Solid-State NMR in Macromolecular Systems: Insights on How Molecular Entities Move

Abstract

The function of synthetic and natural macromolecularsystems critically depends on the packing and dynamics of the individual components of a given system. Not only can solid-state NMR provide structural information with atomic resolution, but it can also provide a way to characterize the amplitude and time scales of motions over broad ranges of length and time. These movements include molecular dynamics, rotational and translational motions of the building blocks, and also the motion of the functional species themselves, such as protons or ions. This Account examines solid-state NMR methods for correlating dynamics and function in a variety of chemical systems. In the early days, scientists thought that the rotationalmotions reflected the geometry of the moving entities. They described these phenomena as jumps about well-defined axes, such as phenyl flips, even in amorphous polymers. Later, they realized that conformational transitions in macromolecules happen in a much more complex way. Because the individual entities do not rotate around well-defined axes, they require much less space. Only recently researchers have appreciated the relative importance of large angle fluctuations of polymers over rotational jumps. Researchers have long considered that cooperative motions might be at work, yet only recently they have clearly detected these motions by NMR in macromolecular and supramolecular systems. In correlations of dynamics and function, local motions do not always provide the mechanism of long-range transport. This idea holds true in ion conduction but also applies to chain transport in polymer melts and semicrystalline polymers. Similar chain motions and ion transport likewise occur in functional biopolymers, systems where solid-state NMR studies are also performed. In polymer science, researchers have appreciated the unique information on molecular dynamics available from advanced solid-state NMR at times, where their colleagues in the biomacromolecular sciences have emphasized structure. By contrast, following X-ray crystallographers, researchers studying proteins using solution NMR introduced the combination of NMR with computer simulation before that became common practice in solid-state NMR. Today's simulation methods can handle partially ordered or even disordered systems common in synthetic polymers. Thus, the multitechnique approaches employed in NMR of synthetic and biological macromolecules have converged. Therefore, this Account will be relevant to both researchers studying synthetic macromolecular and supramolecular systems and those studying biological complexes.

Studying dynamics by magic-angle spinning solid-state NMR spectroscopy: Principles and applications to biomolecules

“…In the model-free approach [61,62] we assume that the correlation function can be expressed by a mono-exponential decay with the functional form (23) which leads to a spectral density function given by (24) In this approach there are two parameters, namely the correlation time τ c which characterizes the time scale of the motion and the order parameter S 2 which characterizes the spatial restriction of the motion. In this case the total spectral-density function is independent of the parameters m and m' and we obtain for an two interactions μ and μ' which are both axially symmetric (25) Because of the orientation dependence of relaxation-rate constants in solids, we would like to discuss some general properties of spin relaxation in MAS NMR, which are illustrated by Figure 3.…”

“…In this review we focus on spin-1/2 nuclei, and deliberately omit methods that use quadrupolar couplings, such as deuterium relaxation and line shape analysis. These approaches, (for biomolecules in particular deuterium-based experiments), have been reviewed elsewhere [19][20][21][22][23]. For additional complementary views of (bio-)molecular dynamics studied by solution-or solid-state NMR spectroscopy, we refer the reader to existing reviews [3,5,[23][24][25][26][27][28][29][30]].…”

Polymer Transport Properties

“…Such notion is illustrated in Table 11, which provides the ratio of gas permeability in amorphous PET to the corresponding gas permeability in various related amorphous polyesters. NMR characterization of polymer ring-flipping motions can be accomplished via multiple methods, where one effective method involves examining the 2 H line-shape spectra for polymers which contain deuterated phenyl rings (245,250). When measured correctly, such line-shapes (also termed Pake patterns or doublets) can provide detailed information regarding the dynamics of phenyl ring motion.…”

Modern solid state NMR techniques and concepts in structural studies of synthetic polymers

“…SSNMR is able to provide a wealth of information regarding the structure, dynamics, and different kind of interactions presented in the solid phase in a nondestructive way. [3][4][5] Some structural features typical for polymers, such as primary and higher order structure, degree of crystallization, formation of polymorphs, chain ends or junctions in linear, block, crosslinked, star, hyperbranched, or grafted (co)-polymers, may be precisely characterized. Moreover, this technique can be used for examination of additives, elucidation of undesired modifications during aging, and so on.…”