Page:Concepts for detection of extraterrestrial life.djvu/37

 CHAPTER VII

This experiment is being studied for NASA by Dr. R. E. Kay and Dr. E. R. Walwick at the Philco Research Laboratories and is designed for use on Mars.

Because of the probable evolutionary history of the Martian environment, it is believed that Martian life will be based on similar chemical constituents and evolutionary principles as life on Earth. On Earth, life resides only in systems which are composed of molecular aggregates (macromolecules) known as proteins, nucleic acids and polysaccharides. Therefore, it is reasonable to assume that the detection on Mars of macromolecules having properties similar to proteins, nucleic acids or carbohydrates, will provide some support for the view that life exists on the planet. When certain dyes interact with macromolecules, color changes (metachromasia) occur which can serve to identify and detect biological materials. The present experiments have been concerned with the changes produced in the absorption spectrum of a dibenzothiacarbocyanine dye when it interacts with trace amounts of biological macromolecules. The spectral changes occurring when this dye reacts with biological macromolecules are unique in regard to the diversity of the changes that occur and the large amount of information which can be deduced.

In this case, the interaction of the dye with biological macromolecules always produces an increase in absorbance at new maxima. There are seven different regions of the spectrum in which absorption maximum are found. These are located at approximately 450, 480, 508, 535, 560, 575, and 650 mμ. The peak in the 650-mμ region is referred to as a J-band, being named after E. E. Jelly who described it in detail. This absorption band is particularly interesting because, of the macromolecules which have been tested, only those of biological origin interact with the dye to produce this absorption band. This is indeed fortunate, since the J-band has properties which make it especially useful in a detection scheme. It lies almost entirely outside the absorption region of the normal dye absorption spectrum, and the absorption coefficient is extremely high. Because of this, an increase in absorbance in the J-band region occurs 27