磁场方向对磁镊的伸长测量噪音的影响
The Effect of Magnetic Field Direction on the Extension Noise in Magnetic Tweezers Measurement

磁镊作为一种单分子操纵技术被广泛地应用于蛋白质和DNA等生物大分子的物理、化学性质和生物功能的探究。通常，磁镊使用反向平行的双磁铁对连接着生物大分子的超顺磁球施加拉力，从而实现对单分子的拉伸。当拉力小于3皮牛且分子构象转变引起的伸长变化小于10 nm时，由磁球热运动引起的在拉力方向上的噪音会使分子伸长的变化难以准确测量。为了提高磁镊在小力下对分子伸长的测量精度，我们使用了单根圆柱状磁铁，以减小磁球转动在拉力方向上引入的噪音。我们使用了双磁铁和单磁铁分别对517 bp的DNA的进行了拉伸实验。通过比较两者的拉力–伸长曲线，我们发现当拉力小于3皮牛的时候，使用柱状单磁铁的实验组在拉力方向上的背景噪音显著小于双磁铁实验组。
As a single-molecule manipulation technology, magnetic tweezers are widely used to explore the properties and biological functions of biological macromolecules such as proteins and DNA. Generally, magnetic tweezers use anti-parallel double magnets to exert a pulling force on the superparamagnetic spheres connected with biological macromolecules, thereby realizing the stretching of single molecules. When the tensile force is less than 3 pN, and the extension change caused by the molecular conformational transition is less than 10 nm, the noise in the tensile direction caused by the thermal motion of the magnetic bead will make the molecular extension change difficult to measure accurately. In order to improve the accuracy of the magnetic tweezers in the measurement of molecular extension under small force, we use a single cylindrical magnet to reduce the noise of the magnetic bead in the direction of tension. We used anti-parallel double magnets and a single magnet to stretch a 517 bp DNA. By comparing the force-extension curves, it is determined that when the tensile force is less than 3 pN, the extension noise in the experiments using a cylindrical single magnet is significantly smaller than that in the experiments using antiparallel double magnets.