18040
J. Phys. Chem. B 2006, 110, 18040-18045
Alzheimer Amyloid β(1-40) Peptide: Interactions with Cationic Gemini and Single-Chain Surfactants Yajuan Li, Meiwen Cao, and Yilin Wang* Key Laboratory of Colloid and Interface Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China ReceiVed: May 24, 2006; In Final Form: July 19, 2006
The aggregation of amyloid β-peptide [Aβ(1-40)] into fibril is a key pathological process associated with Alzheimer’s disease. The effect of cationic gemini surfactant hexamethylene-1,6-bis-(dodecyldimethylammonium bromide) [C12H25(CH3)2N(CH2)6N(CH3)2C12H25]Br2 (designated as C12C6C12Br2) and single-chain cationic surfactant dodecyltrimethylammonium bromide (DTAB) on the Alzheimer amyloid β-peptide Aβ(1-40) aggregation behavior was studied by microcalorimetry, circular dichroism (CD), and atomic force microscopy (AFM) measurements at pH 7.4. Without addition of surfactant, 0.5 g/L Aβ(1-40) mainly exists in dimeric state. It is found that the addition of the monomers of C12C6C12Br2 and DTAB may cause the rapid aggregation of Aβ(1-40) and the fibrillar structures are observed by CD spectra and the AFM images. Due to the repulsive interaction among the head groups of surfactants and the formation of a small hydrophobic cluster of surfactant molecules, the fibrillar structure is disrupted again as the surfactant monomer concentration is increased, whereas globular species are observed in the presence of micellar solution. Different from singlechain surfactant, C12C6C12Br2 has a much stronger interaction with Aβ(1-40) to generate larger endothermic energy at much lower surfactant concentration and has much stronger ability to induce the aggregation of Aβ(1-40).
Introduction Alzheimer’s disease (AD) neuropathology is characterized by extracellular amyloid deposition of senile plaques and neurofibrillary tangles in vulnerable AD brain regions.1 These plaques are composed primarily of fibrils of amyloid β-peptide,2 a small peptide composed of 39-43 amino acids in length.3 The most abundant forms are 40 and 42 amino acids. This kind of peptide is cleaved from a large protein called amyloid precursor protein (APP) by a system of secretases.4 The production of amyloid β-peptide is a normal occurrence, although its function remains unknown. However, in AD patients the peptide forms ordered fibrillar aggregates. This process involves the secondary structure transition from disordered random structure to ordered β-sheet conformation.5 Recently, it was revealed that the early soluble oligomers of amyloid β-peptide can be more neurotoxic than the fibrillar form.6 Thus, an attractive therapeutic approach to AD would be to reduce selectively the levels of potentially synaptotoxic amyloid β-peptide oligomers. Until now, many researchers have focused on the discovery and development of non-peptide inhibitors to fibril formation and to amyloid β-peptide aggregation for clinical evaluation in AD treatment,7 and these works have been summarized in several review articles.8,9 Micelles can produce a heterogeneous amphiphilic environment, which may be a basis for the role of biological membranes.10 Therefore, the effect of surfactants on the propensity of the formation of fibrils from amyloid β-peptide has been extensively described.11-15 For example, Marcinowski et al.13 studied the interaction of amyloid β-peptide fragment Aβ(1-28) with three micelle systems: the anionic sodium * To whom correspondence should be addressed. E-mail: yilinwang@ iccas.ac.cn.
dodecyl sulfate, zwitterionic dodecylphosphocholine, and cationic dodecyltrimethylammonium chloride. Their results showed that the promotion and stabilization of R-helix secondary structure are highly dependent on the surface charge of micelles, indicating the significant role of electrostatic interaction between Aβ(1-28) and surfactants. In addition, Sabate´ and Estelrich11 reported the interaction of Aβ(1-40) with three alkylammonium bromides (dodecyl, tetradecyl, and hexadecyl) at supra- and submicellar concentrations. They observed a dual behavior: the surfactants promoted or retarded fibril formation in a concentration-dependent manner. Below critical micelle concentration (cmc), the surfactants promote the aggregation, presumably by means of electrostatic interactions. Beyond this concentration, the presence of micelles induces a delay of the aggregation. Although the role of conventional single-chain surfactants in the aggregation of amyloid β-peptide has been put forward, the effect of gemini surfactant on the aggregation has not yet been reported. Gemini surfactants are made of two hydrophobic chains and two head groups covalently linked through a spacer group.16-19 This novel class of surfactants has many unique aggregation properties, such as remarkably low cmc, high dependence on spacer structure, unusual aggregation morphologies, strong hydrophobic microdomain, and so on. Therefore, gemini surfactants are expected to show special effects on the aggregation of amyloid β-peptide. In the present work, the effect of gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) ([C12H25(CH3)2N(CH2)6N(CH3)2C12H25]Br2, designated as C12C6C12Br2) as well as single-chain surfactant dodecyltrimethylammonium bromide (DTAB) on the aggregation behavior of amyloid β-peptide [Aβ(1-40)] has been investigated by microcalorimetry, circular dichroism (CD), and atomic force microscopy (AFM) under a physiological pH condition of 7.4. Aβ(1-40)
10.1021/jp063176h CCC: $33.50 © 2006 American Chemical Society Published on Web 08/23/2006
Alzheimer Amyloid β(1-40) Peptide
J. Phys. Chem. B, Vol. 110, No. 36, 2006 18041
Figure 1. Sequence of the Aβ(1-40) used.
has an amphipathic sequence with polar residues 1-28 and hydrophobic residues 29-40. At pH 7.4, Aβ(1-40) bears six negative charges. The influence of surfactant on the aggregation of Aβ(1-40) is monitored as a function of surfactant concentration. Experimental Section Materials. The peptide used was fragment 1-40 of Aβ (trifluoroacetate salt) and was obtained from Sigma (Aβ(140), No. A4473, Lots 043K1348, 085K1804, and 095K1569). The purity of the peptide was greater than 95%. The sequence of the Aβ(1-40) peptide is shown in Figure 1. Gemini surfactant C12C6C12Br2 was synthesized and purified according to the method of Menger and Littau.18,20 The structure of C12C6C12Br2 was confirmed by 1H NMR spectroscopy, and the purity was verified by elemental analysis. DTAB (purity > 99%) from Aldrich was recrystallized twice before use. 1,1,1,3,3,3Hexafluoro-2-propanol (HFIP) was from ACROS. Aβ(1-40) Preparation. Aβ(1-40) in nonaggregated form was prepared as described previously.21 Briefly, a stock solution was prepared by dissolving 1 mg of Aβ(1-40) in 1 mL of HFIP, which was incubated at room temperature for 1 h in sealed vials. Then, the solution was bath-sonicated for 30 min until Aβ(140) dissolved completely. After that, the solution was kept at 4 °C for 30 min to avoid solvent evaporation when aliquoting. Once the sample was aliquoted, HFIP was removed from the disaggregated peptide by evaporation under a gentle stream of nitrogen, leaving a slightly yellow film. Just before initiating the following experiments, an aliquot of Aβ(1-40) was added to 5 mM sodium phosphate buffer of ionic strength 0.01 mol/L at pH 7.4 to reach a final peptide concentration of 0.5 g/L. The molecular weight of Aβ(1-40) was about 4328.9, and the molar concentration of Aβ(1-40) was about 0.116 mM. Isothermal Titration Microcalorimetry (ITC). The calorimetric measurements were conducted using a TAM 2277-201 microcalorimetric system (Thermometric AB, Ja¨rfa¨lla, Sweden) with a stainless steel sample cell of 1 mL at 298.15 ( 0.01 K. The cell was initially loaded with 0.7 mL of pH 7.4 buffer or 0.5 g/L Aβ(1-40) in pH 7.4 buffer. The concentrated surfactant solutions were injected into the sample cell via a 500-µL Hamilton syringe controlled by a 612 Thermometric Lund pump. A series of injections, each 4-10 µL, were made until the desired range of concentration had been covered. The system was stirred at 50 rpm with a gold propeller. The titration rate was 0.5 µL/s, and the interval between two injections was 15 min. The accuracy of the calorimeter was periodically calibrated electrically and verified by measuring the dilution enthalpies of concentrated sucrose solution.22 Raw data were obtained as a plot of heat flow P against time t. The peak for each injection was integrated to obtain a plot of observed enthalpy (∆Hobs) against the final surfactant concentration. Circular Dichroism. CD spectra were recorded on a JASCO J-810 spectrophotometer at room temperature using a 0.1-mm quartz cell. Scans were obtained in a range between 190 and 260 nm by taking points at 1 nm, with an integration time of 0.5 s. Typically, eight spectra were averaged and smoothed to improve the signal-to-noise ratio. The results were represented
Figure 2. Variation of the observed enthalpies (∆Hobs) of dilution of (a) C12C6C12Br2 and (b) DTAB into pH 7.4 phosphate buffer at 298.15 K.
as molar residue ellipticity (Theta, in units of deg cm2 dmol-1), taking a value of 108 as mean residue weight for the Aβ(140) peptide. Atomic Force Microscopy. AFM morphology images were recorded using a tapping mode (Nanoscope IIIa multimode system, Digital Instruments, Santa Barbara, CA) with silicon cantilever probes in air. To prepare specimens for AFM imaging, we incubated 5-10 µL aliquots of Aβ(1-40) solutions in the absence or in the presence of surfactant on the surface of freshly cleaved mica, usually for 30-60 s. Following incubation, the excess solution was dried with a gentle stream of nitrogen. Results The micellization of C12C6C12Br2 and DTAB in 5 mM pH 7.4 phosphate buffer was first studied by microcalorimetry. The calorimetric titration curves of the observed enthalpy (∆Hobs) with the final surfactant concentration (C) are presented in Figure 2. On each plot, a break corresponding to micelle formation is observed, allowing identification of the cmc. When C is below the cmc, the added micelles dissociate into monomers and the monomers are then diluted. When C is above the cmc, the added micelles are only diluted and finally ∆Hobs drops toward zero. The value of the cmc is then determined from the intersection of the two lines extrapolated from the initial portion
18042 J. Phys. Chem. B, Vol. 110, No. 36, 2006
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Figure 3. Calorimetric titration curves for concentrated C12C6C12Br2 and DTAB solution into 0.5 g/L Aβ(1-40) in pH 7.4 phosphate buffer against the final surfactant concentration at 298.15 K. (a) C12C6C12Br2. (b) DTAB. The insets are enlargements of the curves at lower surfactant concentration.
of the curve and of the rapidly decreasing portion of the curve,23,24 as shown in Figure 2a. The cmc values for DTAB and C12C6C12Br2 at pH 7.4 are 14.50 ( 0.5 mM and 0.56 ( 0.05 mM, respectively. The cmc value of C12C6C12Br2 is much lower than that of DTAB, which is consistent with the observation of other authors.20,25 This is mainly because of the two alkyl chains in each gemini surfactant molecule and the resultant stronger hydrophobic interaction.26 To investigate the interaction of Aβ(1-40) with C12C6C12Br2 and DTAB, microcalorimetry was performed to monitor the enthalpy change during the interaction process. The calorimetric titration curves of the variations of the observed enthalpies (∆Hobs) of C12C6C12Br2 and DTAB diluted into Aβ(1-40) with the final concentration (C) of surfactants at 298.15 K are shown in Figure 3, together with the curves for the corresponding surfactants added into the buffer without Aβ(1-40) for comparison. The titration curves for C12C6C12Br2 and DTAB into Aβ(1-40) solution show obvious differences from the curves for the surfactants in the absence of Aβ(140). The differences can be attributed to the interactions between Aβ(1-40) and the surfactants. With the first several injections of the surfactants into Aβ(1-40) solution, ∆Hobs decreases rapidly until the minimum point. Then, with the further addition of the surfactants, ∆Hobs rises until the maximum point. After the maximum point, ∆Hobs decreases and then the curve coincides with the dilution curve of the corresponding surfactant in the absence of Aβ(1-40). As shown in Figure 3, the calorimetric curves are separated into region I, region II, and region III, respectively. In region I, C12C6C12Br2 shows sig-
Figure 4. (A) Far-UV CD spectrum of 0.5 g/L Aβ(1-40) dissolved in phosphate buffer at pH 7.4. (B) AFM image (2 × 2 µm2) of 0.5 g/L Aβ(1-40) dissolved in pH 7.4 phosphate buffer on the surface of freshly cleaved mica.
nificant difference from DTAB in ∆Hobs (i.e., ∆Hobs is much more endothermic for C12C6C12Br2 in this very low concentration region). To aid the explanation on enthalpy changes, the CD and AFM experiments were conducted to study the structural and conformational variation. The selected surfactant concentrations are located in region I, region II, and region III, respectively: 0.05 mM C12C6C12Br2, 0.1 mM C12C6C12Br2, 0.1 mM DTAB, and 0.2 mM DTAB corresponding to region I; 0.3 mM C12C6C12Br2 and 10 mM DTAB corresponding to region II; and 2.5 mM C12C6C12Br2 and 16 mM DTAB corresponding to region III. Aβ(1-40) adopts a predominantly random structure when dissolved in buffer at pH 7.4. This is illustrated by the CD spectrum of the peptide in Figure 4A, in which a single minimum can be observed at about 195 nm.27 The AFM micrograph of the sample shown in Figure 4B reveals the presence of a large number of randomly dispersed globular particles, and the diameter of the particles determined from the height measurement ranges from 1.5 to 3 nm. Walsh et al.28 found that the hydrodynamic radius of fresh Aβ(1-40) aggregates at pH 7.4 is about 1.8 nm determined by dynamic light scattering. Then, they thought that Aβ(1-40) exists as a dimer. As reported by Sabate´ and Estelrich,29 the critical micelle
Alzheimer Amyloid β(1-40) Peptide
J. Phys. Chem. B, Vol. 110, No. 36, 2006 18043
Figure 5. (A) CD spectra for the surfactant concentrations corresponding to regions (a) I, (b) II, and (c) III. (B) AFM images (2 × 2 µm2) of 0.5 g/L Aβ(1-40) in the presence of the surfactants at pH 7.4. (G-1) 0.05 mM C12C6C12Br2; (G-2) 0.1 mM C12C6C12Br2; (G-3) 0.3 mM C12C6C12Br2; (G-4) 2.5 mM C12C6C12Br2; (D-1) 0.1 mM DTAB; (D-2) 0.2 mM DTAB; (D-3) 10 mM DTAB; and (D-4) 16 mM DTAB. The size of the insets in (G-2), (G-3), and (D-2) is 0.25 × 0.25 µm2.
concentration for Aβ(1-40) is 0.017 mM. The reported halfaggregation time is 120 min. At the peptide concentration of 0.116 mM that we used, the peptide molecules should tend to form micelles. However, for all the experiments in the present work, Aβ(1-40) was disaggregated just prior to use. Wood et al.21 also found that the aggregation of Aβ(1-40) requires hours or days at pH 7.4. On the basis of these conclusions, the present globular particles of fresh Aβ(1-40) without the surfactants are thought to be mainly composed of the dimers of Aβ(1-40) and of some other oligomers with a diameter ranging from 1.5 to 3 nm in the time scale of the experiments. CD spectra of 0.5 g/L Aβ(1-40) in the presence of 0.05 mM C12C6C12Br2, 0.1 mM C12C6C12Br2, 0.1 mM DTAB, and 0.2 mM DTAB as well as the spectrum of 0.5 g/L Aβ(1-40) without the surfactants are shown in Figure 5A (a), where all the surfactant concentrations are within region I of the ITC curves. The spectra have a minimum at around 218 nm and a maximum at about 198 nm. The relative intensity at 218 nm increases with increasing surfactant concentration. These suggest that Aβ(1-40) has acquired a substantial degree of β-sheet structure.30 With further increase in the surfactant concentration to 0.3 mM C12C6C12Br2 and 10 mM DTAB (region II), the spectra (Figure 5A (b)) still show the characteristic of β-sheet conformation, but the intensity of molar ellipticity is somewhat decreasing. The appearance of the β-sheet signals for regions I and II is likely from the presence of fibrillar structure, whose AFM images are shown in Figure 5B. Short fibrils ranging from
150 to 250 nm in length and 1.5 to 2.5 nm in diameter are observed in the presence of 0.05 mM C12C6C12Br2. In the presence of 0.1 mM DTAB, the diameter of fibrils ranges from 1 to 2 nm and the length varies from 150 to 250 nm, but the amount of fibrils is significantly less than that in the presence of 0.05 mM C12C6C12Br2. With an increase in the concentration to 0.1 mM for C12C6C12Br2 and 0.2 mM for DTAB, the shape of fibrils is largely changed and expressed in the diameter of the fibrils ranging from 7 to 10 nm with C12C6C12Br2 and ranging from 2 to 3 nm with DTAB. These results suggest that C12C6C12Br2 has stronger ability to induce Aβ(1-40) to form fibrillar structure within region I. Additionally, increasing the concentration of C12C6C12Br2 and DTAB in region I (from 0.05 mM to 0.1 mM for C12C6C12Br2 and from 0.1 mM to 0.2 mM for DTAB), we observe that the amount of fibril is visibly increased as presented in AFM images. Since the surfactant concentration is still far below the cmc values, the results mean that the formation of fibrils is accelerated as the concentration of surfactant monomers is increased. With further increase in the surfactant concentration, the β-structure seems to be disrupted. A large amount of approximate globular aggregates appears in the presence of 0.3 mM C12C6C12Br2 (Figure 5B (G3)), and the quantity of fibril decreases significantly in the presence of 10 mM DTAB accompanying the appearance of some globular Aβ(1-40) aggregates with a diameter of 1015 nm (Figure 5B (D-3)). Obviously, the mixtures of Aβ(1-
18044 J. Phys. Chem. B, Vol. 110, No. 36, 2006 40) with the surfactants exhibit quite different morphologies in the different concentration regions of the surfactants. Figure 5A (c) shows the CD spectra of Aβ(1-40) in the presence of 2.5 mM C12C6C12Br2 and 16 mM DTAB (region III). In this concentration region, the surfactants form micelles. The spectra are still characteristic of β-sheet conformation. However, comparison with the CD spectra of Aβ(1-40) in the presence of the surfactant monomers in regions I and II shows that the intensity of molar ellipticity at 218 nm decreases. Because the far-UV absorption of bromine salt at higher concentration disturbs the absorption of Aβ(1-40), especially in the wavelength below 200 nm, the absorption of Aβ(1-40) in the presence of 16 mM DTAB cannot be recorded precisely. Under these conditions, as indicated by the AFM images, Aβ(1-40) in the presence of micellar solution appears to form globular structure. In the presence of 2.5 mM C12C6C12Br2 (Figure 5B (G-4)), the diameter of globular species ranges from 10 to 14 nm. However, in the presence of 16 mM DTAB (Figure 5B (D-4)), the diameter of globular species ranges from 14 to 20 nm, which is slightly larger than that with addition of 2.5 mM C12C6C12Br2. The globular species may be larger or elongated aggregates of Aβ(1-40), suggesting that Aβ(1-40) is not organized into fibrils, and also not maintained in soluble oligomers due to its interaction with the micelles of C12C6C12Br2 and DTAB. Discussion Under the experimental conditions used, the presence of the monomers of DTAB and C12C6C12Br2 facilitates the fibrillar aggregation of Aβ(1-40). With increase in the surfactant monomer concentration, the fibrillar structure is disrupted again. However, the presence of micelles helps to form the globular aggregates of Aβ(1-40). In region I, the surfactant monomer will bind to Aβ(1-40) accompanying with dehydration from Aβ(1-40) and from the charged groups of the surfactants, as indicated by the large endothermic ∆Hobs and the significantly decreasing tendency of ∆Hobs in the calorimetric curve. The binding of the surfactant monomers with Aβ(1-40) electrostatically will neutralize part of the negative charges on Aβ(1-40) and promote the formation of Aβ(1-40) fibrillar aggregates, indicated by the position of CD signals and the AFM images. In addition, this kind of promotion by C12C6C12Br2 monomer is stronger than that by DTAB monomer, proved with the stronger intensity of CD spectra and the much more endothermic ∆Hobs. The side chains of Asp1, Glu3, Asp7, Glu11, Glu22, and Asp23 are deprotonated at pH 7.4 because the pKa values of Asp and Glu are around 4.3 and 4.5, respectively.31 Aβ(1-40) then bears six negative charges. The molar concentration of Aβ(1-40) used is about 0.116 mM. In this case, the ratio of negative charges on Aβ(1-40) to the positive charges on the surfactant head groups is about 7 for 0.05 mM C12C6C12Br2 and 0.1 mM DTAB. For 0.1 mM C12C6C12Br2 and 0.2 mM DTAB, the ratio is about 3.5. As a result, the surfactant monomers will bind to Aβ(140) through electrostatic attraction and part of the negative charges on Aβ(1-40) is neutralized. On one hand, this partially charged neutralization leads to the reduction of the intrapeptide electrostatic interaction between the opposite charges in Aβ(1-40) itself, which is the force to keep the native structure of Aβ(1-40). Therefore, the unfolding of Aβ(1-40) may be induced by the binding of the surfactant monomers. On the other hand, this charge neutralization shields the repulsive charges between the interacting Aβ(1-40) molecules32 and causes the reduction in the amount of water participating in the hydration
Li et al. of Aβ(1-40)33 and thereby makes a promotion of fibril formation. Additionally, the alkyl chains of the surfactants may associate with the unfolded Aβ(1-40) molecules by hydrophobic interaction, resulting in the formation of the surfactant/Aβ(1-40) aggregates. Once the surfactant/Aβ(1-40) aggregates are formed, interpeptide interactions are likely to develop, making the aggregates in β-sheet more favorable. Consequently, the fibril formation is promoted by the surfactant monomers. For C12C6C12Br2, the promotion of fibril formation is much stronger due to its double charges and double hydrophobic chains. The strong electric field in the doubly charged gemini surfactant ions may induce much stronger dehydration stemming from its strong binding with Aβ(1-40) through strong electrostatic interaction and strong hydrophobic interaction. This may result in a much larger entropy gain, making the fibril formation more favorable in the presence of the monomers of the gemini surfactant. In region II, the fibrillar structure is disrupted with increasing monomer concentration, as indicated by the increasing positive ∆Hobs values, the morphology in the AFM images, and the decreased intensity of CD signals. As the concentration is increased to 0.3 mM for C12C6C12Br2 and 10 mM for DTAB, the ratios of negative charges on Aβ(1-40) to the positive charges of the surfactant head groups decrease to about 1 and 0.07 for C12C6C12Br2 and DTAB, respectively. This means that the negative charges on Aβ(1-40) are nearly completely neutralized. The excess binding of surfactants may lead to unfavorable electrostatic repulsion among the bound surfactant molecules on Aβ(1-40) and destabilize the β-sheet structure. On the other hand, above a certain surfactant monomer concentration, the surfactant monomers may create small hydrophobic clusters. This kind of cluster can surround the surfactant/Aβ(1-40) aggregates and even penetrate the aggregates, which may separate the surfactant/Aβ(1-40) aggregates from each other. Considering the above two factors, the fibrillar structure of Aβ(1-40) is disrupted with an increase in the monomer concentration within this concentration region. The breakage of fibrillar structure should be an endothermic process. Thus, the increase in the positive ∆Hobs values may therefore indicate that fibrillar structure breakage superimposes upon the surfactant binding. In region III, the presence of the micelles of C12C6C12Br2 and DTAB induces the formation of globular aggregates. Phe19 and Phe20 residues are reported to have an important role on forming β-sheet structure in buffer solution.34,35 Ma et al.31 found that there is no β-sheet of Aβ(1-28) in dodecylphosphocholine (DPC) micelle solution. They thought that Phe19 and Phe20 residues may interact weakly with the hydrophobic chains of the DPC micelles in a manner that prevents intermolecular β-aggregation. In the present study, the surfactant concentrations of 2.5 and 16 mM for C12C6C12Br2 and DTAB are beyond the cmc. The ratios of the negative charges on Aβ(1-40) to the positive charges of the surfactant head groups are 0.14 and 0.04 for C12C6C12Br2 and DTAB, respectively. That is to say, the negative charges on Aβ(1-40) have been completely neutralized and more surfactant molecules could not bind with Aβ(1-40) any longer through electrostatic attraction. However, the surfactant micelles produce a strong hydrophobic microdomain. Therefore, the hydrophobic chains of C12C6C12Br2 and DTAB micelles may interact with Phe19 and Phe20 residues through hydrophobic interaction, just like that of DPC micelles. Such binding may inhibit the formation of fibrils by interfering with the complementary alignment and stacking of the β-sheet. An alternate pathway would have the complexes of Aβ(1-40) with
Alzheimer Amyloid β(1-40) Peptide the surfactant micelles aggregate into the globular aggregates in an effort to reduce the exposure of hydrophobic regions. Conclusions This work investigated via ITC, CD, and AFM the effect of single-chain and gemini surfactants, DTAB and C12C6C12Br2, on the aggregation of Aβ(1-40). Our findings demonstrated that C12C6C12Br2 and DTAB can induce the aggregation of Aβ(1-40) in a concentration-dependent fashion. Below the cmc, the surfactant monomers electrostatically bind on Aβ(1-40), which facilitates the conformational transition from random structure to an ordered β-sheet structure. With increase in the monomer concentration, the ordered β-sheet structure starts to be disrupted due to the repulsive interaction among the head groups of surfactants bound on Aβ(1-40) and due to the formation of small hydrophobic cluster of the surfactant molecules. In the micelle solution, the globular aggregates of the complexes of Aβ(1-40) with the surfactant micelles are induced through the hydrophobic interaction between the chains of surfactant micelles and Phe19 and Phe20 residues of Aβ(140). Significantly, as compared with DTAB, C12C6C12Br2 has much stronger binding ability with Aβ(1-40) to induce the fibrillar formation at very low surfactant concentration accompanying with strong dehydration, and the binding of C12C6C12Br2 monomers to Aβ(1-40) produces at least 10 times more endothermic ∆Hobs values at very lower surfactant concentration. Acknowledgment. We are grateful for financial support from the National Natural Science Foundation of China (Grant Nos. 20233010 and 20573123). References and Notes (1) Wolfe, M. S. J. Med. Chem. 2001, 44, 2039-2060. (2) Selkoe, D. J. Annu. ReV. Cell Biol. 1994, 10, 373-403. (3) Glenner, G. G.; Wong, C. W. Biochem. Biophys. Res. Commun. 1984, 120, 885-890. (4) Hardy, J.; Selkoe, D. J. Science 2002, 297, 353-356. (5) Lomakin, A.; Chung, D. S.; Benedek, G. B.; Kirschner, D. A.; Teplow, D. B. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 1125-1129. (6) Walsh, D. M.; Klyubin, I.; Fadeeva, J. V.; Cullen, W. K.; Anwyl, R.; Wolfe, M. S.; Rowan, M. J.; Selkoe, D. J. Nature 2002, 416, 535-539. (7) (a) Kuner, P.; Bohrmann, B.; Tjernberg, L. O.; Naslund, J.; Huber, G.; Celenk, S.; Gruninger-Leitch, F.; Richards, J. G.; Jakob-Roetne, R.; Kemp, J. A.; Nordstedt, C. J. Biol. Chem. 2000, 275, 1673-1678. (b)
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