A study of melaphene interaction with phospholipid membranes

Abstract

218 It is known that presowing treatment of plants with the growth regulator melaphene (melamine salt of bis(oxymethyl)phosphinic acid) at low and ultralow concentrations significantly increases the yield of cereals, legumes, and crucifers, enhancing the stress-resistance of plants under adverse environmental conditions [1, 2]. For substantiated and efficient application of this agent, it is necessary to investigate in detail the mechanism of its action at the molecular and cellular levels. It was shown earlier that the effect of melaphene is determined by the activation of metabolic pathways in stress, which correlated with changes in the microviscosity of annular lipids in cell membranes [3‐5]. The goal of this work was to study the fine structure of model phospholipid membranes under the treatment of melaphene. The range of tested melaphene concentrations was significantly broadened. An additional goal of this study was to reveal possible side effects of melaphene. We studied the interaction of melaphene with the liposomes formed from the individual neutral phospholipid dimyristoylphosphatidylcholine (DMPC) and egg lecithin, which represents a mixture of natural phospholipids. The results of differential microcalorimetric and X-ray diffraction analyses showed that melaphene changes the domain structure of neutral phospholipids in the membrane but has no effect on the next organizational level—the total size of bilayers in multilamellar liposomes formed of natural phospholipids. The effect of melaphene in a broad concentration range (10 ‐21 to 10 ‐3 M) on the conformational rearrangements of the bilayers was assessed by measuring the small-angle X-ray diffraction of egg lecithin liposomes and the thermostability of DMPC liposomes as described in [6‐10]. Small-angle X-ray diffraction analysis of liposomes was performed as described in [8] with some modifications. X-ray diffractograms of dispersed liposomes of phospholipid membranes were obtained using a smallangle X-ray diffractometer equipped with a linear coordinate detector [9]. Diffractograms of dispersed liposomes were normalized by the maximum intensity at the peak of the first diffraction maximum, and the collimation correction for the height of the X-ray beam and detector window was introduced according to [10]. Two diffraction maxima could be distinguished on the liposome dispersion diffractograms of the samples containing melaphene at concentrations 0, 10 ‐21 , 10 ‐18 , 10 ‐12 , and 10 ‐6 M, which represented the first and second orders of reflection of ordered membrane multilayers in liposomes. As seen in Fig. 1, the diffractogram patterns of the studied samples of liposome dispersion coincided, indicating that the membrane structure was the same in all liposome samples. The membrane periodicity interval in liposomes ( D ) was found to be 6.9 nm. The electron density profiles of lipid membranes in the liposomes (Fig. 2) were used to determine the thickness of one membrane, which was calculated as the distance between the electron density peaks corresponding to the position of polar groups in lipids. The membrane thickness in all liposome preparations in the presence of different melaphene concentrations was approximately 4 nm. Thus, the results of small-angle X-ray diffraction analysis showed that melaphene in a broad concentration range ( 10 –21 to 10 –6 M) has no marked effect on the structure of lipid membranes. Differential scanning microcalorimetry of DMPC liposomes was performed by the standard method using a DASM-4 microcalorimeter; the results were subsequently processed using the Microcal Origin 5.0 software [7]. It was found that melaphene at low and ultralow concentrations affects the fine structure of