In addition to listing of the concentrations of all compounds included in each problem, the results of three-element problems have been expressed on a triangular composition diagram for convenience. A coarse grid of 60 points is used to survey all elemental compositions, with finer grids being used in regions of particular interest. The calculated concentrations of the compounds at each composition are stored, and finally a series of triangular diagrams is printed out, each showing the concentrations of as many as four compounds at the grid points.
Figure 2 shows the results obtained in the C, H, and O systems. Organic compounds in concentrations greater than 10-20 mole fraction are found everywhere except where free O2, is present. Solid carbon theoretically becomes stable along the lower dashed line at 500° K. However, reactions producing it are very slow. The supersaturated region beyond the line of potential carbon formation was also investigated. A threshold was found where polynuclear aromatic compounds are sufficiently concentrated to form a liquid phase. These conditions may have been involved in the primordial formation of asphaltic petroleum.
Figure 2.—Equilibrium diagram for the system C-H-O.
Jukes and associates ([ref.154]) at the University of California at Berkeley have been investigating the code for amino acids in protein synthesis, the key for translating the sequence of bases in DNA into the sequence of amino acids in proteins. The amino acid code was solely a matter of theory until Nirenberg and Matthaei ([ref.155]) at the National Institutes of Health carried out a crucial experiment. This experiment bridged the last remaining gap separating theoretical genetics and test-tube biochemistry. It now became experimentally possible to search for codes for all 20 amino acids concerned in the synthesis of proteins.
The amino acid bases of DNA are: A, adenine; C, cytosine; G, guanine; T, thymine; and U, uracil, which replaces thymine in RNA. There are only 16 ways of arranging A, C, G, and T in pairs. For this and other reasons it is thought that a triplet of three consecutive bases is needed to code for each amino acid. The sequences of bases in a strand of DNA are known to be unrestricted with respect to the order in which they occur; apparently any one of the four bases can be next to any of the other four, although, of course, each base must be paired with the corresponding complementary base in the adjacent strand. Since the same freedom is true of the amino acid sequences in the polypeptide chains of proteins, any one of the 20 amino acids can occur next to any other. Moreover, the sequences in DNA are subject to mutational changes in which one base replaces another, or bases are added to or deleted from the DNA. Such rearrangements plus the possibility of lengthening of DNA molecules are numerous enough to account for all the genetics of living forms since the first appearance of life on Earth.
Most of our knowledge is based on experiments with synthetic RNA carried out with extracts of E. coli. The majority of the work has been at Nirenberg's laboratory at the National Institutes of Health and at Ochoa's laboratory at New York University ([ref.155]). Various combinations of A, C, G, and U were used in preparing the synthetic RNA molecules that are used in experiments to explore the code. These molecules are made by incubating a mixture of ribonucleoside diphosphates with a specific enzyme, polynucleotide phosphorylase. An important property of this enzyme is that it condenses the nucleoside diphosphates into polynucleotide strands containing random sequences depending on the proportion of each base. For example, if the enzyme were furnished with a mixture of 5 parts of A and 1 part of C, it would make strands containing, on the average, 25 sequences of AAA, 5 of AAC, 5 of ACA, 5 of CAA, and 1 each of ACC, CAC, and CCA. The proportion of triplets within the strands of a polynucleotide is reflected in the proportion of amino acids in polypeptides that are obtained in the cell-free system. Most of the present knowledge of the amino acid code is based on this concept. All the proposed codes have been discovered by this experimental approach where synthetic RNA molecules are used as "artificial" messenger RNA.
Representative of another class of activities in molecular biology is the examination of passive ion flux across axon membranes. This work is being done by Goldman at the National Naval Medical Center. The question of stimulus transmission by nerve tissue is far from simple, and the ion concentrations associated with nerve membranes is a significant part of the answer. Because the space environment may very well produce alterations in these ion potentials, an investigation of their natures and significance becomes extremely important. A working theory is now being developed as a result of this study.
Vital cell processes, chemical transformations, and mechanisms that provide energy for cell maintenance and activity have been studied by Kiesow (refs. [ref.157] and [ref.158]) at the Naval Medical Research Institute. The common objective of all phases of this project is the elucidation of reaction steps in which energy and matter are transformed in living systems. Compared with photosynthetic organisms, chemosynthetic bacteria offer distinct advantages for the study of energy assimilation. These studies have led to the following experimental findings.
With the energy from oxidation of nitrite, NO2— to nitrate, NO3— as an inorganic source, and with added organic chemical energy from the hydrolysis of adenosinetriphosphate (ATP) to adenosinediphosphate (ADP) and inorganic phosphate, chemosynthetic bacteria are capable of reducing diphosphopyridinenucleotide (DPN+) to DPNH, in a coupled oxidoreduction-dephosphorylation. Thus, in the crucial step of chemosynthesis, ATP is consumed, not produced. However, in simultaneously proceeding cell respiration, the energy donor, DPNH, is oxidized and generates more ATP than is required for DPN+ reduction. This "breeder cycle" for DPNH—with different ratios of cell respiration and biosynthesis—results in a net production of either DPNH, or ATP, or both. Production of DPNH in the cycle leads immediately to the assimilation of C14 from HC14O3—. These observations explain the bacteria's energy source without the classical hypotheses of either direct phosphorylation or direct CO2 reduction by inorganic chemical or electromagnetic energy. The cycle transforms the free energy of nitrite oxidation into the free energy of the organic compounds. Cell respiration and elementary biosynthesis proceed through structure-bound enzyme systems in the same fraction of subcellular particles. Three components, two cytochromes and one flavoprotein, have been identified. A thermodynamic analysis of the DPNH "breeder cycle" appears to be attainable by measurements of redox potentials and calorimetric determinations of heats of reaction.