Molecular Combing of a Flexible Polymer Chain by Simple Spin-Casting

Structural hierarchy of mechanical extensibility in human von Willebrand factor multimers

The von Willebrand factor (VWF) is a multimeric glycoprotein composed of 80- to 120-nm-long protomeric units and plays a fundamental role in mediating platelet function at high shear. The exact nature of the shear-induced structural transitions have remained elusive; uncovering them requires the high-resolution quantitative analysis of gradually extended VWF. Here, we stretched human blood-plasma-derived VWF with molecular combing and analyzed the axial structure of the elongated multimers with atomic force microscopy. Protomers extended through structural intermediates that could be grouped into seven distinct topographical classes. Protomer extension thus progresses through the uncoiling of the C_1-6 domain segment, rearrangements among the N-terminal VWF domains, and unfolding and elastic extension of the A₂ domain. The least and most extended protomer conformations were localized at the ends and the middle of the multimer, respectively, revealing an apparent necking phenomenon characteristic of plastic-material behavior. The structural hierarchy uncovered here is likely to provide a spatial control mechanism to the complex functions of VWF.

Molecular Combing of Various Poly(n-Alkyl Acrylate) Chains on Mica by the Dipping Method

If polymer chains could be deposited on a substrate as a fully extended chain, a procedure known as "molecular combing," the chain structure could be characterized by atomic force microscopy in more detail than has been possible with the measurements available today. We show here, for the first time, that flexible polymers can be molecularly combed to fully extended chains by the dipping method. We studied the molecular combing of a series of poly(n-alkyl acrylate)s on mica from a chloroform solution by the dipping method and found that poly(n-alkyl acrylate)s with an alkyl group longer than n-octyl can be molecularly combed into straight chains under optimized conditions. With increasing alkyl lengths, the number of chains deposited decreases by four orders of magnitude, and chains become molecularly combed under a wider range of conditions. The length of the molecularly combed chains is ∼80% for poly(n-octyl acrylate) but ∼100% of the all-trans conformation for poly(n-alkyl acrylate)s with an alkyl length longer than n-nonyl.