Formation Of Salt Bridge Research Articles

Salt Bridge Formation within the RKS Motif of Histone H3 Detected by NMR Spectroscopy

Histone N‐terminal tails are subject to covalent post‐translational modifications (PTMs) including methylation and phosphorylation which can help signal gene activation. More recently, PTMs are studied for their impact on each other. In many cases, one modification may prevent that of another. We set out to physically characterize the formation of a salt bridge between arginine‐8 and phosphoserine‐10 in histone H3 by nuclear magnetic resonance (NMR) spectroscopy. We hypothesize that both methylated arginine and phosphorylated serine do not co‐exist when in close proximity; phosphorylation of serine‐10 forms a salt bridge with neighboring unmodified arginine‐8, preventing arginine‐8 methylation. To investigate the contributions of these two modifications, 31P NMR was used for salt bridge detection because the natural isotopic abundance of phosphorus (100 %) is high enough to observe without the need for isotopic labeling. To calculate the relative free energy of the salt bridge, we carried out a 31P NMR titration using four modified peptides that correspond to histone H3. Our NMR data confirm the formation of a salt bridge. The detection of a salt bridge provides evidence that this interaction changes the conformation of the histone H3 tail and also prevents subsequent methylation of arginine‐8. This work can lead to mechanistic insights into the modification of histone H3 tails in both healthy and diseased states.Support or Funding InformationNational Science Foundation Louis Stokes Alliance for Minority Participation HDR‐1602210This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Structural and Mechanistic Insights into the Doughnut‐Shaped Lytic Transglycosylase from Campylobacter jejuni

The bacterial cell wall peptidoglycan (PG) layer undergoes constant remodeling. The breakdown of the peptidoglycan layer is, in part, carried out by lytic transglycosylases (LT). The inhibition of lytic transglycosylases has been shown to potentiate β‐lactam antibiotics: this is caused by either a direct effect on cell wall integrity, or indirectly, via downregulation of β‐lactamase expression by altering PG fragment concentrations. A sub‐family of LTs are soluble LTs which harbor an unusual doughnut‐shaped structure. To probe the structure and mechanism of a new member of this family, the Campylobacter jejuni Cj0843c LT, we determined its crystal structure in the absence and presence of the bulgecin A inhibitor, carried out activity measurements and biophysical experiments, and performed molecular dynamics (MD) studies with PG strands. The domain organization of Cj0843c reveals a new variation of the doughnut‐shaped architecture. Its structure is similar to that of the larger E. coli SLT70 yet Cj0843c's U‐shaped U‐domain is shorter and it accomplishes completion of its circular nature by having a separate NU‐domain that is tethered to the U‐domain via a flexible linker. The bulgecin A inhibitor binds to the active site of Cj0843c and causes a tightening of the active site groove concomitant with a 9.5°C increase in thermal stability of Cj0843c. We find that Bulgecin A inhibits Cj0843c activity, as measured using a turbidity assay, and potentiates ampicillin susceptibility of C. jejuni. The electrostatic surface of Cj0843c and similar LTs reveal that their inner ring surfaces are predominantly positively charged. This surface characteristic is likely to complement the charge of the tetrapeptide‐disaccharide units of the PG substrate as each tetrapeptide section harbors three carboxylate groups. Molecular dynamics (MD) simulations of a 4 or 5 disaccharide tetrapeptide unit PG strand in the Cj0843c active site either in a product or substrate binding mode, respectively, revealed key PG interactions. The GlcNAc‐2 and MurNAc‐1 occupy positions and interactions similar to that of bulgecin A's moieties when bound to Cj0843c. The terminal tetrapeptide moiety that is either attached to the 1,6‐anhydroMurNAc+2 in the substrate binding mode or to the 1,6‐anhydroMurNAc‐1 in the product binding mode forms salt bridges with a nearby positively charged pocket. When a PG strand is positioned in a random position mostly outside the central hole of Cj0843c, a 1 μs MD simulation revealed that the PG strand is able to enter the hole and that its terminal tetrapeptide is able to find the positively charged pocket forming stable anchoring interactions throughout the remainder of the simulation. The MD simulations reveal global fluctuations regarding the contracting and dilating of Cj0843c's circumference and key movements in the active site when the PG strand is present in the active site groove. Our results suggest that Cj0843c's doughnut shape favors non‐ or minimally‐crosslinked sections of PG strands and point to a mechanism for the mostly exolytic nature of this class of LTs and potential processivity in degrading PG strands.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Design, Synthesis, and Anti-HIV-1 Evaluation of a Novel Series of 1,2,3,4-Tetrahydropyrimidine-5-Carboxylic Acid Derivatives.

A series of tetrahydropyrimidine derivatives (2a - 2l) were designed, synthesized, and screened for anti-HIV-1 properties based on the structures of HIV-1 gp41 binding site inhibitors, NB-2 and NB-64. A computational study was performed to predict the pharmacodynamics, pharmacokinetics, and drug-likeness features of the studied molecules. Docking studies revealed that the carboxylic acid group in the molecules forms salt bridges with either Lys574 or Arg579. Physiochemical properties (e.g., molecular weight, number of hydrogen bond donors, number of hydrogen bond acceptors, and number of rotatable bonds) of the synthesized compounds confirmed and exhibited that these compounds were within the range set by Lipinski's rule of five. Compounds 2e and 2k with 4-chlorophenyl substituent and 4-methylphenyl group at C(4) position of the tetrahydropyrimidine ring was the most potent one among the tested compounds. This suggests that these compounds may serve as leads for development of novel small-molecule HIV-1 inhibitors.

Probing Adsorption Behaviors of BSA onto Chiral Surfaces of Nanoparticles.

Chiral properties of nanoscale materials are of importance as they dominate interactions with proteins in physiological environments; however, they have rarely been investigated. In this study, a systematic investigation is conducted for the adsorption behaviors of bovine serum albumin (BSA) onto the chiral surfaces of gold nanoparticles (AuNPs), involving multiple techniques and molecular dynamic (MD) simulation. The adsorption of BSA onto both L- and D-chiral surfaces of AuNPs shows discernible differences involving thermodynamics, adsorption orientation, exposed charges, and affinity. As a powerful supplement, MD simulation provides a molecular-level understanding of protein adsorption onto nanochiral surfaces. Salt bridge interaction is proposed as a major driving force at protein-nanochiral interface interaction. The spatial distribution features of functional groups (COO- , NH3+ , and CH3 ) of chiral molecules on the nanosurface play a key role in the formation and location of salt bridges, which determine the BSA adsorption orientation and binding strength to chiral surfaces. Sequentially, BSA corona coated on nanochiral surfaces affects their uptake by cells. The results enhance the understanding of protein corona, which are important for biological effects of nanochirality in living organisms.

Salt-bridge dynamics in intrinsically disordered proteins: A trade-off between electrostatic interactions and structural flexibility

Intrinsically Disordered Proteins (IDPs) are enriched in charged and polar residues; and, therefore, electrostatic interactions play a predominant role in their dynamics. In order to remain multi-functional and exhibit their characteristic binding promiscuity, they need to retain considerable dynamic flexibility. At the same time, they also need to accommodate a large number of oppositely charged residues, which eventually lead to the formation of salt-bridges, imparting local rigidity. The formation of salt-bridges therefore opposes the desired dynamic flexibility. Hence, there appears to be a meticulous trade-off between the two mechanisms which the current study attempts to unravel. With this objective, we identify and analyze salt-bridges, both as isolated as well as composite ionic bond motifs, in the molecular dynamic trajectories of a set of appropriately chosen IDPs. Time evolved structural properties of these salt-bridges like persistence, associated secondary structural ‘order-disorder’ transitions, correlated atomic movements, contribution in the overall electrostatic balance of the proteins have been studied in necessary detail. The results suggest that the key to maintain such a trade-off over time is the continuous formation and dissolution of salt-bridges with a wide range of persistence. Also, the continuous dynamic interchange of charged-atom-pairs (coming from a variety of oppositely charged side-chains) in the transient ionic bonds supports a model of dynamic flexibility concomitant with the well characterized stochastic conformational switching in these proteins. The results and conclusions should facilitate the future design of salt-bridges as a mean to further explore the disordered-globular interface in proteins.

Elucidating the multi-targeted anti-amyloid activity and enhanced islet amyloid polypeptide binding of β-wrapins

β-wrapins are engineered binding proteins stabilizing the β-hairpin conformations of amyloidogenic proteins islet amyloid polypeptide (IAPP), amyloid-β, and α-synuclein, thus inhibiting their amyloid propensity. Here, we use computational and experimental methods to investigate the molecular recognition of IAPP by β-wrapins. We show that the multi-targeted, IAPP, amyloid-β, and α-synuclein, binding properties of β-wrapins originate mainly from optimized interactions between β-wrapin residues and sets of residues in the three amyloidogenic proteins with similar physicochemical properties. Our results suggest that IAPP is a comparatively promiscuous β-wrapin target, probably due to the low number of charged residues in the IAPP β-hairpin motif. The sub-micromolar affinity of β-wrapin HI18, specifically selected against IAPP, is achieved in part by salt-bridge formation between HI18 residue Glu10 and the IAPP N-terminal residue Lys1, both located in the flexible N-termini of the interacting proteins. Our findings provide insights towards developing novel protein-based single- or multi-targeted therapeutics.

Glycosyl-Substituted Dicarboxylates as Detergents for the Extraction, Overstabilization, and Crystallization of Membrane Proteins.

To tackle the problems associated with membrane protein (MP) instability in detergent solutions, we designed a series of glycosyl-substituted dicarboxylate detergents (DCODs) in which we optimized the polar head to clamp the membrane domain by including, on one side, two carboxyl groups that form salt bridges with basic residues abundant at the membrane-cytoplasm interface of MPs and, on the other side, a sugar to form hydrogen bonds. Upon extraction, the DCODs 8 b, 8 c, and 9 b preserved the ATPase function of BmrA, an ATP-binding cassette pump, much more efficiently than reference or recently designed detergents. The DCODs 8 a, 8 b, 8 f, 9 a, and 9 b induced thermal shifts of 20 to 29 °C for BmrA and of 13 to 21 °C for the native version of the G-protein-coupled adenosine receptor A2A R. Compounds 8 f and 8 g improved the diffraction resolution of BmrA crystals from 6 to 4 Å. DCODs are therefore considered to be promising and powerful tools for the structural biology of MPs.

Glycosyl‐Substituted Dicarboxylates as Detergents for the Extraction, Overstabilization, and Crystallization of Membrane Proteins

AbstractTo tackle the problems associated with membrane protein (MP) instability in detergent solutions, we designed a series of glycosyl‐substituted dicarboxylate detergents (DCODs) in which we optimized the polar head to clamp the membrane domain by including, on one side, two carboxyl groups that form salt bridges with basic residues abundant at the membrane–cytoplasm interface of MPs and, on the other side, a sugar to form hydrogen bonds. Upon extraction, the DCODs 8 b, 8 c, and 9 b preserved the ATPase function of BmrA, an ATP‐binding cassette pump, much more efficiently than reference or recently designed detergents. The DCODs 8 a, 8 b, 8 f, 9 a, and 9 b induced thermal shifts of 20 to 29 °C for BmrA and of 13 to 21 °C for the native version of the G‐protein‐coupled adenosine receptor A2AR. Compounds 8 f and 8 g improved the diffraction resolution of BmrA crystals from 6 to 4 Å. DCODs are therefore considered to be promising and powerful tools for the structural biology of MPs.

Caged Molecular Glues as Photoactivatable Tags for Nuclear Translocation of Guests in Living Cells.

We developed dendritic caged molecular glues (CagedGlue-R) as tags for nucleus-targeted drug delivery, whose multiple guanidinium ion (Gu+) pendants are protected by an anionic photocleavable unit (butyrate-substituted nitroveratryloxycarbonyl; BANVOC). Negatively charged CagedGlue-R hardly binds to anionic biomolecules because of their electrostatic repulsion. However, upon exposure of CagedGlue-R to UV light or near-infrared (NIR) light, the BANVOC groups of CagedGlue-R are rapidly detached to yield an uncaged molecular glue (UncagedGlue-R) that carries multiple Gu+ pendants. Because Gu+ forms a salt bridge with PO4-, UncagedGlue-R tightly adheres to anionic biomolecules such as DNA and phospholipids in cell membranes by a multivalent salt-bridge formation. When tagged with CagedGlue-R, guests can be taken up into living cells via endocytosis and hide in endosomes. However, when the CagedGlue-R tag is photochemically uncaged to form UncagedGlue-R, the guests escape from the endosome and migrate into the cytoplasm followed by the cell nucleus. We demonstrated that quantum dots (QDs) tagged with CagedGlue-R can be delivered efficiently to cell nuclei eventually by irradiation with light.

The gating mechanism in cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) channels mediate transduction in several sensory neurons. These channels use the free energy of CNs’ binding to open the pore, a process referred to as gating. CNG channels belong to the superfamily of voltage-gated channels, where the motion of the α-helix S6 controls gating in most of its members. To date, only the open, cGMP-bound, structure of a CNG channel has been determined at atomic resolution, which is inadequate to determine the molecular events underlying gating. By using electrophysiology, site-directed mutagenesis, chemical modification, and Single Molecule Force Spectroscopy, we demonstrate that opening of CNGA1 channels is initiated by the formation of salt bridges between residues in the C-linker and S5 helix. These events trigger conformational changes of the α-helix S5, transmitted to the P-helix and leading to channel opening. Therefore, the superfamily of voltage-gated channels shares a similar molecular architecture but has evolved divergent gating mechanisms.

Spontaneous protein desorption from self-assembled monolayer (SAM)-coated gold nanoparticles.

When nanoparticles enter blood or other biological fluids, they tend to contact with a variety of proteins which may hamper their application and even bring adverse impacts. Such nonspecific protein binding is usually weak and proteins reach dynamic equilibrium between adsorption and desorption. Studies on the spontaneous desorption of weakly bound proteins should not only be helpful to enrich our understanding about such nonspecific interactions but also shed light on the strategy to avoid nonspecific binding of proteins. Here, we use molecular dynamics simulations to investigate the interactions of human serum albumin with the self-assembled monolayer (SAM)-coated gold(111) surface, a typical facet of various gold nanoparticles. The response of the protein interfacial behavior to solution pH, especially the spontaneous desorption, is studied. When the solution pH is relatively low, the protein can adsorb to the SAM surface. Such adhesion is attributed to several salt bridges between acidic residues and SAM's protonated groups, and there are water molecules distributed under the adsorbed protein, i.e. interlayer water. Interestingly, the increase of the solution pH reduces the binding affinity of the protein, which engenders the lateral diffusion of the protein and the increase of interlayer water. When the solution pH is further increased, the enhanced lateral diffusion of the protein and the growth of interlayer water disrupt the formation of salt bridges between the protein and the SAM, and the protein progressively dissociates from the SAM. The spontaneous desorption process of the protein and the corresponding mechanisms illustrated in this work may be helpful to develop an antifouling surface and enhance the biosafety of nanomaterials.

Free energy calculations on the stability of the 14-3-3ζ protein

Mutations of cysteine are often introduced to e.g. avoid formation of non-physiological inter-molecular disulfide bridges in in-vitro experiments, or to maintain specificity in labeling experiments. Alanine or serine is typically preferred, which usually do not alter the overall protein stability, when the original cysteine was surface exposed. However, selecting the optimal mutation for cysteines in the hydrophobic core of the protein is more challenging. In this work, the stability of selected Cys mutants of 14-3-3ζ was predicted by free-energy calculations and the obtained data were compared with experimentally determined stabilities. Both the computational predictions as well as the experimental validation point at a significant destabilization of mutants C94A and C94S. This destabilization could be attributed to the formation of hydrophobic cavities and a polar solvation of a hydrophilic side chain. A L12E, M78K double mutant was further studied in terms of its reduced dimerization propensity. In contrast to naïve expectations, this double mutant did not lead to the formation of strong salt bridges, which was rationalized in terms of a preferred solvation of the ionic species. Again, experiments agreed with the calculations by confirming the monomerization of the double mutants. Overall, the simulation data is in good agreement with experiments and offers additional insight into the stability and dimerization of this important family of regulatory proteins.

Importance of the positively charged residue at position 54 to the chaperoning function, conformational stability and amyloidogenic nature of human αA-crystallin.

Arginine 54 (R54) in αA-Crystallin (αA-Cry) is highly conserved within different species. Recently, three missense mutations at this hot spot position have been reported to cause congenital cataract disorders. To investigate the impact of charge on structural and functional aspects of αA-Cry, R54 was individually substituted with lysine and aspartate. Replacement of R54 with the positively and negatively charged residues led to structural alteration and reduction in the protein conformational and proteolytic stability. Also, these mutations resulted in important increase in the amyloidogenic propensity of αA-Cry. Additionally, all these changes were more pronounced upon R54D mutation. Keeping the positive charge by R54K mutation, the structural integrity and stability of αA-Cry were partially preserved. Our results suggest that arginine 54 may also participate in salt bridge formation and conformational stabilization of αA-Cry. Also, it seems that unique physicochemical properties of arginine 54 may have a prominent role in the structural integrity, conformational stability and functional aspects of human αA-Cry.

Constant pH Molecular Dynamics Reveals How Proton Release Drives the Conformational Transition of a Transmembrane Efflux Pump.

AcrB is the inner-membrane transporter of an E. coli AcrAB-TolC tripartite efflux complex, which plays a major role in the intrinsic resistance to clinically important antibiotics. AcrB pumps a wide range of toxic substrates by utilizing the proton gradient between periplasm and cytoplasm. Crystal structures of AcrB revealed three distinct conformational states of the transport cycle, substrate access, binding, and extrusion or loose (L), tight (T), and open (O) states. However, the specific residue(s) responsible for proton binding/release and the mechanism of proton-coupled conformational cycling remain controversial. Here we use the newly developed membrane hybrid-solvent continuous constant pH molecular dynamics technique to explore the protonation states and conformational dynamics of the transmembrane domain of AcrB. Simulations show that both Asp407 and Asp408 are deprotonated in the L/T states, while only Asp408 is protonated in the O state. Remarkably, release of a proton from Asp408 in the O state results in large conformational changes, such as the lateral and vertical movement of transmembrane helices as well as the salt-bridge formation between Asp408 and Lys940 and other side chain rearrangements among essential residues. Consistent with the crystallographic differences between the O and L protomers, simulations offer dynamic details of how proton release drives the O-to-L transition in AcrB and address the controversy regarding the proton/drug stoichiometry. This work offers a significant step toward characterizing the complete cycle of proton-coupled drug transport in AcrB and further validates the membrane hybrid-solvent CpHMD technique for studies of proton-coupled transmembrane proteins which are currently poorly understood.

Initial Structural Models of the Aβ42 Dimer from Replica Exchange Molecular Dynamics Simulations.

Experimental characterization of the molecular structure of small amyloid (A)β oligomers that are currently considered as toxic agents in Alzheimer’s disease is a formidably difficult task due to their transient nature and tendency to aggregate. Such structural information is of importance because it can help in developing diagnostics and an effective therapy for the disease. In this study, molecular simulations and protein–protein docking are employed to explore a possible connection between the structure of Aβ monomers and the properties of the intermonomer interface in the Aβ42 dimer. A structurally diverse ensemble of conformations of the monomer was sampled in microsecond timescale implicit solvent replica exchange molecular dynamics simulations. Representative structures with different solvent exposure of hydrophobic residues and secondary structure content were selected to build structural models of the dimer. Analysis of these models reveals that formation of an intramonomer salt bridge (SB) between Asp23 and Lys28 residues can prevent the building of a hydrophobic interface between the central hydrophobic clusters (CHCs) of monomers upon dimerization. This structural feature of the Aβ42 dimer is related to the difference in packing of hydrophobic residues in monomers with the Asp23–Lys28 SB in on and off states, in particular, to a lower propensity to form hydrophobic contacts between the CHC domain and C-terminal residues in monomers with a formed SB. These findings could have important implications for understanding the difference between aggregation pathways of Aβ monomers leading to neurotoxic oligomers or inert fibrillar structures.

Publisher’s Note

DNA polymerase β is a 39kDa enzyme that is a major component of Base Excision Repair in human cells. The enzyme comprises two major domains, a 31kDa domain responsible for the polymerase activity and an 8kDa domain, which bind ssDNA and has a deoxyribose phosphate (dRP) lyase activity. DNA polymerase β was shown to be phosphorylated in vitro with protein kinase C (PKC) at serines 44 and 55 (S44 and S55), resulting in loss of its polymerase enzymic activity, but not its ability to bind ssDNA. In this study, we investigate the potential phosphorylation-induced structural changes for DNA polymerase β using molecular dynamics. The simulations show drastic conformational changes of the polymerase structure as a result of S44 phosphorylation. Phosphorylation-induced conformational changes transform the closed (active) enzyme structure into an open one. Further analysis of the results points to a key hydrogen bond and newly formed salt bridges as potential drivers of these structural fluctuations. The changes observed with S44/55 and S55 phosphorylation were less dramatic than S44 and the integrity of the H-bond was not compromised. Thus the phosphorylation of S44 is likely the major contributor to structural fluctuations that lead to loss of enzymatic activity.

Investigation on the Molecular Interactions Stabilizing the Structure of α-synuclein Fibril: An In silico Study.

Amyloid fibrils represent stable form of many misfolded proteins associated with numerous diseases like Parkinson's Disease (PD), Type II diabetes and Alzheimer's disease (AD). α-synuclein protein is the principal constituent of Lewy bodies that are considered to be pathological hallmark of PD. Recently, a high resolution structure of α-synuclein protein that stacks together forming fibrils in brains of PD patients were identified. What structural features drive pathology of PD can now be possibly answered from the fibril structure of protein. To understand the molecular interactions those are responsible for the stability of the α- synuclein fibril structure. To study the molecular interactions stabilizing the α-synuclein fibril, we have used a high resolution amyloid fibril structure (PDB ID 2N0A). The molecular interactions in fibril structure were studied using PDBSum server. We then looked into the destabilization of α-synuclein fibril by disrupting the salt-bridge holding the strands and probable methods to decompose fibril into structurally distinct units using Top-domain web-server. The effect of salt-bridges on the stability of the fibril structure was studied by mutating one of the residues involved in the formation of salt-bridge using molecular dynamics simulation. Our results indicate a finite salt-bridge (E46-K80) is crucial for stability of protofibril. Besides, we observed hydrogen bonds and non-bonded contacts involved in fibril stabilization. We noticed α-synuclein dimer predominantly exists in conformations distinct from fibril. We characterized the salient molecular interactions in α-synuclein fibril and these findings may be useful to design potential inhibitors for the treatment of PD.

Physicochemical and Structural Studies on Shaping of β-hairpin in Proteins as a First Stage of Amyloid Formation.

The aggregation of proteins or their digested fragments through β-sheet structures has a great significance because it leads to neurodegenerative diseases, which are a problem of the aging societies of the developed countries. Short peptides are typically used as models to study the formation of specific structures. However, while the formation of α-helical structure was investigated thoroughly, until recently, there have been much fewer studies on the formation of β-structure. In this review, recent experimental and theoretical studies of β-hairpin-forming peptides, both model alaninebased systems, and those based on the fragments of real proteins, are summarized with regard to the role of hydrophobic, local, and Coulombic interactions. It is demonstrated that the presence of charged residues can induce a bent structure not only owing to the formation of salt bridges if oppositely- charged residues present at the ends of a sequence but also through shielding the hydrophobic interior by like-charged residues at the end of the sequence.

Impact of intermolecular drug-copolymer interactions on size and drug release kinetics from pH-responsive polymersomes

pH-sensitive polymersomes are produced from amphiphilic copolymers of the type mPEG-b-(PMMA-ran-PDMAEMA) obtained via ATRP, so that mPEG with molecular masses of 2k and 5kDa forms the corona of a hydrophobic double layer with 22–28% molar content in protonable DMAEMA. Vesicles obtained via dialysis were loaded with curcumin, 2-naphthole, paclitaxel (PTX) and ampicillin sodium salt, and the release kinetics of the latter studied via UV-vis spectrometry as a function of pH. Overall, the release profiles clearly indicated a dopant-sensitive kinetics and, likely, mechanism depending on molecule-copolymer interactions. Infrared spectrometry highlighted the formation of hydrogen bonds and salt bridges that may be responsible for these findings; support for the formation of the latter are obtained comparing the IR spectrum for ampicillin doped-vesicles with the anharmonic vibrational transition of model salt bridges. Importantly, DLS data indicated that our vesicles appeared to remain stable even at pH 4.4 after 48 h and completely releasing ampicillin. The release profiles of co-loaded curcumin/PTX with ampicillin also suggest that desorption rates of water-soluble species can be modulated by the presence of hydrophobic molecules in the double layer, at least at pH 7.4 and 6.4.

Perturbation of the F19-L34 Contact in Amyloid β (1-40) Fibrils Induces Only Local Structural Changes but Abolishes Cytotoxicity.

We explored structural details of fibrils formed by a mutated amyloid β (Aβ(1-40)) peptide carrying a Phe19 to Lys19 mutation, which was shown to completely abolish the toxicity of the molecule. Computer models suggest that the positively charged Lys19 side chain is expelled from the hydrophobic fibril interior upon fibrillation. This can be accommodated by either a 180° flip of the entire lower β-strand (model M1) or local perturbations of the secondary structure in the direct vicinity of the mutated site (model M2). This is accompanied by the formation of a new salt bridge between Glu22 and Lys28 in model M1. Experimentally, a novel contact between Phe20 and Leu34 as well as the significant structural perturbation of residues 20-23 could be confirmed. However, the mutated fibrils do not show the formation of any salt bridges. This demonstrates that although morphologically very robust, local perturbations of the Aβ(1-40) sequence lead to moderate structural alterations with tremendous impact on the physiological importance of these aggregates, which may suggest alternative strategies for the development of a remedy against Alzheimer's disease.