Page:Concepts for detection of extraterrestrial life.djvu/38

28 in the presence of very low macromolecule concentrations and variations in the reference (dye band) are negligible. Thus, the experiment has been referred to as the “J-band life-detector.” This title is convenient because of its brevity, but it focuses attention on only one aspect of the method. The program is concerned not only with the J-band, but also with other alternations of the dye spectrum which result from the interaction of the dye with macromolecules.

The maxima which appear, and their exact wavelength, are functions of the macromolecule structure and the nature of its functional groups. Thus, for example, interaction of the dye with native deoxyribonucleic acid (DNA) produces a single peak at 575 mμ, whereas its interaction with denatured DNA causes a single peak at 540 mμ. On the other hand, proteins may produce multiple bands which occur in the 650-, 575- and 480-mμ regions of the spectrum. With proteins, a band can always be produced in the 650-mμ region of the spectrum; the exact wavelength of this band appears to be a function of the nature of the protein. Variables, such as change in acidity or alkalinity (pH) and temperature, cause changes in the absorption spectra of macromolecule-dye complexes which are related to the nature of the macromolecule. In general, the method is sensitive to 0.1 to 1 microgram of macromolecule per ml, and the absorbance of the associated complex is proportional to the concentration of the macromolecule.

The positions of the new absorption maxima appear to be a direct property of the spacing of the functional groups on the macromolecule, the rigidity of the macromolecule structure and the sequence of the anionic and cationic sites. The effects produced by changes in temperature and pH are apparently associated with modification in the folding and coiling of the macromolecule, the dye-macromolecule equilibrium and the ionizability of the functional side groups. Thus, by observing the spectral changes which occur when this dye interacts with a macromolecule and by appropriate manipulations of environmental variables, it is possible to detect trace amounts of macromolecules, distinguish between macromolecules which are difficult to differentiate by conventional methods and obtain information about the structure of macromolecules and estimate the macromolecule concentration.

It is expected that on Mars the experiment will be carried out in the following manner: The test capsule will acquire a soil sample, extract the macromolecules and mix them with the dye solution (sample solution and preparation). The absorbance of the dye-macromolecule mixture will then be determined.

In laboratory tests, a number of soil samples have been analyzed for macromolecules by the scheme indicated for the Mars experiment. In each case.