Molecular Changes in Spider Viscid Glue As a Function of Relative Humidity Revealed Using Infrared Spectroscopy

Spider aggregate glue can absorb moisture from the atmosphere to reduce its viscosity and become tacky. The viscosity at which glue adhesion is maximized is remarkably similar across spider species, even though that viscosity is achieved at very different relative humidity (RH) values matching their diverse habitats. However, the molecular changes in the protein structure and the bonding state of water (both referred to here as molecular structure) with respect to the changes in RH are not known. We use attenuated total reflectance-infrared (ATR-IR) spectroscopy to probe the changes in the molecular structure of glue as a function of RH for three spider species from different habitats. We find that the glue retains bound water at lower RH and absorbs liquid-like water at higher RH. The absorption of liquid-like water at high RH plasticizes the glue and explains the decrease in glue viscosity. The changes to protein conformations as a function RH are either subtle or not detectable by IR spectroscopy. Importantly, the molecular changes are reversible over multiple cycles of RH change. Further, separation of glue constituents results in a different humidity response as compared to pristine glue, supporting the standing hypothesis that the glue constituents have a synergistic association that makes spider glue a functional adhesive. The results presented in this study provide further insights into the mechanism of the humidity-responsive adhesion of spider glue.蜘蛛聚合胶（Spider aggregate glue）能够从大气中吸收水分，进而降低自身黏度并产生粘性。不同蜘蛛物种的胶黏附强度达到最大值时的黏度极为相近，尽管该黏度对应的相对湿度（RH）因物种栖息环境的差异而大不相同。然而，关于相对湿度变化所引发的蛋白质结构分子变化以及水的键合状态（二者在本文中统称为分子结构），目前仍未明确。本研究采用衰减全反射红外（ATR-IR）光谱法，针对三种栖息环境各异的蜘蛛，探究其胶的分子结构随相对湿度变化的规律。研究发现，在较低相对湿度下，胶会保留结合态水；而在较高相对湿度下，则会吸收类液态水。高相对湿度下类液态水的吸收会使胶发生增塑作用，这也解释了胶黏度下降的原因。蛋白质构象随相对湿度的变化要么极为细微，要么无法通过红外光谱法检测到。值得注意的是，这些分子变化在多次相对湿度循环变化过程中均可逆。此外，与天然原始胶相比，分离后的胶组分表现出不同的湿度响应特性，这支持了当前主流假说：胶组分之间存在协同关联，使得蜘蛛聚合胶成为具备功能性的黏合剂。本研究所得结果，进一步阐明了蜘蛛聚合胶的湿度响应型黏附机制。