PolyQ peptides teeter between polyproline II (PPII) and beta-sheet conformations. In tandem polyQ-polyP peptides, the polyP segment tips the balance toward PPII, increasing the threshold number of Gln residues needed for fibrillation. To investigate the mechanism of cis-inhibition by flanking polyP segments on polyQ fibrillation, we examined short polyQ, polyP, and tandem polyQ-polyP peptides. These polyQ peptides have only three glutamines and cannot form beta-sheet fibrils. We demonstrate that polyQ-polyP peptides form small, soluble oligomers at high concentrations (as shown by size exclusion chromatography and diffusion coefficient measurements) with PPII structure (as shown by circular dichroism spectroscopy and (3)J(HN-C alpha) constants of Gln residues from constant time correlation spectroscopy NMR). Nuclear Overhauser effect spectroscopy and molecular modeling suggest that self-association of these peptides occurs as a result of both hydrophobic and steric effects. Pro side chains present three methylenes to solvent, favoring self-association of polyP through the hydrophobic effect. Gln side chains, with two methylene groups, can adopt a conformation similar to that of Pro side chains, also permitting self-association through the hydrophobic effect. Furthermore, steric clashes between Gln and Pro side chains to the C-terminal side of the polyQ segment favor adoption of the PPII-like structure in the polyQ segment. The conformational adaptability of the polyQ segment permits the cis-inhibitory effect of polyP segments on fibrillation by the polyQ segments in proteins such as huntingtin.

QS-21 is one of the most promising new adjuvants for immune response potentiation and dose-sparing in vaccine therapy given its exceedingly high level of potency and its favorable toxicity profile. Melanoma, breast cancer, small cell lung cancer, prostate cancer, HIV-1, and malaria are among the numerous maladies targeted in more than 80 recent and ongoing vaccine therapy clinical trials involving QS-21 as a critical adjuvant component for immune response augmentation. QS-21 is a natural product immunostimulatory adjuvant, eliciting both T-cell- and antibody-mediated immune responses with microgram doses. Herein is reported the synthesis of QS-21A(api) in a highly modular strategy, applying novel glycosylation methodologies to a convergent construction of the potent saponin immunostimulant. The chemical synthesis of QS-21 offers unique opportunities to probe its mode of biological action through the preparation of otherwise unattainable nonnatural saponin analogues.

Polyglutamine expansion in the exon 1 domain of huntingtin leads to aggregation into β-sheet-rich insoluble aggregates associated with Huntington's Disease. We assessed eight polyglutamine peptides with different permutations of N-methylation of backbone and side chain amides as potential inhibitors of polyglutamine aggregation. Surprisingly, the most effective inhibitor, 5QMe 2 (Anth-K-Q-Q(Me2)-Q-Q(Me 2 )-Q-CONH 2 , Anth = N-methyl anthranilic acid, Q(Me 2 ) = side chain N-methyl Q) has only side chain methylations at alternate residues, highlighting the importance of side chain interactions in polyglutamine fibrillogenesis. Above a 1:1 stoichiometric ratio, 5QMe 2 can completely prevent fibrillation of a synthetic aggregating peptide YAQ 12 A; it also shows significant inhibition at substoichiometric ratios. Surface plasmon resonance (SPR) measurements show a moderate K d with very fast k on and k off . Sedimentation equilibrium analytical ultracentrifugation indicates that 5QMe 2 is predominantly or entirely monomeric at concentrations up to 1 mM, and that it forms a 1:1 stoichiometric complex with a fibril-forming target, YAQ 12 A. 5QMe 2 inhibits not only nucleation of YAQ 12 A, but also fibril extension, as shown by the fact that it also inhibits seeded fibril growth where the nucleation steps are bypassed. 5QMe 2 acts on its targets only when they are in the PPII-like conformation, but not after they undergo a transition to β-sheets. Thus 5QMe 2 does not disassemble pre-formed YAQ 12 A; this contrasts with our previously described, backbone N-methylated inhibitors of β-amyloid aggregation (16,17). The mode of action of 5QMe 2 is reminiscent of chaperones, since it binds and releases its targets very rapidly, and maintains them in a non-aggregation-prone, monomeric state, in this case, the polyproline II (PPII)-like conformation, as shown by CD spectroscopy.Expanded polyglutamine (polyQ) tracts are responsible for at least nine neurodegenerative diseases, including Huntington's Disease (HD). HD occurs when the polyQ domain of exon † We acknowledge NIH Medical Scientist Training Program Grant (T32 GM07281 JDL), NIH Cardiovascular Pathophysiology Training Grant (HL07237 JDL) and NIH (NS042852 SCM) and the Alzheimer's Association (IIRG-06-27794). * To whom correspondence should be addressed: Department of Pathology, 5841 S. Maryland Ave., Chicago, IL 60637. Tel: 773-702-1267. Fax: 773-834-5251. scmeredi@uchicago.edu . Supporting Information Available The following items are available as supporting information: A derivation of the equations and other details needed for analysis of equilibrium sedimentation analytical ultracentrifugation of two dissimilar interacting species; determination of for YAQ 12 A and 5QMe 2 ; reverse phase HPLC isocratic analysis of YAQ 12 A remaining in solution, in the presence or absence of 5QMe 2 ; experiments showing the effect of added fibril seeds on fibrillation of YAQ 12 A; electron micrograph of fibril seed slurry used in preceding experiments; example of analytical ...