The Nanoscale World

Biopolymers such as DNA and proteins, of which living organisms are composed, express high-level functions indispensable for life support by forming a singular structure typically consisting of helices and self-organizing it in a specific point of time and space. In 1995, as a step to control helical polymers artificially, Eiji Yashima et al. found a new method for free formation of helices of desired direction (helicity) using polymers with various structures (helicity induction). They discovered a very peculiar phenomenon that the induced helix self-repairs its shape and the information on the repair is stored in its memory for a long time.

Helical structure and its direction (helicity) are the most essential and most important issue in the research study of macromolecular helical structure. However, it is not easy to directly determine the helicity and pitch using a microscope, even today and even for DNA polymers that are enormous molecules and on which extensive researches have been and are being carried out. Kumaki’s group in the Yashima Super-structured Helix Project (Japan Science and Technology Agency (ERATO, JST)), utilizing AFM observation technology and ingenious ideas, succeeded in directly determining the helical structures of many artificial helical polymers. They were further successful in determining variegated helical structures of tactic poly(methyl methacrylate) (PMMA), which is a general-purpose polymer commonly used for a large variety of applications.

Before their success, there had been no successful case of microscopic visualization of macromolecular helical structures in the world. Most probably, this is because much of the focus had concentrated on the observation of isolated, irregular polymer chains adsorbed onto the substrate. Their approach was different this time. They centered their focus on the observation of helical molecules self-organized into two-dimensional crystals on the solid board. This novel approach stemmed from the measurement of atomic images in the tapping mode operation of the atomic force microscope (AFM), and allowed them to be first to directly visualize macromolecular helical structures with a microscope. 

It is not easy to detect one atom placed on the substrate in contact mode atomic force microscope (AFM), as the radius of curvature of its probe is typically some nanometers.  Detection of one atom in contact mode AFM is, however, relatively easy if the atom has a surface with periodic structure, even though it is of sub-angstrom level in size and the radius of curvature of its probe is mere nanometers. In this case, there is a hundredfold or more difference in size between them.

With the conception as above, Kumaki, Sakurai, and Yashima thought that measurements might be easier if multiple molecules with periodic structure were arranged on the substrate, rather attempting to measure an individual molecule in pursuit of true atomic resolution. Another challenge was in preparing the sample. It was required to lay out each individual molecular layer in a very flat way. The question was to figure out what type of structure would meet this requirement.

Having cleared these conditions, they still had to jump over the last hurdle: the microscope had to be able to provide stable control of very weak forces at high resolution, while being capable of starting measurements without any damage to the probe tip.

Veeco’s MultiMode® Atomic Force Microscope with a NanoScope® Controller enabled Kumaki et al. to continually measure images without disturbance to achieve brilliant research results (see figure 1).

Figure 1. AFM height images of 2D self-assembled poly-Aib cast on HOPG. The polymer stands with clearly identifiable right- and left-handed helical blocks are shown in red and blue, respectively. The cross-section height profile, taken along the white dashed line, and right- (red) and left-handed (blue) helical polymer models constructed on the basis on the X-ray analysis are also shown. Reproduced with permission, ©2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The MultiMode gave Kumaki et al. quick, clear information on the number of helix inversion cycles, inversion points, and the number of helices and their length, and allowed them to make the crucial absolute molecular weight determination.

Making the best use of these successful results, Kumaki is progressing with various follow-up research, including the world’s first AFM observation of the reptant or creeping motion (molecular dance) of synthetic polymer monomolecular chains (see figure 2).

Figure 2. Movements of an it-PMMA chain on mica at lower humidity. (a) Time-lapse AFM images of it-PMMA chains at 34% RH. The arrow indicates movements of a loop along the chain. Scale: 121 x 264nm; scan direction: right to left. (b) Chain images from 3D AFM image of (a). Reproduced with permission, ©2006 American Chemical Society.

More recently, Yashima et al. are researching possible application of synthetic helical polymers to liquid crystal technology. If this research is successful, readily-controllable liquid crystal devices can be supplied at very reasonable prices on a global scale. Yashima says, “The results from this research can be achieved only when certain conditions are met, that is, we should have an expert in free control of synthesization of helix polymers and another who is capable of versatile assessment of the synthesized samples from various aspects, and finally if we can go ahead with the research united as a project team. Otherwise, we can not achieve the expected results.”

References

1. Shin-ichiro Sakurai, et al., “Two-Dimensional Helix Bundles Formation of a Dynamic Helical Poly(phenylacetylene) with Achiral Pendant Groups on Graphite,” Angew. Chem. Int. Ed. 2007, 46, 7605-7608.

2. Jiro Kumaki, et al., "’Reptational’ Movements of Single Sythetic Polymer Chains on Substrate Observed by in-Situ Atomic Force Microscopy,” Macromolecules 2006, 39, 1209-1215.