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The folding of globular proteins occurs through intermediate states whose characterisation provides information about the mechanism of folding. A major class of intermediate,states is the compact 'molten globule', whose characteristics have been studied intensively in those conditions in which it is stable (at acid pH, high temperatures and intermediate concentrations of strong denaturants). In studies involving bovine carbonic anhydrase, human a-lact-albumin, bovine /3-lactoglobulin, yeast phosphoglycerate kinase, D-lactamase from Staphylococcus aurew and recombinant human interleukin l/I, we have demonstrated that a transient intermediate which accumulates during refolding is compact and has the properties of the 'molten globule' state. We show that it is formed within 0.1-0.2 s. These proteins belong to different structural types (8, a+/3 and a/S), with and without disulphide bridges and they include proteins with quite different times of complete folding (from seconds to decades of minutes).We propose that the formation of the transient molten globule state occurs early on the pathway of folding of all globular proteins. Protein folding; Folding intermediate; Folding kinetics; Framework modelThe stable molten globule states obtained under mild denaturing conditions have been shown to be consistently much more compact than state U by viscosity, sedimentation, diffuse X-ray scattering, quasielastic light scattering and urea gradient electrophoresis for CAB [4], ,8Lase [5,6] and aLA [1,2,7]. It is almost as compact as state N [l-7] and has a pronounced secondary structure [l-4,6,8].This secondary structure can be N-like and the molten globule may have some features of the N fold [9]. However, this state differs from state N by the absence of close packing throughout the molecule and by a substantial increase of fluctuations in side chains as well as of larger parts of the molecule [l-3]. In agreement with these data, the equilibrium molten globule states for PLase [lo] and CAB both have Ves on FPLC gel exclusion that are intermediate between the Ves for N and U states. This permits the use of FPLC not only for the evaluation of the compactness of these states but also to monitor the kinetics of the formation of state N in refolding experiments [lo].

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The temperature-induced coil-globule transition has been studied in dilute aqueous solutions (with 200 mg/L SDS) for different fractions of polyW-isopropylacrylamide) (PNIPAM) and poly-CZV-isopropylmethacrylamide) (PNIPMAM) using scanning microcalorimetry, diffusion, and size-exclusion chromatography (FPLC). It has been shown that both these polymers undergo a coil-globule transition upon temperature increase. This transition is accompanied by cooperative heat absorption and a decrease of heat capacity, which makes it similar to the cold denaturation of globular proteins. The globule-coil transition is an "all-or-none" process only for the fractions with the lowest molecular weights (~10 x 103) while fractions of larger molecular weights behave as if they consist of quasi-independent cooperative units, the "domains". The number of "domains" in a macromolecule is proportional to the molecular weight of the polymer. This suggests that the "domain" character of cooperative transitions in large proteins does not, in principle, need evolutionary-selected amino acid sequences but can occur even in homopolymers.