<sup>S</sup>PECTRAL-FLUORESCENT AND POLARIZING CHARACTERISTICS OF <i>Z</i>,<i>Z</i>-BILIRUBIN IXα
We have investigated the spectral-fluorescent and polarizing characteristics of Z,Z-bilirubin IXα at the room temperature in chloroform and in a water buffer medium, being incorporated into an equilibrium complex with human serum albumin (HSA), as well as under the conditions of low temperatures (T = -100 °C) in isobutyl alcohol. The batochromic shift of the fluorescence spectra is uncovered which is most pronounced for bilirubin-albumin complex. Among possible reasons of the observable dependence of the position of the fluorescence spectra (excitation of fluorescence) on the excitation wavelength (registration), one may consider the following: structural and spectral nonidentity of chromophore, forming a molecule of Z,Z-bilirubin IXα; conformational heterogeneity of a pigment in a solution; the contribution of molecules which have not finished the process of the vibronic relaxation; nonhomogenous orientational broadening of levels; heterogeneity of a microenvironment chromophore in the albuminous matrix. The presence of the polarized fluorescence of bilirubin at the room temperature is shown, which is caused by the anomalously short lifetime τ (picosecond or subpicoseconds ranges) of fluorescence. Despite such a short τ, the data of the polarization spectra on absorption and emission are indicative of the presence of intramolecular nonradiative singlet-singlet energy transfer when exciting bilirubin to high vibronic sublevels of the S1-state (the degree of polarization p = 0.11-0.12). The energy transfer is absent at the excitation of fluorescence at the long-wave slope of the absorption spectrum - the degree of polarization (p = 0.46-0.47) is close to the limiting one (p = 0.50). The role of excitonic interactions between chromophores in a molecule of bilirubin at its excitation is discussed.