Energy Migration in Protein Molecules

Intermolecular migration of energy in biological systems has been established in a whole series of cases. A critical review of the data relating to this may be found in A. N. Terenin’s review[^1]. As for intramolecular migration of energy in molecules of biological significance, and above all in proteins, the few data available up to now could not be regarded as especially convincing. In one experiment[^2] the inactivation spectrum of the enzyme urease was measured. The latter consists of a protein component and an active, so-called prosthetic, group, and the absorption spectra of these two components differ from one another. Measurement of the action spectrum of inactivation showed that it is similar to the absorption spectrum, which means that the quantum yield does not depend on the wavelength of the radiation. These data were regarded by their authors as evidence for the transfer of energy absorbed by the protein to the prosthetic group, since for inactivation of the enzyme precisely this group must be affected.

In two other experiments[^3][^4] the dissociation spectrum of the carbon monoxide–protein complex was investigated. In the later work[^4] the CO₂–myoglobin complex was used. The enzyme myoglobin consists of a protein and a hemin group. Carbon monoxide binds to hemin and, consequently, for dissociation the energy must be transferred to this group. It turned out (as also in work[^3]) that, in the absorption interval of the radiation by the protein and by hemin, the quantum yield does not depend on wavelength.

Thus one could think that the energy absorbed by the protein is transferred to the hemin, in which the act of dissociation directly takes place.

A. N. Terenin and A. A. Krasnovsky[^5] pointed out that experiments on the dissociation of the enzyme–CO₂ complex do not prove the presence of migration of electronic-excitation energy from the protein to hemin, since the energy of such dissociation is so small (~15 kcal) in comparison with the energy of the absorbed quantum (~90 kcal) that dissociation may in fact be caused by the transition of part of the electronic-excitation energy into vibrational energy of the molecule. Such “internal heating” of the molecule, with the transfer of a noticeable fraction of the energy from one place in the molecule to another, is especially probable in molecules having a structure similar to that of protein molecules. In the opinion of the cited authors, transfer of the excitation quantum without substantial losses would be proved if it were possible to detect sensitized fluorescence of a molecule fixed on the protein under the action of light absorbed by the protein.

The paper reviewed[^6] reports the performance of such an experiment. The object of investigation was the blue chromoprotein phycocyanin (a compound of a protein with the chromophore phycocyan).

The experiment consisted in investigating the fluorescence action spectrum upon irradiating a solution of phycocyanin with ultraviolet rays in the range from 254 to 405 mµ. Comparison of this action spectrum with the absorption spectrum of phycocyanin showed that, in the indicated wavelength interval, the relative quantum yield of fluorescence, with an accuracy of up to 10%, does not depend on the wavelength of the exciting light. Separate recording of the absorption curve of the protein and of the chromophore showed that at 275 mµ about 46% of the absorption is due to the protein and the remaining 54% to the chromophore. The protein does not absorb at all in the region with λ = 320 mµ.

Thus, the data obtained indicate that rays absorbed by the protein cause fluorescence with the same efficiency as rays absorbed by the chromophore itself. Very important is the circumstance that the data obtained prove the transfer of large portions of energy, which in any case are not less than the energy of a quantum of red fluorescen-

of phycocyanin (the fluorescence maximum is at 610 mμ). It is very probable that practically the entire ultraviolet quantum is transferred to the chromophore, in which conversion into a red quantum takes place. On the other hand, it is also possible that only part of the energy of the ultraviolet quantum reaches the chromophore. Of considerably greater interest is to determine which of these possibilities occurs. However, this question cannot be resolved on the basis of the data obtained in the experiment under consideration.

L. B.

CITED LITERATURE

1. A. N. Terenin, UFN 43, 347 (1951).
2. F. Kubowitz, E. Haas, Biochem. Zeits. 257, 337 (1932).
3. O. Warburg, Angew. Chem. 45, 1 (1932).
4. T. Bücher, J. Kaspers, Biochim. et Biophys. Acta 1, 21 (1947).
5. A. N. Terenin, A. A. Krasnovskii, UFN 37, 65 (1949).
6. T. Bannister, Arch. Biochim. and Biophis. 49, No. 1, 222 (1954).