Passage of light through a suspension is accompanied by a competition of processes of absorption and scattering for each individual particle. As a result, a hypochromism phenomenon (a decrease in the extinction coefficient) takes place. The hypochromism value increases with the growth of a particle sizes or its refractive index. Since Tyndall's light scattering in a suspension where the size of every particle is consid erably larger than the wavelength weakly depends on the wavelength, the absorption (or excitation) spectrum is almost uniformly attenuated. A simple method for determination of the true extinction coefficients of the absorption and excitation spectra of diluted suspensions which have no multiple light scattering is proposed. Experimental data on the spectra of hemoglobin in erythrocytes, actinomycin in DNA, and flavines in mito chondria are discussed.
We have investigated a number of complexes of 7-aminoactinomycin D (7AAMD), with its potential carriers: caffeine, folic acid (FA), purine bases-guanine and adenine, pyrimidine base-thymine and with fragmented DNA to determine more stable and suitable complex. The process of binding of the fluorescent antibiotic with clusters of caffeine, guanine, adenine, thymine and with fragmented DNA was accompanied by a considerable long-wavelength shift in excitation spectrum. The energy of interaction between phenoxazine hetero-cycle of 7AAMD and chromophores of the carriers studied has been found. In the case of 7AAMD with guanine, adenine, thymine and caffeine, the energy is about of 7 kcal/mol, which is a little lower than in the case with DNA (7.7 kcal/mol). On the basis of emission spectra, in all examined compounds, with the exception DNA, the 7AAMD molecule emits photons from water phase, not from a cluster, since photo-excitation leads to desorption of the antibiotic from a cluster surface. We observed also the mutual fluorescence quenching of 7AAMD and FA in their complex. It may well be that this complex forms due to interaction of peptide-lactone rings of 7AAMD with system of FA. In the case of DNA, the complex with 7AAMD has very high stability that is determined not only by interaction between phenoxazine of 7AAMD and the DNA bases, but it is largely owing to the interaction between two peptide-lactone rings of 7AAMD and the DNA deoxyribose-phosphate chains.
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