A macromolecular history

The study of large molecules follows two strands that have alternately diverged and intertwined over the subject's history. The first strand explores the natural macromolecules of biology, including proteins, polysaccharides and nucleic acids. The second is concerned with synthetic macromolecules, the invention of which in the early twentieth century launched industries based on plastics such as nylon, polyethylene and Perspex. In Giant Molecules, biophysicist Walter Gratzer weaves together both stories.

Initially the two strands developed together, sharing experimental methods and theoretical approaches. The simpler chemistry of the synthetic materials offered tractable analogues of the natural systems. Then, as the inherent complexity of biological molecules became central to their understanding, the fields split. The flowering of structural biology in the late 1950s and 1960s was driven by the technique of X-ray diffraction and owed little to polymer science.

Gratzer describes the history engagingly, and includes many anecdotes. He explains how German chemist Hermann Staudinger's concept of polymers as giant molecules became accepted amid controversy, rancour and the ugly academic politics of German universities in the period up to the Second World War. Not all of the anecdotes he chooses are reliable: he includes, for example, the widely held but incorrect notion that the windows in medieval cathedrals are thicker at their base because the glass has flowed. This lapse is symptomatic of a general weakness in the book when it comes to the physical science of macromolecules.

Recent developments in the physics and chemistry of macromolecules get short shrift. The book's discussion of ways of measuring the size of polymer molecules, for example, is many years out of date, and the influential work of those such as Nobel laureate Pierre-Gilles de Gennes, who brought theoretical physics concepts to molecular science, is not mentioned. New methods of polymer chemistry, such as living polymerization, ring-opening metathesis and solid-phase peptide synthesis, all of which allow unparalleled control of the size and architecture of synthetic macromolecules, are not mentioned, despite yielding Nobel prizes.

Tiny capsules made of synthetic polymers can be used to deliver drugs for slow release in the body. 

In recent years, the two strands of macromolecular science have converged again. Techniques such as laser tweezers and single-molecule force spectroscopy have allowed us to study the behaviour of biological macromolecules as individual physical objects. Aspects of protein behaviour, such as their mechanical unfolding and the structures they form when they misfold, re-emphasize the analogies between biological and synthetic macromolecules. The increasing ability of chemists to control the architecture of synthetic polymers has made new applications possible, especially in nanotechnology. The new forms of carbon — fullerenes, nanotubes and graphene — earn their place in the book.

Gratzer covers the promise of polymer nanotechnology in brief. Some applications — such as glues inspired by shellfish; drug-delivery devices based on self-assembled polymer vesicles; and scaffolds for tissue engineering — are directly inspired by biology. Others, such as the plastic electronics made possible by semi-conducting polymers, use properties of macromolecules that have not been exploited by nature.

Arguably, DNA is the most important macromolecule, and I share the author's particular fascination with its potential uses, for example as the basis of synthetic molecular motors, for information processing and to make intricate self-assembled nano-objects. Only time will tell whether such beautiful laboratory demonstrations will yield practical technologies that have the impact in the twenty-first century that plastics had in the twentieth.