We have used fluorescence spectroscopy methods to show that imidacloprid and its structural analogs form complexes with human serum albumin (HSA). The nature of the spectral changes in the ligand×protein systems and the calculated complexation parameters suggest that these low molecular weight compounds mainly bind to a specific section of the protein molecule, near the tryptophan residue in the 214 position of the polypeptide chain. We have found that the association constants are on the order of 10 4 M -1 , and the affinity of the ligands for HSA varies in the series 6-chloronicotinic acid > 6-methoxynicotinic acid = imidacloprid > the keto analog of imidacloprid. The major contribution to the complexation energy probably comes from hydrophobic interaction forces with participation of the aromatic pyridine ring of the ligands, while additional enhancement of ligand-protein affinity can be provided by the nitroimine group of imidacloprid.Key words: human serum albumin, neonicotinoid, imidacloprid, quenching of fluorescence, ligand-protein binding.Introduction. Neonicotinoids are a new and very important class of insecticides with selective action which are increasingly aggressively displacing pyrethroids, organophosphorus compounds, and methylcarbamates from the area of application for protection of plants from insect pests. The selectivity of action of neonicotinoids is determined by the properties of their molecular target: the nicotinic acetylcholine receptor. These pesticides have high affinity for the insect receptor and bind weakly to other types of receptors, in particular exhibiting low toxicity for mammals [1,2]. The molecular aspects of high affinity and selectivity for binding of neonicotinoids and their analogs by receptor proteins and the physicochemical nature of the interactions in the complexes formed have been studied in many papers. As a result, some structural elements and also chemical groups have been identified in the insecticides and the receptor that are involved in complexation, and computer models of the ligand×protein interaction have been proposed based on a quantum chemical method [1,[3][4][5][6].Another important direction in studies of the biological properties of pesticides is directly related to protection of the health of humans and agricultural animals, and includes study of the interactions between these xenobiotics and proteins, enzymes, and receptors in blood plasma and tissues. The chemical structure of many pesticides, including insecticides in the neonicotinoid class, contains sections and groups capable of forming electrostatic, hydrophobic, and hydrogen bonds as well as other types of bonds typical of endogenous ligands in their complexes with proteins. Therefore many pesticides can play the role of exogenous ligands and alter their own properties within the composition of protein complexes, as is true for natural bioregulators such as hormones. These changes can involve metabolic parameters and biological effects of the pesticides in the human body and in the bodies of...

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The phytosteroids 313-hydroxy-(24S)-stigmast-5,22E-dien-7-one and (24S)-stigmasta-5,22E-diene-3{3, 713-diol have been synthesized from stigmasterol. Stigmasterol (la) is one of the main steroids of plants. Together with/3-sitosterol and campesterol, this substance ensures the functioning of plant cell membranes. Moreover, the biosynthesis of various biologically active steroids necessary for the normal vital activity of plants starts from phytosterols, including stigmasterol. Compounds of this type include, for example, the hydroxysteroids (3a-4a) detected in plants [1, 2], the structures of which are characterized by the same carbon skeleton as in the stigmasterol molecule. It has been established by numerous investigations [4][5][6][7][8][9] that phytosterol derivatives with oxygen-containing functions at C-7, including steroids (2a-4a), possess a pronounced antitumoral activity. This circumstance has induced us to undertake the synthesis of these substances from the commercially available stigmasterol, which is used as a raw material in drug manufacture. In the present paper we give the results of this investigation, which is a continuation of syntheses of 6-oxophytosteroids from/3-sitosterol [ 10] and stigmasterol [ 11] that we have performed previously. In the stigmasterol molecule, the H-7 atom is in an allyl position in relation to the 5(6) double bond, and, for this reason, it is activated_ Although precisely the same situation exists for the H-4 atom, the selective oxidation of 5(6)-unsaturated steroids at C-7 is possible, for example, with the complex of chromium trioxide and pyridine, which has the structure of pyridinium dichromate [12]. Since, under these conditions, the 3~-hydroxy group in the stigmasterol molecule would also be oxidized, it was necessary to protect it. For this purpose, we first converted stig-rnasterol by acetylation with acetic anhydride in pyridine into the known acetate (lb) in quantitative yield.

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