Ribezzi-Crivellari M, Huguet J M, Ritort F
Departamento de Fisica Fonamental, Universitat de Barcelona, Barcelona, Spain.
Rev Sci Instrum. 2013 Apr;84(4):043104. doi: 10.1063/1.4799289.
We present a dual-trap optical tweezers setup which directly measures forces using linear momentum conservation. The setup uses a counter-propagating geometry, which allows momentum measurement on each beam separately. The experimental advantages of this setup include low drift due to all-optical manipulation, and a robust calibration (independent of the features of the trapped object or buffer medium) due to the force measurement method. Although this design does not attain the high-resolution of some co-propagating setups, we show that it can be used to perform different single molecule measurements: fluctuation-based molecular stiffness characterization at different forces and hopping experiments on molecular hairpins. Remarkably, in our setup it is possible to manipulate very short tethers (such as molecular hairpins with short handles) down to the limit where beads are almost in contact. The setup is used to illustrate a novel method for measuring the stiffness of optical traps and tethers on the basis of equilibrium force fluctuations, i.e., without the need of measuring the force vs molecular extension curve. This method is of general interest for dual trap optical tweezers setups and can be extended to setups which do not directly measure forces.
我们展示了一种双阱光镊装置，它利用线性动量守恒直接测量力。该装置采用反向传播几何结构，允许分别对每束光的动量进行测量。此装置的实验优势包括全光操作导致的低漂移，以及由于力测量方法而具有的稳健校准（与捕获物体或缓冲介质的特性无关）。尽管这种设计无法达到一些同向传播装置的高分辨率，但我们表明它可用于执行不同的单分子测量：在不同力下基于波动的分子刚度表征以及分子发夹的跳跃实验。值得注意的是，在我们的装置中，可以操纵非常短的系链（例如带有短柄的分子发夹），直至珠子几乎接触的极限。该装置用于说明一种基于平衡力波动来测量光阱和系链刚度的新方法，即无需测量力与分子伸长曲线。这种方法对于双阱光镊装置具有普遍意义，并且可以扩展到不直接测量力的装置。
