Concepts for detection of extraterrestrial life/Chapter 8
Optical Rotation
To determine the presence of life on other planets through remotely controlled instruments, two factors must be considered. First, we postulate that life on other planets has a chemistry similar to ours. This is an obvious first guess and may well prove to be approximately correct. As was shown in the introduction, there is reason to believe that the chemical events occurring on the primitive Earth also occurred on other planets of the solar system. Secondly, what is life and how do you know when you have found it?
“This,” says Dr. Ira Blei, “brings us to the heart of the problem of the search for life on other planets.” Dr. Blei, of Melpar, is the scientist in charge of the optical rotatory experiment for NASA.
How can one design single experiments which will provide enough information to permit a decision to be made concerning the existence of biologically significant molecules? Life has become very difficult to define in just a few words. The highest form of life on Earth, man, is a collection of very complex molecules having certain life-like properties associated with them. At the other end of the scale are the many types of simple chemical substances—sugars for example—which are obviously not alive. Further, somewhere between the two extremes are systems which are sometimes “alive” and sometimes not: the viruses.
So, we may look for a substance or property which is common to all life. One measurable property which has consistently been found in all living systems is optical rotation. A substance is said to possess optical activity when a “flat ribbon,” or plane wave, of light (polarized) passing through this substance is twisted, or rotated, so that the flat ribbon of light emerges in a new plane.
This ability to rotate the plane of polarized light is associated with molecular structure in a unique manner, just as the absorption spectroscopic characteristics are unique. Not all materials are capable of rotating the plane of polarized light; however, nucleic acids, proteins, and carbohydrates, all associated with life, do.
To measure the ability of a substance to rotate plane polarized light, it is necessary to place the substance between two polarizing filters. Plane polarized light passes through the substance into the second filter, which has been rotated so that no light can penetrate. Now, if the natural material can cause the plane ray to twist, some light will begin to leak through the second, or analyzer, filter. To restore the initial condition of the incident light leak, the analyzer must be rotated through a certain angle. The extent to which the analyzer is rotated is the measure of net optical rotation. Figure
Figure 12.—Optical system for optical-rotatory device.
12 shows the general layout of the optical component of the optical rotation device.
An important feature of optical rotation is that it is thousands of times more sensitive near a spectroscopic absorption band of the substance than at wavelengths removed from it. Such biological molecules as nucleic acids and aromatic amino acids maximally absorb in the 2600 Å and 2800 Å regions, respectively. Organic compounds formed by chemical synthesis generally consist of mixtures which rotate light in opposite directions and thus neutralize each other. It is a characteristic property of living things to select and synthesize forms which rotate polarized light in one direction.