Astaxanthin and its esters, which are classified as secondary carotenoids, are synthesized under stress conditions in Haematococcus pluvialis cells that undergo transformation from vegetative cells to hematocysts. Stress factors can affect not only the rate and the direction of synthesis, but also the conformational changes in the pigment molecule, causing conversion of trans-isomers to cis-isomers. The aim of this study was to evaluate the effects of different factors (the polarity of the medium and light) on the process of trans–cis isomerization of astaxanthin. Investigation of the formation of 9-cis- and 13-cis-isomers from all-trans astaxanthin solutions at 50°C showed that medium polarity solvents stabilized cis-isomers more efficiently than highly polar solvents. Nonpolar solvents did not contribute to the stabilization of cis-isomers. The presence of π-bonds in the solvent molecule also stimulated isomerization. Analysis of the effects of the solvent refractive index on polarizability of astaxanthin revealed differences between the dependencies for polar and nonpolar solvents. Supposedly, solvents of medium polarity facilitate the process of trans–cis astaxanthin isomerization by decreasing the energy of the torque and stabilizing the cis-conformation of the molecule due to diminished dipole–dipole repulsion between the methyl groups in the polyene chain and the ionone ring. Exposure of astaxanthin solutions to light at 25°C induced formation of cis-isomers, which were more abundant within the first 4 h of exposure than after 6 h or later. Exposure to irradiation for longer periods caused astaxanthin decomposition and formation of degradation and oxidation products.

中文翻译：

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溶剂性质和光照对虾青素异构化和氧化降解的影响

虾青素及其酯（被分类为类胡萝卜素）是在应激条件下在雨生红球菌细胞中合成的，该细菌经历了从营养细胞到血囊的转化。应激因素不仅可以影响合成的速率和方向，而且可以影响颜料分子中的构象变化，从而导致反式异构体转化为顺式异构体。这项研究的目的是评估虾青素的反式-顺式异构化过程中不同因素（介质和光的极性）的影响。从全反式形成9-顺式和13-顺式异构体的研究虾青素溶液在50°C下显示，中极性溶剂比高极性溶剂更有效地稳定了顺式异构体。非极性溶剂无助于稳定顺式异构体。溶剂分子中π键的存在也刺激了异构化。分析溶剂折射率对虾青素极化率的影响，发现极性和非极性溶剂的依赖性之间存在差异。据称，中等极性的溶剂可通过降低扭矩能量并稳定顺式来促进反式-顺式虾青素的异构化过程。由于多烯链中的甲基与紫罗兰酮环之间的偶极-偶极排斥力减弱，使分子的构象变迁。将虾青素溶液在25°C的光照下会诱导顺式异构体的形成，在暴露的头4小时内比在6小时或更晚之后含量更高。长时间暴露在辐射下会导致虾青素分解并形成降解和氧化产物。