Cooper Christopher D, Clementi Natalia C, Barba Lorena A
Mechanical Engineering, Boston University, Boston, Massachusetts 02215, USA.
Mechanical and Aerospace Engineering, The George Washington University, Washington, DC 20052, USA.
J Chem Phys. 2015 Sep 28;143(12):124709. doi: 10.1063/1.4931113.
Protein-surface interactions are ubiquitous in biological processes and bioengineering, yet are not fully understood. In biosensors, a key factor determining the sensitivity and thus the performance of the device is the orientation of the ligand molecules on the bioactive device surface. Adsorption studies thus seek to determine how orientation can be influenced by surface preparation, varying surface charge, and ambient salt concentration. In this work, protein orientation near charged nanosurfaces is obtained under electrostatic effects using the Poisson-Boltzmann equation, in an implicit-solvent model. Sampling the free energy for protein G B1 D4' at a range of tilt and rotation angles with respect to the charged surface, we calculated the probability of the protein orientations and observed a dipolar behavior. This result is consistent with published experimental studies and combined Monte Carlo and molecular dynamics simulations using this small protein, validating our method. More relevant to biosensor technology, antibodies such as immunoglobulin G are still a formidable challenge to molecular simulation, due to their large size. With the Poisson-Boltzmann model, we obtained the probability distribution of orientations for the iso-type IgG2a at varying surface charge and salt concentration. This iso-type was not found to have a preferred orientation in previous studies, unlike the iso-type IgG1 whose larger dipole moment was assumed to make it easier to control. Our results show that the preferred orientation of IgG2a can be favorable for biosensing with positive charge on the surface of 0.05 C/m(2) or higher and 37 mM salt concentration. The results also show that local interactions dominate over dipole moment for this protein. Improving immunoassay sensitivity may thus be assisted by numerical studies using our method (and open-source code), guiding changes to fabrication protocols or protein engineering of ligand molecules to obtain more favorable orientations.
蛋白质与表面的相互作用在生物过程和生物工程中普遍存在，但尚未得到充分理解。在生物传感器中，决定设备灵敏度进而决定其性能的一个关键因素是配体分子在生物活性设备表面的取向。因此，吸附研究旨在确定表面制备、表面电荷变化和环境盐浓度如何影响取向。在这项工作中，使用泊松-玻尔兹曼方程，在隐式溶剂模型中，在静电作用下获得了带电纳米表面附近蛋白质的取向。通过对蛋白质G B1 D4'相对于带电表面在一系列倾斜和旋转角度下的自由能进行采样，我们计算了蛋白质取向的概率，并观察到偶极行为。这一结果与已发表的实验研究以及使用这种小蛋白质的蒙特卡罗和分子动力学模拟相结合的结果一致，验证了我们的方法。与生物传感器技术更相关的是，诸如免疫球蛋白G之类的抗体由于其尺寸较大，对分子模拟来说仍然是一个巨大的挑战。利用泊松-玻尔兹曼模型，我们获得了不同表面电荷和盐浓度下同种型IgG2a的取向概率分布。与同种型IgG1不同，此前的研究认为同种型IgG1较大的偶极矩使其更容易控制，而这种同种型在以前的研究中并未发现有优先取向。我们的结果表明，对于表面正电荷为0.05 C/m²或更高且盐浓度为37 mM的情况，IgG2a的优先取向可能有利于生物传感。结果还表明，对于这种蛋白质，局部相互作用比偶极矩更占主导。因此，使用我们的方法（和开源代码）进行数值研究可能有助于提高免疫测定的灵敏度，指导制造方案的改变或配体分子的蛋白质工程，以获得更有利的取向。
