Klaus Schmidt-Rohr

Microstructure of poly(vinyl alcohol) hydrogels produced by freeze/thaw cycling

To understand the reversible gelation and subsequent aging of hydrogels prepared by freeze/thaw processing of poly(vinyl alcohol) (PVOH) solutions, the microstructures of gels prepared by different freeze/thaw protocols and aged to varying extents are studied by cryogenic transmission electron microscopy, solid-state nuclear magnetic resonance, X-ray scattering, and differential scanning calorimetry (DSC). As discussed in the literature, gelation by the freeze/thaw process occurs as a homogeneous aqueous poly(vinyl alcohol) solution is cycled, perhaps multiple times, between temperatures above 0 °C and well below 0 °C. The current investigation has determined that a few percent of chain segments crystallize during the first cycle, organizing themselves into 3-8 nm primary crystallite junctions separated on an irregular mesh by an average spacing of ∼ 30 nm. Aging or imposition of additional freeze/thaw cycles augments the level of crystallinity and transforms the as-formed liquid-like microstructure, characterized in the electron microscope by rounded ∼ 30 nm pores, into a fibrillar network. Observation that the transformation occurs at fixed mesh spacing and approximately constant average crystallite size suggests the formation of secondary crystallites that do not affect network connectivity. Dendritic ice crystallization and possibly spinodal decomposition superimpose on this nanoscale structure a matrix of much larger pores.

Principles of centerband-only detection of exchange in solid-state nuclear magnetic resonance, and extension to four-time centerband-only detection of exchange

Theoretical principles and experimental details of the centerband-only detection of exchange (CODEX) nuclear magnetic resonance (NMR) experiment for characterizing slow segmental dynamics in solids are described. The experiment, which is performed under magic-angle spinning, employs recoupling of the chemical-shift anisotropy before and after a long mixing time during which molecular reorientations may occur. By an analysis in terms of the difference tensor of the chemical shifts before and after the mixing time, the dependence on the reorientation angle is obtained analytically for uniaxial interactions, and a relation to two-dimensional exchange NMR patterns is established; the same theory can also be applied for analyzing stimulated-echo and pure-exchange NMR data. A favorable linear dependence is derived generally for small rotations, which makes the experiment suitable for detecting small-amplitude motions. Quantification is excellent because the peaks are narrow and intense, unlike the broad powder ...

IDENTIFICATION AND MOBILITY OF DEUTERATED RESIDUES IN PEPTIDES AND PROTEINS BY 2H-13C SOLID-STATE NMR

Abstract We present a solid-state NMR approach for 13 C chemical-shift identification and 2 H lineshape characterization of amino-acid residues in peptides and proteins deuterated by amide hydrogen/deuteron exchange. The technique exploits heteronuclear 13 C – 2 H dipolar couplings to correlate 13 C nuclei and nearby deuterons. Magic-angle spinning provides high sensitivity and 13 C chemical-site resolution. The simplest version of the experiment, which is closely related to REDOR, yields a 13 C spectrum permitting identification of the deuterated residues. In the full two-dimensional experiment, segmental dynamics are characterized in terms of 2 H -NMR lineshapes. The technique is demonstrated on dipeptides and a 14-kDa protein, with 13 C in natural abundance.