Biological host molecules such as β-cyclodextrins (β-CDs) have been used to remove cholesterol guests from membranes and artery plaques. In this work, we calibrated the host–guest intermolecular mechanical forces (IMMFs) between cholesterol and cyclodextrin complexes by combining single-molecule force spectroscopy in optical tweezers and computational molecular simulations for the first time. Compared to native β-CD, methylated beta cyclodextrins complexed with cholesterols demonstrated higher mechanical stabilities due to the loss of more high-energy water molecules inside the methylated β-CD cavities. This result is consistent with the finding that methylated β-CD is more potent at solubilizing cholesterols than β-CD, suggesting that the IMMF can serve as a novel indicator to evaluate the solubility of small molecules such as cholesterols. Importantly, we found that the force spectroscopy measured in such biological host–guest complexes is direction-dependent: pulling from the alkyl end of the cholesterol molecule resulted in a larger IMMF than that from the hydroxyl end of the cholesterol molecule. Molecular dynamics coupled with umbrella sampling simulations further revealed that cholesterol molecules tend to enter or leave from the wide opening of cyclodextrins. Such an orientation rationalizes that cyclodextrins are rather efficient at extracting cholesterols from the phospholipid bilayer in which hydroxyl groups of cholesterols are readily exposed to the hydrophobic cavities of cyclodextrins. We anticipate that the IMMF measured by both experimental and computational force spectroscopy measurements help elucidate solubility mechanisms not only for cholesterols in different environments but also to host–guest systems in general, which have been widely exploited for their solubilization properties in drug delivery, for example.

中文翻译：

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胆固醇和环糊精主客体复合物中单分子力的测量

生物宿主分子，例如 β-环糊精 (β-CDs)，已被用于从膜和动脉斑块中去除胆固醇客体。在这项工作中，我们首次通过结合光镊中的单分子力谱和计算分子模拟来校准胆固醇和环糊精复合物之间的主客体分子间机械力 (IMMF)。与天然β-CD相比，与胆固醇复合的甲基化β-环糊精表现出更高的机械稳定性，因为甲基化β-CD腔内失去了更多高能水分子。该结果与甲基化 β-CD 在溶解胆固醇方面比 β-CD 更有效的发现一致，表明 IMMF 可以作为评估小分子（如胆固醇）溶解度的新指标。重要的是，我们发现在这种生物主客体复合物中测量的力谱是方向依赖性的：从胆固醇分子的烷基端拉出比从胆固醇分子的羟基端拉出更大的 IMMF。分子动力学与伞形采样模拟相结合进一步揭示了胆固醇分子倾向于从环糊精的宽开口进入或离开。这种取向合理化了环糊精在从磷脂双层中提取胆固醇方面相当有效，其中胆固醇的羟基很容易暴露于环糊精的疏水空腔中。