About 200 years ago the German poet and philosopher J. W. Goethe noted that Nature is not only a great artist but also a skillful master. The contemporary generation of scientists who work in the field of molecular biology can appreciate the external beauty of nature, the internal perfection of biological structures and physicochemical processes taking place in nature, and the enormous difficulty of studying them.

Modern molecular biology faces extremely complicated experimental problems. Proteins, biological membranes, nucleic acids, polysaccharides, and other ingredients of a biological cell interact, form sophisticated structures, and accomplish numerous catalytic, regulatory, and other functions. Many of the specific problems to be solved arise in the study of these systems.

In investigations of biological systems, one uses a broad arsenal of physical and chemical methods. Of particular importance in this arsenal is the approach of selective modification of biological objects with various labels capable of providing information on their structure, molecular dynamics, and mechanisms of actions. The necessity of such an approach is caused by the specificity of biological systems. Rather than seek complete information, a researcher usually aims to learn the main structural and dynamic properties important in the functional activity of a system.

These days the method of physical labeling is used to solve many structural problems in biophysical and biochemical laboratories all over the world. The most popular methods use spin and fluorescent labels and probes. The achievements in this field in the 1970s have been summarized in a number of monographs and reviews.