Page:Concepts for detection of extraterrestrial life.djvu/36

26 Now, if we study the absorption of electromagnetic energy by proteins, it is seen that a portion of the spectrum in the far ultraviolet region (around 1950 Å) is characteristically absorbed by the protein. Further study shows that the particular vibration in resonance with the ultraviolet is somewhere in the peptide bond. A tripeptide absorbs twice as much as a dipeptide. A polypeptide with 100 bonds (101 amino acids) absorbs one hundred times as much energy as a dipeptide. As a protein is hydrolyzed we see that the absorption of ultraviolet light decreases, and when it is completely hydrolyzed and all the peptide bonds are broken, there is no more absorption of this region of the ultraviolet.

Unfortunately, peptides are not the only substances that absorb in this region, and some confusion might result. A study of the absorption by other substances shows, so far, that hydrolysis does not affect it. Thus, it can be said for the present that if the substance absorbs far ultraviolet before and after hydrolysis, it is not a peptide, but if hydrolysis reduces the absorption, it may be a peptide.

If further research warrants it, a small, rugged spectrophotometer will be built. The instrument will be able to collect a sample of Martian soil, treat it with solvents and place a portion in each of two quartz vessels. One sample will be hydrolyzed and the other will not. Then the instrument will take spectrophotometric readings of both samples. If the readings are different, a message will be sent to scientists on Earth that peptides exist on Mars.

The ultraviolet absorption spectra of three different concentrations of alanylglycylglycine at pH 1 are shown in figure 9. Figure 10 illustrates the ultraviolet absorption spectra for sodium hydroxide extracts of soil (ratios are amounts of NaOH to H$2$O in extracting solutions).