environment and, because we have the initial scans to create a starting point, we can monitor the charts long term as a preventive method to detect any color shift while they are housed in their enclosures.  We will also be working with the NIH Mechanical Design and Fabrication section to build an attachment that will allow us to monitor the environment inside the frames with existing equipment and ensure that the enclosures remain anoxic over time. Each frame will create a sealed microenvironment for the enclosed document, so while people will easily be able to view the charts, they will be protected against the damaging factors associated with a normal air environment and preserved for new generations of researchers.

This article is part of a series that commemorates the 50th anniversary of the Genetic Code Charts.

Kristi Wright is a contract conservator for the Conservation Program of the History of Medicine Division at the National Library of Medicine and principal of Wright Conservation and Framing.

Holly Herro is Conservation Librarian for the History of Medicine Division at the National Library of Medicine.

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