Todd M. Doran

Effect of C-Terminal Modification on the Self-Assembly and Hydrogelation of Fluorinated Fmoc-Phe Derivatives

The development of hydrogels resulting from the self-assembly of low molecular weight (LMW) hydrogelators is a rapidly expanding area of study. Fluorenylmethoxycarbonyl (Fmoc) protected aromatic amino acids derived from phenylalanine (Phe) have been shown to be highly effective LMW hydrogelators. It has been found that side chain functionalization of Fmoc-Phe exerts a significant effect on the self-assembly and hydrogelation behavior of these molecules; fluorinated derivatives, including pentafluorophenylalanine (F(5)-Phe) and 3-F-phenylalanine (3-F-Phe), spontaneously self-assemble into fibrils that form a hydrogel network upon dissolution into water. In this study, Fmoc-F(5)-Phe-OH and Fmoc-3-F-Phe-OH were used to characterize the role of the C-terminal carboxylic acid on the self-assembly and hydrogelation of these derivatives. The C-terminal carboxylic acid moieties of Fmoc-F(5)-Phe-OH and Fmoc-3-F-Phe-OH were converted to C-terminal amide and methyl ester groups in order to perturb the hydrophobicity and hydrogen bond capacity of the C-terminus. Self-assembly and hydrogelation of these derivatives was investigated in comparison to the parent carboxylic acid compounds at neutral and acidic pH. It was found that hydrogelation of the C-terminal acids was highly sensitive to solvent pH, which influences the charge state of the terminal group. Rigid hydrogels form at pH 3.5, but at pH 7 hydrogel rigidity is dramatically weakened. C-terminal esters self-assembled into fibrils only slowly and failed to form hydrogels due to the higher hydrophobicity of these derivatives. C-terminal amide derivatives assembled much more rapidly than the parent carboxylic acids at both acidic and neutral pH, but the resultant hydrogels were unstable to shear stress as a function of the lower water solubility of the amide functionality. Co-assembly of acid and amide functionalized monomers was also explored in order to characterize the properties of hybrid hydrogels; these gels were rigid in unbuffered water but significantly weaker in phosphate buffered saline. These results highlight the complex nature of monomer/solvent interactions and their ultimate influence on self-assembly and hydrogelation, and provide insight that will facilitate the development of optimal amino acid LMW hydrogelators for gelation of complex buffered media.

Complementary π–π Interactions Induce Multicomponent Coassembly into Functional Fibrils

Noncovalent self-assembled materials inspired by amyloid architectures are useful for biomedical applications ranging from regenerative medicine to drug delivery. The selective coassembly of complementary monomeric units to provide ordered multicomponent fibrils is a possible strategy for enhancing the sophistication of these noncovalent materials. Herein we report that complementary π-π interactions can be exploited to promote the coassembly of phenylalanine (Phe) derivatives that possess complementary aromatic side-chain functionality. Specifically, equimolar mixtures of Fmoc-Phe and Fmoc-F(5)-Phe, which possess side-chain groups with complementary quadrupole electronics, readily coassemble to form two-component fibrils and hydrogels under conditions where Fmoc-Phe alone fails to self-assemble. In addition, it was found that equimolar mixtures of Fmoc-Phe with monohalogenated (F, Cl, and Br) Fmoc-Phe derivatives also coassembled into two-component fibrils. These results collectively indicate that face-to-face quadrupole stacking between benzyl side-chain groups does not account for the molecular recognition between Phe and halogenated Phe derivatives that promote cofibrillization but that coassembly is mediated by more subtle π-π effects arising from the halogenation of the benzyl side chain. The use of complementary π-π interactions to promote the coassembly of two distinct monomeric units into ordered two-component fibrils dramatically expands the repertoire of noncovalent interactions that can be used in the development of sophisticated noncovalent materials.

Amyloid-binding small molecules efficiently block SEVI (semen-derived enhancer of virus infection)- and semen-mediated enhancement of HIV-1 infection.

Semen was recently shown to contain amyloid fibrils formed from a self-assembling peptide fragment of the protein prostatic acid phosphatase. These amyloid fibrils, termed semen-derived enhancer of virus infection, or SEVI, have been shown to strongly enhance HIV infectivity and may play an important role in sexual transmission of HIV, making them a potential microbicide target. One novel approach to target these fibrils is the use of small molecules known to intercalate into the structure of amyloid fibrils, such as derivatives of thioflavin-T. Here, we show that the amyloid-binding small molecule BTA-EG6 (the hexa(ethylene glycol) derivative of benzothiazole aniline) is able to bind SEVI fibrils and effectively inhibit both SEVI-mediated and semen-mediated enhancement of HIV infection. BTA-EG6 also blocks the interactions of SEVI with HIV-1 virions and HIV-1 target cells but does not cause any inflammation or toxicity to cervical epithelial cells. These results suggest that an amyloid-binding small molecule may have utility as a microbicide, or microbicidal supplement, for HIV-1.

Seminal Plasma Accelerates Semen-derived Enhancer of Viral Infection (SEVI) Fibril Formation by the Prostatic Acid Phosphatase (PAP248–286) Peptide

Background: SEVI is an amyloid fibril that enhances HIV infectivity. To date, it has been produced from its precursor peptide only under nonphysiologic conditions. Results: Seminal plasma (SP) accelerates SEVI formation and protects SEVI from proteolytic degradation. Conclusion: SEVI forms spontaneously from its precursor peptide under physiologic conditions in SP. Significance: These findings may explain the presence of SEVI in human semen. Amyloid fibrils contained in semen, known as SEVI, or semen-derived enhancer of viral infection, have been shown to increase the infectivity of HIV dramatically. However, previous work with these fibrils has suggested that extensive time and nonphysiologic levels of agitation are necessary to induce amyloid formation from the precursor peptide (a proteolytic cleavage product of prostatic acid phosphatase, PAP248–286). Here, we show that fibril formation by PAP248–286 is accelerated dramatically in the presence of seminal plasma (SP) and that agitation is not required for fibrillization in this setting. Analysis of the effects of specific SP components on fibril formation by PAP248–286 revealed that this effect is primarily due to the anionic buffer components of SP (notably inorganic phosphate and sodium bicarbonate). Divalent cations present in SP had little effect on the kinetics of fibril formation, but physiologic levels of Zn2+ strongly protected SEVI fibrils from degradation by seminal proteases. Taken together, these data suggest that in the in vivo environment, PAP248–286 is likely to form fibrils efficiently, thus providing an explanation for the presence of SEVI in human semen.

Utility of Redundant Combinatorial Libraries in Distinguishing High and Low Quality Screening Hits

Large one-bead one-compound (OBOC) combinatorial libraries can be constructed relatively easily by solid-phase split and pool synthesis. The use of resins with hydrophilic surfaces, such as TentaGel, allows the beads to be used directly in screens for compounds that bind selectively to labeled proteins, nucleic acids, or other biomolecules. However, we have found that this method, while useful, has a high false positive rate. In other words, beads that are scored as hits often display compounds that prove to be poor ligands for the target of interest when they are resynthesized and carried through validation trials. This results in a significant waste of time and resources in cases where putative hits cannot be validated without resynthesis. Here, we report that this problem can be largely eliminated through the use of redundant OBOC libraries, where more than one bead displaying the same compound is present in the screen. We show that compounds isolated more than once are likely to be high quality ligands for the target of interest, whereas compounds isolated only once have a much higher likelihood of being poor ligands. While the use of redundant libraries does limit the number of unique compounds that can be screened at one time in this format, the overall savings in time, effort, and materials makes this a more efficient route to the isolation of useful ligands for biomolecules.

Role of amino acid hydrophobicity, aromaticity, and molecular volume on IAPP(20–29) amyloid self‐assembly

Aromatic amino acids strongly promote cross‐β amyloid formation; whether the amyloidogenicity of aromatic residues is due to high hydrophobicity and β‐sheet propensity or formation of stabilizing π–π interactions has been debated. To clarify the role of aromatic residues on amyloid formation, the islet amyloid polypeptide 20–29 fragment [IAPP(20–29)], which contains a single aromatic residue (Phe 23), was adopted as a model. The side chain of residue 23 does not self‐associate in cross‐β fibrils of IAPP(20–29) (Nielsen et al., Angew Chem Int Ed 2009;48:2118–2121), allowing investigation of the amyloidogenicity of aromatic amino acids in a context where direct π–π interactions do not occur. We prepared variants of IAPP(20–29) in which Tyr, Leu, Phe, pentafluorophenylalanine (F5‐Phe), Trp, cyclohexylalanine (Cha), α‐naphthylalanine (1‐Nap), or β‐naphthylalanine (2‐Nap) (in order of increasing peptide hydrophobicity) were incorporated at position 23 (SNNXGAILSS‐NH2), and the kinetic and thermodynamic effects of these mutations on cross‐β self‐assembly were assessed. The Tyr, Leu, and Trp 23 variants failed to readily self‐assemble at concentrations up to 1.5 mM, while the Cha 23 mutant fibrillized with attenuated kinetics and similar thermodynamic stability relative to the wild‐type Phe 23 peptide. Conversely, the F5‐Phe, 1‐Nap, and 2‐Nap 23 variants self‐assembled at enhanced rates, forming fibrils with greater thermodynamic stability than the wild‐type peptide. These results indicate that the high amyloidogenicity of aromatic amino acids is a function of hydrophobicity, β‐sheet propensity, and planar geometry and not the ability to form stabilizing or directing π–π bonds. Proteins 2012;.

Turn nucleation perturbs amyloid β self-assembly and cytotoxicity.

The accumulation of senile plaques composed of amyloid β (Aβ) fibrils is a hallmark of Alzheimers disease, although prefibrillar oligomeric species are believed to be the primary neurotoxic congeners in the pathogenesis of Alzheimers disease. Uncertainty regarding the mechanistic relationship between Aβ oligomer and fibril formation and the cytotoxicity of these aggregate species persists. β-Turn formation has been proposed to be a potential rate-limiting step during Aβ fibrillogenesis. The effect of turn nucleation on Aβ self-assembly was probed by systematically replacing amino acid pairs in the putative turn region of Aβ (residues 24-27) with d-ProGly ((D)PG), an effective turn-nucleating motif. The kinetic, thermodynamic, and cytotoxic effects of these mutations were characterized. It was found that turn formation dramatically accelerated Aβ fibril self-assembly dependent on the site of turn nucleation. The cytotoxicity of the three (D)PG-containing Aβ variants was significantly lower than that of wild-type Aβ40, presumably due to decreased oligomer populations as a function of a more rapid progression to mature fibrils; oligomer populations were not eliminated, however, suggesting that turn formation is also a feature of oligomer structures. These results indicate that turn nucleation is a critical step in Aβ40 fibril formation.

Reversible photocontrol of self-assembled peptide hydrogel viscoelasticity

Peptide hydrogels are promising biomaterials for applications ranging from drug delivery to tissue engineering. Peptide hydrogels that change their physical properties in response to an exogenous stimulus are advantageous as biomaterials that can be temporally controlled. Herein, we report the use of an azobenzene turn mimetic, [3-(3-aminomethylphenylazo)phenyl]acetic acid (AMPP), to engineer a light-responsive β-hairpin into the center of a hydrogel-forming peptide. In the trans state, AMPP exists in a β-arc conformation, and the peptide forms a rigid self-supporting gel. The peptide hydrogel rigidity is reduced upon trans–cis azobenzene isomerization, which promotes formation of putative β-hairpin assemblies. This process is reversible in that hydrogel viscoelasticity is restored upon reverse cis–trans photoisomerization. TEM imaging and spectroscopic data reveal that the loss in rigidity is a result of disruption of the well-ordered macromolecular structure and not due to disassembly of the constituent self-assembled β-sheet fibrils. These findings provide insight into the effect of β-arc and β-hairpin turns on the emergent properties of self-assembled peptide hydrogels and provide a basis for temporal control of hydrogel rigidity using near-UV light.

An azobenzene photoswitch sheds light on turn nucleation in amyloid-β self-assembly.

Amyloid-β (Aβ) self-assembly into cross-β amyloid fibrils is implicated in a causative role in Alzheimers disease pathology. Uncertainties persist regarding the mechanisms of amyloid self-assembly and the role of metastable prefibrillar aggregates. Aβ fibrils feature a sheet-turn-sheet motif in the constituent β-strands; as such, turn nucleation has been proposed as a rate-limiting step in the self-assembly pathway. Herein, we report the use of an azobenzene β-hairpin mimetic to study the role turn nucleation plays on Aβ self-assembly. [3-(3-Aminomethyl)phenylazo]phenylacetic acid (AMPP) was incorporated into the putative turn region of Aβ42 to elicit temporal control over Aβ42 turn nucleation; it was hypothesized that self-assembly would be favored in the cis-AMPP conformation if β-hairpin formation occurs during Aβ self-assembly and that the trans-AMPP conformer would display attenuated fibrillization propensity. It was unexpectedly observed that the trans-AMPP Aβ42 conformer forms fibrillar constructs that are similar in almost all characteristics, including cytotoxicity, to wild-type Aβ42. Conversely, the cis-AMPP Aβ42 congeners formed nonfibrillar, amorphous aggregates that exhibited no cytotoxicity. Additionally, cis-trans photoisomerization resulted in rapid formation of native-like amyloid fibrils and trans-cis conversion in the fibril state reduced the population of native-like fibrils. Thus, temporal photocontrol over Aβ turn conformation provides significant insight into Aβ self-assembly. Specifically, Aβ mutants that adopt stable β-turns form aggregate structures that are unable to enter folding pathways leading to cross-β fibrils and cytotoxic prefibrillar intermediates.

A Liquid Array Platform For the Multiplexed Analysis of Synthetic Molecule-Protein Interactions

Synthetic molecule microarrays, consisting of many different compounds spotted onto a planar surface such as modified glass or cellulose, have proven to be useful tools for the multiplexed analysis of small molecule- and peptide-protein interactions. However, these arrays are technically difficult to manufacture and use with high reproducibility and require specialized equipment. Here we report a more convenient alternative composed of color-encoded beads that display a small molecule protein ligand on the surface. Quantitative, multiplexed assay of protein binding to up to 24 different ligands can be achieved using a common flow cytometer for the readout. This technology should be useful for evaluating hits from library screening efforts, the determination of structure activity relationships, and certain types of serological analyses.