There is considerable interest in small proteins that fold very rapidly. These proteins have become attractive targets for both theoretical and computational studies. The independently folded 36-residue villin headpiece subdomain has been the subject of a number of such studies and is predicted to fold quickly. We demonstrate using dynamic NMR line-shape analysis that the protein folds on the time scale of 10 mus. Folding rates were directly estimated between 56 and 78 degrees C using resolved protein resonances from three different residues at both 500 and 700 MHz. The rates estimated using different residues and different field strengths agree well with each other. The estimated folding rate lies between 0.5 and 2.0 x 105 s-1 over this temperature range. The folding rate depends only weakly on temperature.

One of the characteristics of Alzheimer disease is the presence of neurotoxic deposits in brain tissue, which are largely made up of a short, 39 -42-amino acid-long peptide referred to as amyloid ␤ (A␤).2 〈␤ peptide plays a major role in Alzheimer disease pathogenesis (1). The deposits (plaques) are an active subject of investigation in many research centers in an attempt to design preventative and therapeutic approaches to the disease (2-14). The molecular structures and morphologies of the fibrils comprising the plaques have been studied at various levels with a variety of imaging and spectroscopic techniques including neutron diffraction, atomic force microscopy, cryoelectronmicroscopy,magneticresonancespectroscopy,andfluorescence (8,(11)(12)(13)(15)(16)(17)(18)(19)(20)(21)(22).A typical characteristic of the fibrils is the presence of ribbon-like ␤-sheets with the strands close to perpendicular to the axis of the fibril, whereas the hydrogen bonds between different strands run parallel to the axis. In vitro studies of the full-length fibrils (that comprise 40 -42-residue peptides) demonstrate multiple morphological possibilities for fibril structures that are very much dependent on the details of the growth conditions (23-25). However, the characteristics of molecular structures are largely preserved (11). For example, in the wild-type sequence, only structures that contain parallel ␤-sheets are observed ( Fig. 1) (9, 24 -28). By contrast, antiparallel ␤-sheet structures can found in the D23N (Iowa) mutant (6,13,29,30), which is associated with an early onset of the neurodegeneration process (6). The polymorphs of D23N with the antiparallel ␤-sheet structures are "protofibrils"; i.e. they are relatively short and curved fibril-like intermediates. They are metastable and eventually convert to mature D23N fibrils, which have parallel ␤-sheet structures that are very similar to wild-type A␤ fibrils (13,22,29).Although both parallel and antiparallel structures can display cytotoxicity (13), the level of toxicity varies greatly depending on the morphological form (11,31). These polymorphs are often viewed as twisted or parallel shapes using transmission electron microscopy (11,25,32,33). The antiparallel ␤-sheet structure represents perhaps the most extreme variability in morphology among polymorphs identified up to date. Effective drug development has not been achieved so far, and one of the possible reasons is the complexity of the polymorphs and interconversions between them as well as the presence of conformational diversity within each polymorph.The intrinsic conformational diversity of the A␤ monomer is well known. In solution, A␤ lacks regular ␣-helical or ␤-stranded structure and is recognized as an intrinsically disordered protein (33-36). However, studies of dynamics in the fibril forms at site-specific level are relatively sparse (37). The most common view is that the hydrophobic core of the fibrils is rigid, complemented by other relative flexible regions (33, 38 -40). The flexibility of non-core re...

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