М А Rizakhanov
Thermally stimulated luminescence of plant structures excited by light at 77 K was studied. The luminescence is an integral Z-band with maximum at Tmax ≈ 118 K. It is postulated that the Z-trap is a structurally complex dipole acting as an electron center with polyenergetic spectrum and two-level excitation in the red and blue ranges. It is represented by chlorophyll-protein complexes of the Chl- + Z1…Zj…Zm + D+ type. They include a reduced form of chlorophyll Chl-, an oxidized donor D+ at the periphery, and a protein component Z1…Zj…Zm consisting of a set of bound particles Zj (j = 1,2, …, m). The protein component is characterized by imperfections, such as failure of electron transport and non-uniform spatial distribution. Chlorophyll Chl- provides primary photoexcitation of the electron system and acts as a recombination center during its reduction. The direct function of the electron trap is implemented by a subsystem: protein component Z1…Zj…Zm + donor D+. The key role is played by Z₁, the first particle from the chlorophyll, that is a universal center for localization of the electron released during photoexcitation of Chl-. The full spectrum of the Z-trap is formed as a superposition of the states of the Z-1 centers, the energy spread of which is determined by the variability of the influence of the electrostatic field of the D+ donor. Photoexcitation at Tех = 77 K oxidizes Chl-. Due to intracenter events, the complexes make a transition from the equilibrium dipole state to the excited thermoluminescently active state Chl + Z-₁…Zj…Zm + D+, in which the photodipoles Z-₁…Zj…Zm + D+ have a reduced center Z-₁. Thermal deactivation of the multilevel Z1- center leads to the reduction of Chl, which is accompanied by thermoluminescent emission in the form of an integral Z-band. A series of daughter bands Zv, observed when the Tех point is "violet shifted" in the high-temperature slope of the Z-band, is a consequence of the increased fraction of photodipoles with high oxidation energy in the general population of the activated complexes. A hypothesis about the biological function of the Z trap is put forward. From the standpoint of biological expediency, the Z trap can be considered as an element of the photoprotective mechanism.