Physica B 385–386 (2006) 865–867 Contrast variation SANS for the solution structure of the b-amyloid peptide 1–40 influenced by SDS surfactants U.-Ser Jeng a , Tsang-Lang Lin b,Ã , J.M. Lin b , Derek L. Ho c a National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan b Department of Engineering and System Science, National Tsing Hua University, 101, Sec. 2, Kuang Fu Road, Hsinchu 30043, Taiwan c National Institute of Standards and Technology, Gaithersburg, MD 02899, USA Abstract Using small-angle neutron scattering (SANS), we have studied the suppression of fibril formation of b-amyloid peptide (A b ), a 1–40 amino acid peptide fragment derived from proteolytic cleavage of a large amyloid precursor protein, by an ionic surfactant, SDS. In comparison with the pure peptide in aqueous solutions which forms long and thin fibrils, A b forms smaller complex with SDS, which hinders partially the growth of long fibrils. With a selected deuteration of SDS for a contrast variation in SANS, we have extracted the structural information of the SDS/peptide complex, including a short rod-like shape, size, and an association ratio between SDS and the peptide. r 2006 Elsevier B.V. All rights reserved. PACS: 87.15.Nn Keywords: SANS; Amyloid peptide; SDS; Complex aggregate 1. Introduction The b-amyloid peptide (A b ), of 1–40 amino acids, fragment derived from proteolytic cleavage of a large amyloid precursor protein folds into a random coil, b-helix, or oligomeric b-sheet structure in aqueous solutions. Without buffering, A b evolves into a b-sheet structure, and further forms fibril that can possibly deposit on membranes, as that found in the Alzheimer disease. A pioneer work using small-angle neutron scattering (SANS) and small-angle X-ray scattering for the structure of pure A b aggregates was done by Thiyagarajan et al. [1]. Here, we study the suppression effect on the fibril structure of b- amyloid by an inhibitor, an ionic surfactant sodium dodecyl sulfate (SDS). Using SANS, we have found a suppression effect of long fibril aggregates of A b in aqueous solutions with SDS. Substituting SDS with deuterated SDS (d-SDS) in the system for a contrast variation, we have identified the formation of peptide–surfactant complex. Using a model analysis for the contrast SANS data, we are able to extract a detailed structural information of the peptide/surfactant complex. 2. Scattering model and contrast variation Small-angle scattering profiles for colloidal aggregates of a monodisperse size can be modeled as [2] I ðQÞ¼ I o PðQÞSðQÞ, (1) where P(Q) is the normalized form factor with P(0) ¼ 1 and S(Q) is the structure factor. The scattering wave vector Q is defined by 4p sin(y/2)/l, with y and l the scattering angle and wavelength of incident neutrons, respectively. The forward scattering, I o (Q ¼ 0) ¼ n p (rr w ) 2 V 2 , is de- termined by the number density n p and the volume V of the scattering particles. The scattering-length-density for the particles and the solvent are denoted by r and r w , respectively. For homogeneous rod-like particles of radius a and length L, the spatial-orientation averaged form ARTICLE IN PRESS www.elsevier.com/locate/physb 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.05.128 Ã Corresponding author. Tel.: +886 3 5742671; fax: +886 3 5728445. E-mail addresses: usjeng@nsrrc.org.tw (U.-S. Jeng), tllin@mx.nthu. edu.tw (T.-L. Lin).