Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial membrane lipids that are ubiquitous in the environment. Although the exact source organism is unknown, the distribution of brGDGTs in mineral soils, peats, and lake sediments is correlated with temperature through a decrease in the degree of methylation with increasing temperature. This empirical observation forms the basis of the brGDGT paleothermometer, one of the most important and widely used organic proxies to reconstruct terrestrial temperatures in the past. However, a mechanistic understanding to underpin this empirical correlation between the degree of methylation of brGDGT lipids and temperature is lacking, hindering a holistic understanding of the brGDGT paleothermometer as well as the membrane dynamics of their bacterial producers. To address this, here we present the first molecular dynamics simulations of membranes consisting of brGDGTs. Using intact polar lipid (IPL) brGDGTs with two sugar headgroups, our simulations demonstrate that increasing the degree of methylation modulates membrane order and packing, rendering the membrane less rigid and more fluid. These results indicate that the empirically observed correlation between the degree of methylation and temperature allows brGDGT-producing bacteria to maintain adequate membrane fluidity. Our simulations provide the first molecular simulation data to support the hypothesis that the brGDGT paleothermometer is based on homeoviscous adaptation.

支化甘油二烷基甘油四醚 (brGDGT) 是环境中普遍存在的细菌膜脂。尽管确切来源生物体未知，但 brGDGT 在矿质土壤、泥炭和湖泊沉积物中的分布通过甲基化程度随温度升高而降低而与温度相关。这一经验观察形成了 brGDGT 古温度计的基础，这是过去重建地球温度的最重要和最广泛使用的有机代理之一。然而，缺乏支持 brGDGT 脂质甲基化程度与温度之间这种经验相关性的机械理解，阻碍了对 brGDGT 古温度计及其细菌生产者的膜动力学的整体理解。为了解决这个问题，在这里，我们展示了由 brGDGT 组成的膜的第一个分子动力学模拟。使用具有两个糖头基团的完整极性脂质 (IPL) brGDGT，我们的模拟表明，增加甲基化程度会调节膜的顺序和堆积，使膜的刚性降低，流动性更强。这些结果表明，根据经验观察到的甲基化程度与温度之间的相关性允许产 brGDGT 的细菌保持足够的膜流动性。我们的模拟提供了第一个分子模拟数据，以支持 brGDGT 古温度计基于同源粘性适应的假设。我们的模拟表明，增加甲基化程度会调节膜的顺序和堆积，使膜的刚性降低，流动性更强。这些结果表明，根据经验观察到的甲基化程度与温度之间的相关性允许产 brGDGT 的细菌保持足够的膜流动性。我们的模拟提供了第一个分子模拟数据，以支持 brGDGT 古温度计基于同源粘性适应的假设。我们的模拟表明，增加甲基化程度会调节膜的顺序和堆积，使膜的刚性降低，流动性更强。这些结果表明，根据经验观察到的甲基化程度与温度之间的相关性允许产 brGDGT 的细菌保持足够的膜流动性。我们的模拟提供了第一个分子模拟数据，以支持 brGDGT 古温度计基于同源粘性适应的假设。