The refolding of apo bovine alpha-lactalbumin has been monitored in real time by NMR spectroscopy following rapid in situ dilution of a chemically denatured state. By examining individual resonances in the time-resolved NMR spectra, the native state has been shown to emerge in a cooperative manner from an intermediate formed in the dead-time of the experiments. The kinetics of folding to the native state are closely similar to those observed by stopped-flow fluorescence and near-UV circular dichroism. The NMR spectrum of the transient intermediate resembles closely that of the well characterized stable molten globule state formed at low pH. The results suggest that NMR can play a key role in describing at an atomic level the structural transitions occurring during protein folding.

No abstract

The cold-shock protein CspB (from Bacillus subtilis), a very small protein of 67 residues, folds extremely fast in a reversible N ª U two-state reaction. Both unfolding and refolding are strongly decelerated when the viscosity of the solvent is increased by adding ethylene glycol or sucrose. The folding of CspB thus seems to follow Kramers' model for reactions in which the reactants must diffuse together. It indicates that the compaction of the protein chain occurs in the rate-limiting step of folding. Chain diffusion to a productively collapsed form and the crossing of a high energy barrier are thus tightly coupled in this folding reaction, and the measured reaction rate depends on both the diffusion of the protein chain in the solvent and the magnitude of the activation energy. We suggest that in protein folding an energetic barrier is essential to separate the native from the unfolded conformations of a protein. This barrier protects the ordered structure of a native protein against continuous unfolding by diffusive chain motions and leads to apparent two-state behavior.Most protein chains reach their native conformations during folding rapidly and with high precision, even though the native state is only marginally stable and even though an unfolded protein can adopt very many conformations. Often it is assumed that the folding process is so efficient, because it occurs in two distinct stages (1-4). In the first stage the extended protein chain collapses rapidly into a compact form (often called a molten globule), which is already native-like, but still loosely packed (4, 5). In the second, slow stage the protein chain rearranges to the native state, possibly by a restricted search through the compact conformations. This stage shows a high energy barrier and determines the overall rate of folding. Until recently it was assumed that the rapid formation of compact intermediates is a prerequisite for fast and efficient folding (2,(4)(5)(6)(7)(8).Several small proteins fold extremely fast within a millisecond or even less, but no partially structured intermediates could be detected in these folding reactions (9-17). This seems puzzling. Either these proteins do not follow the two-stage model in their folding, or the initial collapse is so specific that the second stage becomes extremely fast. Thus, the compact intermediate would not accumulate, and the diffusive collapse would become rate-limiting for the entire folding reaction. In this case folding should not follow a monoexponential time course. As a third possibility, chain compaction and crossing of the energy barrier could be tightly coupled in folding. Kramers (18) developed a kinetic model for reactions in which the reactants diffuse together in the rate-limiting step, and he found that the time constants of such processes should depend linearly on the viscosity of the medium. Kramers' theory was used by Karplus and Weaver (19,20) when they formulated the diffusion-collision model for protein folding. Folding reactions that are limited in rate by...

No abstract

Supramolecular polyisobutylenes (PIB) bearing mono- and bifunctional chain ends with hydrogen-bonding units were prepared, and their association behavior in the melt state was investigated by dynamic rheology and compared to aggregation in solution, aiming at determining association dynamics in the solid state. A preparation combining living cationic polymerization with either azide/alkyne “click” reactions or nucleophilic substitution reactions enabled a full end group transformation to the final PIB polymers, modified with either thymine or 2,6-diaminotriazine end groups as proven by NMR and MALDI methods with molecular weights of ∼3500 and ∼10 000 g/mol. Stoichiometric mixtures of these polymers bearing specifically interacting thymine/triazine moieties were prepared by solution blending and the temperature-dependent dynamics investigated by rheological measurements. At temperatures of 20−60 °C all samples display strongly thermoreversible aggregation with sheet-type or partially cross-linked structures, which deaggregate at temperatures of ∼80 °C. More complex aggregates with bridged micellar structure were formed from the respective bifunctional PIB’s bearing thymine and 2,6-diaminotriazine moieties. Thus, in addition to specific linear aggregates, the formation of clusters and aggregates of different architecture has to be taken into account to understand and control structure and mechanical properties of supramolecular chains in the melt.