Concepts for detection of extraterrestrial life/Chapter 6
The Ultraviolet Spectrophotometer
The use of ultraviolet spectrophotometry for detecting peptide bonds is being studied to determine whether it can be applied to the search for life on other planets. This program is under the direction of Dr. Sol Nelson at Melpar. Since all proteins contain peptide bonds, and since all living things on Earth contain proteins, the detection of extraterrestrial peptide bonds might be consistent with the presence of living things.
Before going any further, it would be worthwhile to explain what ultraviolet spectrophotometry is, and what proteins and peptides are.
Suppose a child is in a swing that is moving at a rate of two swings each second. We can say that the frequency is 2 cycles per second. If you stand behind him and push the swing at the same frequency (2 times per second), the swing absorbs the energy and its amplitude increases. Your pushing frequency is said to be in resonance with the swinging frequency. If you try to push at a frequency of 3 cycles per second, it is obvious that the swing will not absorb the energy as efficiently.
As another example, let us consider the case of sound. Picture a room with two pianos, one with a full set of strings and one with only one string—middle C. Now, if the keys of the complete piano are struck one at a time, you will note that the single string on the incomplete piano emits a sound only when middle C (or a note of which it is an overtone) is struck on the complete piano. Here, too, the single string absorbs energy and begins to vibrate when it is in resonance with the energy. Thus, striking a D does not cause the C string to vibrate because their vibration frequency is different.
Although the examples deal with mechanical energy, the same phenomenon occurs with electromagnetic energy which includes X-rays, ultraviolet, visible light, infrared, radar and radio waves. Each of these terms applies to a range of electromagnetic energy. The highest frequencies are in the X-rays, and the lowest frequencies are in the radio. Now, if we recall that matter consists of molecules, and that the molecules, their atoms and their electrons are always vibrating, then we can expect to find some of these vibrations to be in resonance with the vibrations of a portion of the electromagnetic spectrum.
If we want to study the absorption of ultraviolet light by substances, we need an ultraviolet spectrophotometer. This is an instrument that can separate ultraviolet light into bands of very narrow frequency ranges, and can measure the amount of ultraviolet passing through a substance. Thus, if we isolate a 
Figure 9.—Absorption spectra at pH 1; Alanylglycylglycine (tripeptide). 
Figure 10.—Absorption spectra of soil extract (NaOH). band of light and pass it through a substance and find that 100 percent of the light has passed through, we say that the ultraviolet was not absorbed. This means that the substance does not have vibrations of the same frequency as the ultraviolet. Now, if we pass a band of ultraviolet of another frequency and we find that only 10 percent of the light passes through the substance, then we know that the light was absorbed and that the substance had a vibration of the same frequency as that particular band of ultraviolet.
Proteins are large, complex molecules consisting of amino acids linked together in long chains that are folded into characteristic shapes. The amino acids are held together by peptide bonds, and their combination joined in such a fashion is called a peptide. A combination of two amino acid molecules is a dipeptide; three are a tripeptide; many are a polypeptide. A protein is a large polypeptide, or a combination of several polypeptides. When a protein is hydrolyzed, it is split into peptides, and these are then split into amino acids. (This is what happens in the stomach and intestine when proteins are digested.)
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 H2O in extracting solutions).