DIRECTIONAL BROWNIAN MOTION IN UNIFORM HEAT BATH

1991 Fiscal Year Final Research Report Summary

• Project/Area Number: 02804021
• Research Category: Grant-in-Aid for General Scientific Research (C)
• Allocation Type: Single-year Grants
• Research Field: 物理学一般
• Research Institution: KANAZAWA UNIVERSITY
• Principal Investigator: YAMAZAKI Masatoshi KANAZAWA UNIVERSITY, DEPT. OF PHYSICS, PROFESSOR, 理学部, 教授 (00019449)
• Co-Investigator(Kenkyū-buntansha): ANDO Toshio KANAZAWA UNIVERSITY, DEPT. OF PHYSICS, ASSOCIATE PROFESSOR, 理学部, 助教授 (50184320)
• Project Period (FY): 1990 – 1991
• Keywords: Brownian Motion / Frictional Force / Directional Motion / Fluctuation / Heat Bath / Asymmetry

Research Abstract

We studied one-dimensional Brownian motion of a rod-shaped polymer in solution. We named the each end of the polymer by 'plus-end' and 'minus-end', respectively. The polymer experiences frictional force from the solvent against its movement along the long axis. Suppose that the frictional coefficient against the movement toward the plus-end is smaller than that against the movement toward the minus-end, the polymer seems to move, on long-time average, progressively toward the plus-end. In a macroscopic body which has asymmetric frictional coefficients, such one-directional movement really takes place by receiving random forces. The purpose of the present study is to examine whether or not one-directional Brownian motion occurs even in microscopic world. As a long polymer, we used actin filaments decorated with myosin subfragment-1 (S-1). S-1 shaped like a pear binds to the filamentous actin, making an angle of roughly 45ﾟ against the long axis of the actin filament, and thus the actin- S-1 complex has asymmetry along the long axis. In order to realize the one-dimensional motion along the long axis of the complex, methyl-cellulose was added to the solvent. To visualize the complex under a fluorescence microscope, actin was stained by fluorescent dye. To visualize the polarity of the actin filament, the barbed-end side and the pointed-end side of the filament were stained different fluorescent dyes, or by the same dye but with different degree. We successfully observed not only the Brownian motion but also the relation of the movement direction and the actin's polarity. The actin-S-1 complex moved nearly in one-dimension. It was, however, quite difficult to measure with high accuracy the distance of movement.
This was mainly due to the fact that the complex was quite flexible and therefore scarcely positioned on a focal plane of the objective lens over the total length. Even when such incident occurred at some moment, the complex gradually moved out the focal plane after a few second. So, the measured mass center contained error. We analyzed the one-dimensional motion, but could not detect, within the poor precision, predominant direction of the movement. To overcome this difficulty, we tried to make the actin-S-1 complex more rigid by attaching tropomyosin to actin, or by chemically treating actin. Certainly the complex became rigid, but still was moving out the focal plane in moments. In conclusion, we need to have an instrument which allows us to chase the objects moving in three dimension. Such an instrument will be developed in near future.