Prototype for a new family of De Novo proteins

Molecular vessels from preorganised natural building blocks

Supramolecular vessels emerged as tools to mimic and better understand compartmentalisation, a central aspect of living matter. However, many more applications that go beyond those initial goals have been documented in recent years, including new sensory systems, artificial transmembrane transporters, catalysis, and targeted drug or gene delivery. Peptides, carbohydrates, nucleobases, and steroids bear great potential as building blocks for the construction of supramolecular vessels, possessing complexity that is still difficult to attain with synthetic methods - they are rich in functional groups and well-defined stereogenic centers, ready for noncovalent interactions and further functions. One of the options to tame the functional and dynamic complexity of natural building blocks is to place them at spatially designed positions using synthetic scaffolds. In this review, we summarise the historical and recent advances in the construction of molecular-sized vessels by the strategy that couples synthetic predictability and durability of various scaffolds (cyclodextrins, porphyrins, crown ethers, calix[n]arenes, resorcin[n]arenes, pillar[n]arenes, cyclotriveratrylenes, coordination frameworks and multivalent high-symmetry molecules) with functionality originating from natural building blocks to obtain nanocontainers, cages, capsules, cavitands, carcerands or coordination cages by covalent chemistry, self-assembly, or dynamic covalent chemistry with the ultimate goal to apply them in sensing, transport, or catalysis.

From steroids to aqueous supramolecular chemistry: an autobiographical career review

The focus of my group's research is aqueous supramolecular chemistry; we try to understand how chemical entities interact with water and consequently how they interact with each other. This personal history recounts my career experiences that led to his involvement with this fascinating area of science.

X-ray crystal analysis of a TASP: structural insights of a cavitein dimer

Cavitein Q4 is a template assembled synthetic protein designed for X-ray crystallographic analysis. It is based on a previous monomeric helical bundle cavitein (N1GG) that consists of four identical parallel helical peptides. Crystals that were grown in the presence of bromide ions were used to solve the initial phases via single-wavelength anomalous dispersion (SAD). A 1.4 A resolution data set was then refined starting with the SAD phases to provide the crystal structure of cavitein Q4. The crystal structure revealed cavitein Q4 as an asymmetric dimer, although the cavitein appears to be largely monomeric in solution. A comparative analysis is carried out to discern any intrinsic differences between Q4 and its parent cavitein N1GG. We present herein the first X-ray crystal structure of a TASP system and relate this structure to the solution data for both Q4 and its parent N1GG.

Optimal Attachment Position and Linker Length Promote Native-like Character of Cavitand-Based Template-Assembled Synthetic Proteins (TASPs)

We have designed, synthesised and characterised a series of template-assembled de novo four-helix bundles, each differing in the linker length between the template and the peptides. The helix is based on an earlier peptide sequence: EELLKKLEELLKKLG (first-generation sequence), which was designed to link the hydrophilic/hydrophobic interface of the helices. Increasing or decreasing the linker length by one glycine residue had a significant effect on the structure and properties of the template-assembled synthetic proteins (TASPs). Here, the effect of the linker length is further probed by linking the peptides closer to the hydrophobic face by using the second-generation sequence, AEELLKKLEELLKKG, in an effort to improve the packing between the helices and to better understand the helical bundles. The peptides were synthesised with 0-4 Gly linker residues and linked onto a cavitand template. The proteins were found to be alpha-helical, stable to guanidine hydrochloride (GuHCl) and to unfold cooperatively. However, their stabilities toward GuHCl, propensity to self-aggregate and structural specificity differed. The two-glycine variant of the second-generation series demonstrated the highest stability and most native-like character of all the mononeric TASPs in both the first- and second-generation series. The structural specificity of this two glycine variant is comparable to that of other known native-like de novo proteins. Molecular dynamics simulations showed that the two-glycine variant contains helices that are tilted with respect to the cavitand template and may account for its unique properties.

Analysis of peptide design in four-, five-, and six-helix bundle template assembled synthetic protein molecules

Four-, five-, and six-helix bundle template assembled synthetic proteins (TASPs) have been synthesized using disulfide bonds between cavitand templates and peptides, and characterized in terms of stability and structural specificity. The peptide sequence (CGGGEELLKKLEE LLKKG) used was originally designed for a four-helix bundle. The TASPs were analyzed using CD spectroscopy, chemical denaturation studies, NMR spectroscopy, sedimentation equilibria studies, and hydrophobic dye binding studies to determine the effect of a single peptide sequence when incorporated into bundles with different numbers of helices. If the design was indeed idealized for a four-helix bundle, then the five- and six-helix bundles should be less stable and manifest lower conformational specificity. The TASPs all demonstrated high stability and cooperative unfolding. However, the four-helix bundle was found to be significantly more stable and nativelike compared to the five- and six-helix bundles. This suggests that the peptide sequence is specific to the four-helix bundle, as designed. This result demonstrates the ability to design de novo proteins with specified structure, not just generic stability.

Four-helix bundle cavitein reveals middle leucine as linchpin

A template-assembled de novo four-helix bundle is used to examine the hydrophobic effect within the bundle interior. Leu to Ala variants of the basis sequence GG-EELLKKLEELLKKG were characterized by GuHCl denaturation, NMR dispersion, and N-H/D exchange experiments. The results show that the middle leucine (L7) is imperative in maintaining bundle stability. The average leucine was found to contribute 1.8 kcal mol(-1) toward stability, whereas the middle leucines contribute 2.7 kcal mol(-1) each. Substituting alanine into the middle position (7) constitutes a striking 95% reduction of the overall cavitein stability.

Characterization of de novo four-helix bundles by molecular dynamics simulations

We have investigated the structure and dynamics of three cavitand-based four-helix bundles (caviteins) by computer simulation. In these systems, designed de novo, each of the four helices contain the identical basis sequence EELLKKLEELLKKG (N1). Each cavitein consists of a rigid macrocycle (cavitand) with four aryl linkages, to each of which is connected an N1 peptide by means of a linker peptide. The three caviteins studied here differ only in the linker peptide, which consist of one, two, or three glycine residues. Previous experimental work has shown that these systems exhibit very different behavior in terms of stability and oligomerization states despite the small differences in the linker peptide. Given that to date no three-dimensional structure is available for these caviteins, we have undertaken a series of molecular dynamics (MD) simulations in explicit water to try to rationalize the large differences in the experimentally observed behavior of these systems. Our results provide insight, for the first time, into why and how the cavitein with a single glycine linker forms dimers. In addition, our results indicate why although the two- and three-glycine-linked caviteins have similar stabilities, they have different native-like characteristics: the cavitein with three glycines can form a supercoiled helix, whereas the one with two glycines cannot. These findings may provide a useful guide in the rational de novo design of novel proteins with finely tunable structures and functions in the future.

Design of proteins using rigid organic macrocycles as scaffolds

We have designed and synthesized new three-helix template-assembled synthetic proteins (TASPs) 1a-c. The template was the rigid cyclotribenzylene (CTB) macrocycle 2, which has C3 symmetry. Thiol moieties on the CTB template were used to link cysteine-containing peptide strands 3a-c via disulfide bonds. With designed peptide strands of 15 and 18 residues in length, the structure of TASPs 1a-c were determined to be helical in water according to circular dichroism (CD) spectroscopy. The helicities of TASPs 1a-c were unchanged over large ranges of pH (2-12) and salt concentrations (0-2 M KCl). TASPs 1a-c were also extremely resistant to chemical denaturants: it requires a guanidine hydrochloride (GnHCl) concentration of 7.4 M for TASPs 1a-c to lose 50% of their helicity. The major force for stabilization of TASPs 1a-c is the hydrophobic bundling of the helices.