Graduate School of Science , Osaka University , 1-1 Machikaneyama , Toyonaka , Osaka 560-0043 , Japan.
School of Science and Engineering , Tokyo Denki University , Ishizaka, Hatoyama, Hiki , Saitama 350-0394 , Japan.
J Phys Chem B. 2018 Dec 27;122(51):12322-12330. doi: 10.1021/acs.jpcb.8b10026. Epub 2018 Dec 14.
To protect themselves, insects cover their bodies with what is called cuticular lipid. The cuticular lipid of an American cockroach has a unique lipid content; the most abundant is a cis-alkadiene, cis, cis-6,9-heptacosadiene, amounting to about 70%, which is followed by a branched alkane 3-methylpentacosane. In order to clarify the structural features of the unique lipid composition below the critical temperature, the cuticular lipid was studied by Fourier transform infrared (FTIR) spectroscopy in combination with an attenuated total reflection (ATR) sampling technique. The infrared spectra measured on an extracted lipid sample at 20 °C suggested that the lipid keeps an appreciable level of conformational and lateral packing regularity, in spite of a high cis-unsaturated lipid content, and also a highly disordered condition around the methyl terminals and cis-olefin groups. The CH scissoring and the CH rocking regions showed the characteristics of the O⊥ subcell. The same characteristics were observed also by in situ measurements on a forewing of the American cockroach. Combining the spectral features of these bands and the physicochemical properties of each component, it can be inferred that saturated lipids form highly ordered domains within the liquid containing the cis, cis-diene as the main component. For comparison, the cuticular lipid of a male cricket, which consisted of many different hydrocarbons, including 15% of unsaturated hydrocarbons, showed a lower regularity both in the conformation and in the lateral packing of hydrocarbon chains. These results imply that not only the degree of cis-unsaturation but also the chemical structure diversity of hydrocarbons are the important factors to determine the physicochemical properties of cuticular lipid.
为了保护自己，昆虫会在身体表面覆盖一层被称为表皮脂质的物质。美洲大蠊的表皮脂质具有独特的脂质组成，其中最丰富的是顺式-长链二烯，顺式，顺式-6,9-二十碳二烯，约占 70%，其次是支链烷烃 3-甲基二十五烷。为了阐明低于临界温度下独特脂质组成的结构特征，采用傅里叶变换红外（FTIR）光谱结合衰减全反射（ATR）采样技术对表皮脂质进行了研究。在 20°C 下测量的提取脂质样品的红外光谱表明，尽管脂质具有高含量的顺式不饱和脂质，但仍保持着相当水平的构象和层状堆积规则，并且甲基末端和顺式烯烃基团周围的状态高度无序。CH 剪式振动和 CH 摇摆区域显示出 O⊥亚单元的特征。在美洲大蠊前翅上进行的原位测量也观察到了相同的特征。结合这些带的光谱特征和每个组分的物理化学性质，可以推断出饱和脂质在以顺式，顺式二烯为主要成分的液体中形成高度有序的区域。相比之下，由许多不同的碳氢化合物组成的雄性蟋蟀的表皮脂质，包括 15%的不饱和碳氢化合物，在碳氢化合物链的构象和层状堆积方面的规则性都较低。这些结果表明，不仅顺式不饱和程度，而且碳氢化合物的化学结构多样性都是决定表皮脂质物理化学性质的重要因素。
