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AD (Alzheimer's disease) is linked to Abeta (amyloid beta-peptide) misfolding. Studies demonstrate that the level of soluble Abeta oligomeric forms correlates better with the progression of the disease than the level of fibrillar forms. Conformation-dependent antibodies have been developed to detect either Abeta oligomers or fibrils, suggesting that structural differences between these forms of Abeta exist. Using conditions which yield well-defined Abeta-(1-42) oligomers or fibrils, we studied the secondary structure of these species by ATR (attenuated total reflection)-FTIR (Fourier-transform infrared) spectroscopy. Whereas fibrillar Abeta was organized in a parallel beta-sheet conformation, oligomeric Abeta displayed distinct spectral features, which were attributed to an antiparallel beta-sheet structure. We also noted striking similarities between Abeta oligomers spectra and those of bacterial outer membrane porins. We discuss our results in terms of a possible organization of the antiparallel beta-sheets in Abeta oligomers, which may be related to reported effects of these highly toxic species in the amyloid pathogenesis associated with AD.

Attenuated total reflection Fourier-transform infrared spectroscopy of thin hydrated films of soluble and membrane protein included in a phospholipid bilayer is shown to provide useful information as to the secondary structure of the protein. The analysis of the amide I band of deuterated samples by Fourier self-deconvolution followed by a curve fitting was performed by a new procedure in which all the input parameters are generated by the computer rather than by the investigator. The results of this analysis provide a correct estimation of the a-helix and P-sheet structure content with a standard deviation of 8.6% when X-ray structures are taken as a reference. We also show that the orientation of the different secondary structures resolved by the Fourier self-deconvolution/ curve-fitting procedure and of the phospholipid acyl chains can be simultaneously evaluated for membrane proteins reconstituted in a lipid bilayer. Of special interest for reconstitution of membrane proteins, the lipid/ protein ratio can be accurately and quickly determined from the infrared spectrum.Among the spectroscopic techniques currently used for the determination on secondary structures of proteins and peptides (circular dichroism, NMR, X-ray diffraction analysis, etc.) infrared spectroscopy has recently become a prominent one. Although its potential usefulness has been recognized for some time [l, 21, the recognition of the different components of conformation-sensitive amides bands (particularly amide I band) was difficult due to the resulting overlap of bands originating from the different secondary structures such as a-helix, P-sheet, p-turns and random. Pointing out the different components of amide I has been made possible by the advent of numerical analysis of the spectra. The computed second and fourth derivatives [3, 41 for instance replace a broad featureless band by peaks of well-resolved components. More recently, the use of Fourier self-deconvolution has provided the same type of result. The advantage of Fourier selfdeconvolution is that it does not change the integrated areas of the different components of the amide envelope [5]. It must be noted that the Fourier self-deconvolution does not achieve an actual resolution enhancement but rather enhances selected frequencies of the spectrum. The actual resolution of the spectrum is defined by the spectrophotometer settings. Once the number and the approximate frequency of the different components are known, curve-fitting procedures can be applied to quantify the area of the different components of the amide envelope (much care must be taken then to prevent the appearance of artefacts due to the numerical treatment). ItCorrespondence to E. Goormaghtigh, UniversitC Libre de Bruxelles, Campus Plaine CP 206/2, B-1050 Bruxelles, BelgiumAbbreviations. ATR, attenuated total reflection; FTIR, Fouriertransform infrared spectroscopy; Myr2GroPCho, dimyristoylphosphatidylcholine; OcGlc, octyl-glucopyranoside; FWHH, full width at half height.was only in 1986 that the first comprehensi...

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Amyloid refers to insoluble protein aggregates that are responsible for amyloid diseases but are also implicated in important physiological functions (functional amyloids). The widespread presence of protein aggregates but also, in most of the cases, their deleterious effects explain worldwide efforts made to understand their formation, structure and biological functions. We emphasized the role of FTIR and especially ATR-FTIR techniques in amyloid protein and/or peptide studies. The multiple advantages provided by ATR-FTIR allow an almost continuous structural view of protein/peptide conversion during the aggregation process. Moreover, it is now well-established that infrared can differentiate oligomers from fibrils simply on their spectral features. ATR-FTIR is certainly the fastest and easiest method to obtain this information. ATR-FTIR occupies a key position in the analysis and comprehension of the complex aggregation mechanism(s) at the oligomer and/or fibril level. These mechanism(s) seem to present strong similarities between different amyloid proteins and might therefore be extremely important to understand for both disease-associated and functional amyloid proteins. This article is part of a Special Issue entitled: FTIR in membrane proteins and peptide studies.

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Amphipols (APols) are short amphipathic polymers that can substitute for detergents to keep integral membrane proteins (MPs) water soluble. In this review, we discuss their structure and solution behavior; the way they associate with MPs; and the structure, dynamics, and solution properties of the resulting complexes. All MPs tested to date form water-soluble complexes with APols, and their biochemical stability is in general greatly improved compared with MPs in detergent solutions. The functionality and ligand-binding properties of APol-trapped MPs are reviewed, and the mechanisms by which APols stabilize MPs are discussed. Applications of APols include MP folding and cell-free synthesis, structural studies by NMR, electron microscopy and X-ray diffraction, APol-mediated immobilization of MPs onto solid supports, proteomics, delivery of MPs to preexisting membranes, and vaccine formulation.

Fourier-transform infrared spectroscopy is a method of choice for the experimental determination of protein secondary structure. Numerous approaches have been developed during the past 15 years. A critical parameter that has not been taken into account systematically is the selection of the wavenumbers used for building the mathematical models used for structure prediction. The high quality of the current Fourier-transform infrared spectrometers makes the absorbance at every single wavenumber a valid and almost noiseless type of information. We address here the question of the amount of independent information present in the infrared spectra of proteins for the prediction of the different secondary structure contents. It appears that, at most, the absorbance at three distinct frequencies of the spectra contain all the nonredundant information that can be related to one secondary structure content. The ascending stepwise method proposed here identifies the relevance of each wavenumber of the infrared spectrum for the prediction of a given secondary structure and yields a particularly simple method for computing the secondary structure content. Using the 50-protein database built beforehand to contain as little fold redundancy as possible, the standard error of prediction in cross-validation is 5.5% for the alpha-helix, 6.6% for the beta-sheet, and 3.4% for the beta-turn.