Ben-Tal N, Honig B, Bagdassarian C K, Ben-Shaul A
Department of Biochemistry, George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv 69978, Israel.
Biophys J. 2000 Sep;79(3):1180-7. doi: 10.1016/S0006-3495(00)76372-7.
The association of two species to form a bound complex, e.g., the binding of a ligand to a protein or the adsorption of a peptide on a lipid membrane, involves an entropy loss, reflecting the conversion of free translational and rotational degrees of freedom into bound motions. Previous theoretical estimates of the standard entropy change in bimolecular binding processes, DeltaS(o), have been derived from the root-mean-square fluctuations in protein crystals, suggesting DeltaS(o) approximately -50 e.u., i.e., TDeltaS degrees approximately -25 kT = -15 kcal/mol. In this work we focus on adsorption, rather than binding processes. We first present a simple statistical-thermodynamic scheme for calculating the adsorption entropy, including its resolution into translational and rotational contributions, using the known distance-orientation dependent binding (adsorption) potential. We then utilize this scheme to calculate the free energy of interaction and entropy of pentalysine adsorption onto a lipid membrane, obtaining TDeltaS(o) approximately -1.7 kT approximately -1.3 kcal/mol. Most of this entropy change is due to the conversion of one free translation into a bound motion, the rest arising from the confinement of two rotational degrees of freedom. The smaller entropy loss in adsorption compared to binding processes arises partly because a smaller number of degrees of freedom become restricted, but mainly due to the fact that the binding potential is much "softer."
两个物种结合形成结合复合物，例如配体与蛋白质的结合或肽在脂质膜上的吸附，涉及熵的损失，这反映了自由平移和旋转自由度向束缚运动的转化。之前对双分子结合过程中标准熵变ΔS⁰的理论估计，是从蛋白质晶体中的均方根波动推导出来的，表明ΔS⁰约为 -50熵单位，即TΔS⁰约为 -25kT = -15千卡/摩尔。在这项工作中，我们关注的是吸附，而不是结合过程。我们首先提出一个简单的统计热力学方案，用于计算吸附熵，包括将其分解为平移和旋转贡献，使用已知的距离 - 取向依赖的结合（吸附）势。然后我们利用这个方案来计算五赖氨酸吸附到脂质膜上的相互作用自由能和熵，得到TΔS⁰约为 -1.7kT约为 -1.3千卡/摩尔。这种熵变大部分是由于一个自由平移转化为束缚运动，其余部分来自两个旋转自由度的限制。与结合过程相比，吸附中较小的熵损失部分是因为受限制的自由度数量较少，但主要是由于结合势要“软”得多。
