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Background and aims: Cosmic radiation is one of the main hazards for manned space exploration. Uncertainty in radiation risk estimates for crews of long-term missions are very high, and direct biological measurements are necessary. We measured chromosomal aberrations in peripheral blood lymphocytes from 33 cosmonauts involved in space missions during the past 11 years. Methods: Blood lymphocytes from the cosmonauts were stimulated to grow in vitro and were harvested at their first mitosis. Slides were either stained with Giemsa stain for dicentrics analysis, or painted with whole-chromosome DNA probes for translocation analysis (FISH). Results: A statistically significant increase in the yield of chromosomal aberrations was measured following long-term space missions in lymphocytes from cosmonauts at their first flight. No significant changes in aberration frequencies were observed for short-term taxi flights. The increase in long-term missions was consistent with the values calculated from physical dosimetry data. However, for cosmonauts involved in two or more space flights, the yield of interchromosomal exchanges was not related to the total duration of space sojourn or integral absorbed dose. Indeed, the yield of aberrations at the end of the last mission was generally in the range of background frequencies measured before the first mission. Conclusions: Chromosome aberration dosimetry can detect radiation damage during space flight, and biological measurements support the current risk estimates for space radiation exposure. However, for cosmonauts involved in multiple space missions the frequency of chromosomal aberrations is lower than expected, suggesting that the effects of repeated space flights on this particular endpoint are not simply additive. Changes in the immune system in microgravity and/or adaptive response to space radiation may explain the apparent increase in radioresistance after multiple space flights.

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The effects of alkylation of the 4-amino group, polarity and viscosity of a solvent, and temperature on the fluorescence of N-(2,4-dimethylphenyl)imides of 4-aroino-1,8-naphthalenedicarboxylic acid were studied. A decrease in the temperature and polarity of a solvent and an increase in its viscosity cause the rate of radiationless deactivation of 4-dimethylaminonaphthalimide to decrease and have almost no effect on the rate of eroissive deactivation of excited state. The abnormally low efficiency of fluorescence of 4-dimethylaminonaphthalimide in polar low-viscosity media at room temperature is associated with the formation of a nonfluorescent charge-transfer state. The increase in the rate of radiationless deactivation of 4-aminonaphthalimide in the alkylation of its amino group is associated with the increase in its electron-donor ability.Key words: 4-aminonaphthalimide, fluorescence, radiationless deactivation, donor-acceptor interaction.Derivatives of 4-aminonaphthalimides are used as active laser media I and fluorescent labels, z The luminescent properties of these compounds are strongly dependent on the structure of the molecule and solvent. The fluorescence of naphthalimides containing the 4-amino-or 4-monoalkylamino group is characterized by a high quantum yield independent of the solvent, whereas the quantum yield of fluorescence of 4-(N,Ndialkylamino)naphthalimides is strongly dependent on the nature of the solvent, z,3 The nineteen-fold decrease in the quantum yield of fluorescence in ethanol on going from 4-amino-to 4-(N,N-dialkylamino)naphthalimide has been explained by the violation of planarity of the dialkylaminonaphthalimide molecule) The strong effect of substitution in the amino group on luminescent properties of derivatives of 4-aminobenzonitriles, coumarins, and other compounds is associated with a change in the extent of the charge transfer in the excited state 4-~ and in the molecular structure (with a change in hybridization 5 and rotation of the fragments4,6). A rearrangement of the closest environment of the molecule or a change in its structure in the excited state results in the dependence of the efficiency of fluorescence of such systems on the viscosity and polarity of the medium. 4,6 In this work, the effects of the structure of the 4-amino substituent, of the polarity and viscosity of a solvent, as well as the effect of temperature on the fluorescent properties of N-(2,4-dimethylphenyl)imides of 4-amino-1,8-naphthalenedicarboxylic acid were studied to elucidate the reasons for the different fluorescent ExperimentalAbsorption and fluorescence spectra were recorded on a Shimadzu UV-3100 spectrophotometer and an Elyumin 2M spectrofluorimeter, respectively. The kinetics of fluorescence was studied on a nanosecond ORTEC spectrometer by photon counting. The quantum yield of fluorescence (g~) was determined by comparing the areas under the curves of corrected fluorescence spectra of the compound under study with those of a solution of quinine bisulfate in I N H2SO 4 (cp = 0.546). 7 Polym...