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...

With the goal of investigating dynamical features of hydrophobic cores of proteins over a wide range of temperatures, the chicken villin headpiece subdomain protein (HP36) was labeled at a "single" site corresponding to any one of the two C(delta)D(3) groups of leucine-69, which is located in a key position of the core. The main techniques employed are deuteron NMR quadrupolar echo line shape analysis, and T(1Z) (Zeeman) and T(1Q) (quadrupolar order) relaxation experiments performed at 11.7 and 17.6 T over the temperature range of 112 to 298 K. The experimental data are compared with computer simulations. The deuteron line shapes give an excellent fit to a three-mode motional model that consists of (a) fast three-site rotational jumps about the pseudo C(3) methyl spinning axis, (b) slower reorientation of the spinning axis, described by diffusion along a restricted arc, and (c) large angle jumps between traces of rotameric conformers. Relaxation behavior is described by a phenomenological distribution of activation energies for three-site hops at high temperatures that collapses to a single, distinctly smaller value for lower temperatures.

We have investigated microsecond to millisecond time scale dynamics in several key hydrophobic core methyl groups of chicken villin headpiece subdomain protein (HP36) using a combination of single-site labeling, deuteron solid-state NMR line shape analysis, and computational modeling. Deuteron line shapes of hydrated powder samples are dominated by rotameric jumps and show a large variability of rate constants, activation energies, and rotameric populations. Site-specific activation energies vary from 6 to 38 kJ/mol. An additional mode of diffusion on a restricted arc is significant for some sites. In dry samples, the dynamics is quenched. Parameters of the motional models allow for calculations of configurational entropy and heat capacity, which, together with the rate constants, allow for observation of interplay between thermodynamic and kinetic picture of the landscape. Mutations at key phenylalanine residues at both distal (F47L&F51L) and proximal (F58L) locations to a relatively rigid side-chain of L69 have a pronounced effect on alleviating the rigidity of this side chain at room temperature and demonstrate the sensitivity of the hydrophobic core environment to such perturbations.