Diagonal Interactions between Glutamate and Arginine Analogs with Varying Side-Chain Lengths in a ß-Hairpin.

Cross-strand interactions are important for the stability of ß-sheet structures. Accordingly, cross-strand diagonal interactions between glutamate and arginine analogs with varying side-chain lengths were studied in a series of ß-hairpin peptides. The peptides were analyzed by homonuclear two-dimensional nuclear magnetic resonance methods. The fraction folded population and folding free energy of the peptides were derived from the chemical shift data. The fraction folded population trends could be rationalized using the strand propensity of the constituting residues, which was not the case for the peptides with lysine analogs, highlighting the difference between the arginine analogs and lysine analogs. Double-mutant cycle analysis was used to derive the diagonal ion-pairing interaction energetics. The most stabilizing diagonal cross-strand interaction was between the shortest residues (i.e., Asp2-Agp9), most likely due to the least side-chain conformational penalty for ion-pair formation. The diagonal interaction energetics in this study involving the arginine analogs appears to be consistent with and extend beyond our understanding of diagonal ion-pairing interactions involving lysine analogs. The results should be useful for designing ß-strand-containing molecules to affect biological processes such as amyloid formation and protein-protein interactions.

The Effects of Charged Amino Acid Side-Chain Length on Diagonal Cross-Strand Interactions between Carboxylate- and Ammonium-Containing Residues in a ß-Hairpin.

The ß-sheet is one of the common protein secondary structures, and the aberrant aggregation of ß-sheets is implicated in various neurodegenerative diseases. Cross-strand interactions are an important determinant of ß-sheet stability. Accordingly, both diagonal and lateral cross-strand interactions have been studied. Surprisingly, diagonal cross-strand ion-pairing interactions have yet to be investigated. Herein, we present a systematic study on the effects of charged amino acid side-chain length on a diagonal ion-pairing interaction between carboxylate- and ammonium-containing residues in a ß-hairpin. To this end, 2D-NMR was used to investigate the conformation of the peptides. The fraction folded population and the folding free energy were derived from the chemical shift data. The fraction folded population for these peptides with potential diagonal ion pairs was mostly lower compared to the corresponding peptide with a potential lateral ion pair. The diagonal ion-pairing interaction energy was derived using double mutant cycle analysis. The Asp2-Dab9 (Asp: one methylene; Dab: two methylenes) interaction was the most stabilizing (-0.79 ± 0.14 kcal/mol), most likely representing an optimal balance between the entropic penalty to enable the ion-pairing interaction and the number of side-chain conformations that can accommodate the interaction. These results should be useful for designing ß-sheet containing molecular entities for various applications.

Structural impact of thioamide incorporation into a ß-hairpin.

The thioamide is a naturally-occurring single atom substitution of the canonical amide bond. The exchange of oxygen to sulfur alters the amide's physical and chemical characteristics, thereby expanding its functionality. Incorporation of thioamides in prevalent secondary structures has demonstrated that they can either have stabilizing, destabilizing, or neutral effects. We performed a systematic investigation of the structural impact of thioamide incorporation in a ß-hairpin scaffold with nuclear magnetic resonance (NMR). Thioamides as hydrogen bond donors did not increase the foldedness of the more stable "YKL" variant of this scaffold. In the less stable "HPT" variant of the scaffold, the thioamide could be stabilizing as a hydrogen bond donor and destabilizing as a hydrogen bond acceptor, but the extent of the perturbation depended upon the position of incorporation. To better understand these effects we performed structural modelling of the macrocyclic folded HPT variants. Finally, we compare the thioamide effects that we observe to previous studies of both side-chain and backbone perturbations to this ß-hairpin scaffold to provide context for our observations.

Using Slippage to Construct a Prototypical Molecular "Lock & Lock" Box.

We report a new slippage system based on p-tert-butylbenzyl-terminated imidazolium ions and di(ethylene glycol)-containing macrocycles and their use as linking units for the construction of a prototypical molecular "Lock & Lock" box from a resorcinarene-based cavitand "bowl" and a porphyrin "cover". The multivalent structure with four slippage linkers provided the molecular box with high stability, yet the system dissociated into its two components upon application of suitable external stimuli.

Longer charged amino acids favor ß-strand formation in hairpin peptides.

Interactions between charged amino acids significantly influence the structure and function of proteins. The encoded charged amino acids Asp, Glu, Arg, and Lys have different number of hydrophobic methylenes linking the backbone to the charged functionality. It remains to be fully understood how does this difference in the number of methylenes affect protein structure stability. Protein secondary structures are the fundamental three-dimensional building blocks of protein structures. ß-Sheet structures are particularly interesting, because these structures have been associated with a number of protein misfolding diseases. Herein, we report the effect of charged amino acid side chain length at two ß-strand positions individually on the stability of a ß-hairpin. The charged amino acids include side chains with a carboxylate, an ammonium, or a guanidinium group. The experimental peptides, fully folded reference peptides, and fully unfolded reference peptides were synthesized by solid phase peptide synthesis and analyzed by 2D NMR methods including TOCSY, DQF-COSY, and ROESY. Sequence specific assignments were performed for all peptides. The chemical shift data were used to derive the fraction folded population and the folding free energy for the experimental peptides. Results showed that the fraction folded population increased with increasing charged amino acid side chain length. These results should be useful for developing functional peptides that adopt the ß-conformation.

Swapping the Positions in a Cross-Strand Lateral Ion-Pairing Interaction between Ammonium- and Carboxylate-Containing Residues in a ß-Hairpin.

Cross-strand lateral ion-pairing interactions are important for antiparallel ß-sheet stability. Statistical studies suggested that swapping the position of cross-strand lateral residues should not significantly affect the interaction. Herein, we swapped the position of ammonium- and carboxylate-containing residues with different side-chain lengths in a cross-strand lateral ion-pairing interaction in a ß-hairpin. The peptides were analyzed by 2D-NMR. The fraction folded population and folding free energy were derived from the chemical shift data. The ion-pairing interaction energy was derived using double mutant cycle analysis. The general trends for the fraction folded population and interaction energetics remained similar upon swapping the position of the interacting charged residues. The most stabilizing cross-strand interactions were between short residues, similar to the unswapped study. However, the fraction folded populations for most of the swapped peptides were higher compared to the corresponding unswapped peptides. Furthermore, subtle differences in the ion-pairing interaction energy upon swapping were observed, most likely due to the "unleveled" relative positioning of the interacting residues created by the inherent right-handed twist of the structure. These results should be useful for developing functional peptides that rely on lateral ion-pairing interactions across antiparallel ß-strands.

Insertion of Pro-Hyp-Gly provides 2 kcal mol-1 stability but attenuates the specific assembly of ABC heterotrimeric collagen triple helices.

Collagen is a major structural component of the extracellular matrix and connective tissue. The key structural feature of collagen is the collagen triple helix, with a Xaa-Yaa-Gly (glycine) repeating pattern. The most frequently occurring triplet is Pro (proline)-Hyp (hydroxyproline)-Gly. The reversible thermal folding and unfolding of a series of heterotrimeric collagen triple helices with varying number of Pro-Hyp-Gly triplets were monitored by circular dichroism spectroscopy to determine the unfolding thermodynamic parameters Tm (midpoint transition temperature), ΔHTm (unfolding enthalpy), and ΔGunfold (unfolding free energy). The Tm and ΔGunfold of the heterotrimeric collagen triple helices increased with increasing number of Pro-Hyp-Gly triplets. The ΔGunfold increased by 2.0 ± 0.2 kcal mol-1 upon inserting one Pro-Hyp-Gly triplet into all three chains. The Tm difference between the most stable ABC combination and the second most stable BCC combination decreased with increasing number of Pro-Hyp-Gly triplets, even though the ΔGunfold difference remained the same. These results should be useful for tuning the stability of collagen triple helical peptides for hydrogel formation, recognition of denatured collagen triple helices as diagnostics and therapeutics, and targeted drug delivery.

Effects of Arginine Deimination and Citrulline Side-Chain Length on Peptide Secondary Structure Formation.

Post-translational modifications expand the chemical functionality of peptides and proteins beyond that originating from the encoded amino acids, but studies on the structural effects of these modifications have been limited. Arginine undergoes deimination to give citrulline (Cit), converting the positively charged guanidinium moiety into a neutral urea group. Herein, we report the effect of Arg deimination on secondary structure formation. To understand the reason for the number of methylene units in Cit, the effect of Cit side-chain length on secondary structure formation was also studied. Ala-based peptides and ß-hairpin peptides were used to study α-helix and ß-sheet formation, respectively. Peptides containing Cit analogues were prepared by an orthogonal protecting group strategy coupled with solid-phase carbamylation. The CD data for the Ala-based peptides were analyzed by using modified Lifson-Roig theory, showing that the helix propensity of Arg decreased upon deimination and that either shortening or lengthening Cit also decreased the helix propensity. The ß-hairpin peptides were analyzed by NMR methods, showing minimal change in strand formation energetics upon Arg deimination. Altering the Cit side-chain length did not affect strand formation energetics either. These results should be useful for the preparation of urea-bearing systems and the design of peptides incorporating urea-bearing residues with varying side-chain length.

[2]Catenanes Displaying Switchable Gin-Trap-Like Motion.

Sodium ion-controlled switching from "folded" to "linear" states results in significant changes in the molecular shape of a [2]catenane, such that it mimics the operation of a gin trap, with a fluorescent alarm signal appearing when pyrene side arms were present on its two macrocyclic components.

Na+ Ions Induce the Pirouetting Motion and Catalytic Activity of [2]Rotaxanes.

We have prepared [2]rotaxanes, the behavior of which as switchable catalysts depends on their pirouetting motion, which can be controlled through the addition and removal of Na+ ions. At least three sequential on/off cycles of a Michael reaction can be performed in situ when using the NaTFPB/[2.2.2]cryptand reagent pair to switch "on" and "off" the catalytic ability of the [2]rotaxanes.

Effect of lysine methylation and acetylation on the RNA recognition and cellular uptake of Tat-derived peptides.

The two lysine (Lys) residues in the human immunodeficiency virus trans-activator of transcription protein (HIV Tat protein) basic region (residues 47-57) are crucial for two bioactivities: RNA recognition and cellular uptake. Since the post-translational modifications of these two Lys residues affect the biological function of the Tat protein, we investigated the effect of methylation and acetylation of Lys50 and Lys51 in Tat-derived peptides on the two bioactivities. Tat-derived peptides, in which each lysine was replaced with a methylated- or acetylated-Lys, were synthesized by solid phase peptide synthesis. TAR RNA recognition of the peptides was studied by electrophoretic mobility shift assays. Cellular uptake of the peptides into Jurkat cells was determined by flow cytometry. Our results showed that acetylation of either Lys residue attenuated both bioactivities. In contrast, the effect of Lys methylation on the bioactivities depended on position and number of methyl groups. These findings should be useful for the development of functional molecules containing ammonium groups for RNA recognition to affect biological processes and for cellular uptake for drug delivery.

Probing the polarity and water environment at the protein-peptide binding interface using tryptophan analogues.

7-Azatryptophan and 2,7-diazatryptophan are sensitive to polarity changes and water content, respectively, and should be ideal for studying protein-protein and protein-peptide interactions. In this study, we replaced the tryptophan in peptide Baa (LKWKKLLKLLKKLLKLG-NH2) with 7-azatryptophan or 2,7-diazatryptophan, forming (7-aza)Trp-Baa and (2,7-aza)Trp-Baa, to study the calmodulin (CaM)-peptide interaction. Dramatic differences in the (7-aza)Trp-Baa and (2,7-aza)Trp-Baa fluorescence properties between free peptide in water and calmodulin-bound peptide were observed, showing a less polar and water scant environment at the binding interface of the peptide upon calmodulin binding. The affinity of the peptides for binding CaM followed the trend Baa (210±10 pM)<(7-aza)Trp-Baa (109±5 pM)<(2,7-aza)Trp-Baa (45±2 pM), showing moderate increase in binding affinity upon increasing the number of nitrogen atoms in the Trp analogue. The increased binding affinity may be due to the formation of more hydrogen bonds upon binding CaM for the Trp analogue with more nitrogen atoms. Importantly, the results demonstrate that (7-aza)Trp and (2,7-aza)Trp are excellent probes for exploring the environment at the interface of protein-peptide interactions.

Attenuating HIV Tat/TAR-mediated protein expression by exploring the side chain length of positively charged residues.

RNA is a drug target involved in diverse cellular functions and viral processes. Molecules that inhibit the HIV TAR RNA-Tat protein interaction may attenuate Tat/TAR-dependent protein expression and potentially serve as anti-HIV therapeutics. By incorporating positively charged residues with mixed side chain lengths, we designed peptides that bind TAR RNA with enhanced intracellular activity. Tat-derived peptides that were individually substituted with positively charged residues with varying side chain lengths were evaluated for TAR RNA binding. Positively charged residues with different side chain lengths were incorporated at each Arg and Lys position in the Tat-derived peptide to enhance TAR RNA binding. The resulting peptides showed enhanced TAR RNA binding affinity, cellular uptake, nuclear localization, proteolytic resistance, and inhibition of intracellular Tat/TAR-dependent protein expression compared to the parent Tat-derived peptide with no cytotoxicity. Apparently, the enhanced inhibition of protein expression by these peptides was not determined by RNA binding affinity, but by proteolytic resistance. Despite the high TAR binding affinity, a higher binding specificity would be necessary for practical purposes. Importantly, altering the positively charged residue side chain length should be a viable strategy to generate potentially useful RNA-targeting bioactive molecules.

Effect of arginine methylation on the RNA recognition and cellular uptake of Tat-derived peptides.

Arginine (Arg) methylation is a common post-translational modification that regulates gene expression and viral infection. The HIV-1 Tat protein is an essential regulatory protein for HIV proliferation, and is methylated in the cell. The basic region (residues 47-57) of the Tat protein contains six Arg residues, and is responsible for two biological functions: RNA recognition and cellular uptake. In this study, we explore the effect of three different methylation states at each Arg residue in Tat-derived peptides on the two biological functions. The Tat-derived peptides were synthesized by solid phase peptide synthesis. TAR RNA binding of the peptides was assessed by electrophoresis mobility shift assays. The cellular uptake of the peptides into Jurkat cells was determined by flow cytometry. Our results showed that RNA recognition was affected by both methylation state and position. In particular, asymmetric dimethylation at position 53 decreased TAR RNA binding affinity significantly, but unexpectedly less so upon asymmetric dimethylation at position 52. The RNA binding affinity even slightly increased upon methylation at some of the flanking Arg residues. Upon Arg methylation, the cellular uptake of Tat-derived peptides mostly decreased. Interestingly, cellular uptake of Tat-derived peptides with a single asymmetrically dimethylated Arg residue was similar to the native all Arg peptide (at 120 µM). Based on our results, TAR RNA binding apparently required both guanidinium terminal NH groups on Arg53, whereas cellular uptake apparently required guanidinium terminal NH2 groups instead. These results should provide insight into how nature uses arginine methylation to regulate different biological functions, and should be useful for the development of functional molecules with methylated arginines.

Effect of charged amino acid side chain length on lateral cross-strand interactions between carboxylate- and guanidinium-containing residues in a ß-hairpin.

ß-Sheet is one of the major protein secondary structures. Oppositely charged residues are frequently observed across neighboring strands in antiparallel sheets, suggesting the importance of cross-strand ion pairing interactions. The charged amino acids Asp, Glu, Arg, and Lys have different numbers of hydrophobic methylenes linking the charged functionality to the backbone. To investigate the effect of side chain length of guanidinium- and carboxylate-containing residues on lateral cross-strand ion pairing interactions at non-hydrogen-bonded positions, ß-hairpin peptides containing Zbb-Agx (Zbb = Asp, Glu, Aad in increasing length; Agx = Agh, Arg, Agb, Agp in decreasing length) sequence patterns were studied by NMR methods. The fraction folded population and folding energy were derived from the chemical shift deviation data. Peptides with high fraction folded populations involved charged residue side chain lengths that supported high strand propensity. Double mutant cycle analysis was used to determine the interaction energy for the potential lateral ion pairs. Minimal interaction was observed between residues with short side chains, most likely due to the diffused positive charge on the guanidinium group, which weakened cross-strand electrostatic interactions with the carboxylate side chain. Only the Aad-Arg/Agh interactions with long side chains clearly exhibited stabilizing energetics, possibly relying on hydrophobics. A survey of a non-redundant protein structure database revealed that the statistical sheet pair propensity followed the trend Asp-Arg < Glu-Arg, implying the need for matching long side chains. This suggested the need for long side chains on both guanidinium-bearing and carboxylate-bearing residues to stabilize the ß-hairpin motif.

Effect of side chain length on intrahelical interactions between carboxylate- and guanidinium-containing amino acids.

The charge-containing hydrophilic functionalities of encoded charged amino acids are linked to the backbone via different numbers of hydrophobic methylenes, despite the apparent electrostatic nature of protein ion pairing interactions. To investigate the effect of side chain length of guanidinium- and carboxylate-containing residues on ion pairing interactions, α-helical peptides containing Zbb-Xaa (i, i + 3), (i, i + 4) and (i, i + 5) (Zbb = carboxylate-containing residues Aad, Glu, Asp in decreasing length; Xaa = guanidinium residues Agh, Arg, Agb, Agp in decreasing length) sequence patterns were studied by circular dichroism spectroscopy (CD). The helicity of Aad- and Glu-containing peptides was similar and mostly pH independent, whereas the helicity of Asp-containing peptides was mostly pH dependent. Furthermore, the Arg-containing peptides consistently exhibited higher helicity compared to the corresponding Agp-, Agb-, and Agh-containing peptides. Side chain conformational analysis by molecular mechanics calculations showed that the Zbb-Xaa (i, i + 3) and (i, i + 4) interactions mainly involved the χ 1 dihedral combinations (g+, g+) and (g-, g+), respectively. These low energy conformations were also observed in intrahelical Asp-Arg and Glu-Arg salt bridges of natural proteins. Accordingly, Asp and Glu provides variation in helix characteristics associated with Arg, but Aad does not provide features beyond those already delivered by Glu. Importantly, nature may have chosen the side chain length of Arg to support helical conformations through inherent high helix propensity coupled with stabilizing intrahelical ion pairing interactions with the carboxylate-containing residues.

Effect of each guanidinium group on the RNA recognition and cellular uptake of Tat-derived peptides.

The six arginine (Arg) residues in the human immunodeficiency virus transactivator of transcription protein (HIV Tat protein) basic region (residues 47-57) are crucial for two bioactivities: RNA recognition and cellular uptake. Herein, we report a systematic study to investigate the role of the guanidinium group on Arg at each position in Tat-derived peptides for the two bioactivities. Tat-derived peptides, in which each guanidinium-bearing arginine was replaced with a urea-bearing citrulline (Cit) or an ammonium-bearing Lys, were synthesized by solid phase peptide synthesis. RNA recognition of the peptides was studied by electrophoretic mobility shift assays, and cellular uptake into Jurkat cells was determined by flow cytometry. Our results showed that removing the positive charge and altering the hydrogen bonding capacity of Arg affect the two biological functions differently. Furthermore, the effects are position dependent. These findings should be useful for the development of functional molecules containing guanidinium, urea, and ammonium groups for RNA recognition to affect biological processes and for cellular uptake for drug delivery.

Effect of charged amino acid side chain length on lateral cross-strand interactions between carboxylate-containing residues and lysine analogues in a ß-hairpin.

ß-Sheets are one of the fundamental three-dimensional building blocks for protein structures. Oppositely charged amino acids are frequently observed directly across one another in antiparallel sheet structures, suggesting the importance of cross-strand ion pairing interactions. Despite the apparent electrostatic nature of ion pairing interactions, the charged amino acids Asp, Glu, Arg, Lys have different numbers of hydrophobic methylenes linking the charged functionality to the backbone. Accordingly, the effect of charged amino acid side chain length on cross-strand ion pairing interactions at lateral non-hydrogen bonded positions was investigated in a ß-hairpin motif. The negatively charged residues with a carboxylate (Asp, Glu, Aad in increasing length) were incorporated at position 4, and the positively charged residues with an ammonium (Dap, Dab, Orn, Lys in increasing length) were incorporated at position 9. The fraction folded population and folding free energy were derived from the chemical shift deviation data. Double mutant cycle analysis was used to determine the interaction energy for the potential lateral ion pairs. Only the Asp/Glu-Dap interactions with shorter side chains and the Aad-Orn/Lys interactions with longer side chains exhibited stabilizing energetics, mostly relying on electrostatics and hydrophobics, respectively. This suggested the need for length matching of the interacting residues to stabilize the ß-hairpin motif. A survey of a nonredundant protein structure database revealed that the statistical sheet pair propensity followed the trend Asp-Lys < Glu-Lys, also implying the need for length matching of the oppositely charged residues.

Enhanced non-endocytotic uptake of mesoporous silica nanoparticles by shortening the peptide transporter arginine side chain.

Mesoporous silica nanoparticles (MSNs) are multifunctional nanocarriers with potential biomedical applications. However, MSNs are frequently trapped in endosomes upon cellular uptake through endocytosis, requiring endosomal escape. Herein, enhanced nonendocytosis was observed for 300 nm MSNs by conjugating peptides with noncanonical arginine analogs.

Effect of charged amino acid side chain length at non-hydrogen bonded strand positions on ß-hairpin stability.

ß-Sheets have been implicated in various neurological disorders, and ∼20% of protein residues adopt a sheet conformation. Therefore, studies on the structural origin of sheet stability can provide fundamental knowledge with potential biomedical applications. Oppositely charged amino acids are frequently observed across one another in antiparallel ß-sheets. Interestingly, the side chains of natural charged amino acids Asp, Glu, Arg, Lys have different numbers of hydrophobic methylenes linking the backbone to the hydrophilic charged functionalities. To explore the inherent effect of charged amino acid side chain length on antiparallel sheets, the stability of a designed hairpin motif containing charged amino acids with varying side chain lengths at non-hydrogen bonded positions was studied. Peptides with the guest position on the N-terminal strand and the C-terminal strand were investigated by NMR methods. The charged amino acids (Xaa) included negatively charged residues with a carboxylate group (Asp, Glu, Aad in increasing length), positively charged residues with an ammonium group (Dap, Dab, Orn, Lys in increasing length), and positively charged residues with a guanidinium group (Agp, Agb, Arg, Agh in increasing length). The fraction folded and folding free energy for each peptide were derived from the chemical shift deviation data. The stability of the peptides with the charged residues at the N-terminal guest position followed the trends: Asp > Glu > Aad, Dap < Dab < Orn ∼ Lys, and Agb < Arg < Agh < Agp. The stability of the peptides with the charged residues at the C-terminal guest position followed the trends: Asp < Glu < Aad, Dap ∼ Dab < Orn ∼ Lys, and Agb < Arg ∼ Agp < Agh. These trends were rationalized by thermodynamic sheet propensity and cross-strand interactions.