An artificial molecular machine that builds an asymmetric catalyst.

Biomolecular machines perform types of complex molecular-level tasks that artificial molecular machines can aspire to. The ribosome, for example, translates information from the polymer track it traverses (messenger RNA) to the new polymer it constructs (a polypeptide) 1 . The sequence and number of codons read determines the sequence and number of building blocks incorporated into the biomachine-synthesized polymer. However, neither control of sequence2,3 nor the transfer of length information from one polymer to another (which to date has only been accomplished in man-made systems through template synthesis) 4 is easily achieved in the synthesis of artificial macromolecules. Rotaxane-based molecular machines5-7 have been developed that successively add amino acids8-10 (including ß-amino acids 10 ) to a growing peptide chain by the action of a macrocycle moving along a mono-dispersed oligomeric track derivatized with amino-acid phenol esters. The threaded macrocycle picks up groups that block its path and links them through successive native chemical ligation reactions 11 to form a peptide sequence corresponding to the order of the building blocks on the track. Here, we show that as an alternative to translating sequence information, a rotaxane molecular machine can transfer the narrow polydispersity of a leucine-ester-derivatized polystyrene chain synthesized by atom transfer radical polymerization 12 to a molecular-machine-made homo-leucine oligomer. The resulting narrow-molecular-weight oligomer folds to an α-helical secondary structure 13 that acts as an asymmetric catalyst for the Juliá-Colonna epoxidation14,15 of chalcones.

Sequence-Specific ß-Peptide Synthesis by a Rotaxane-Based Molecular Machine.

We report on the synthesis and operation of a three-barrier, rotaxane-based, artificial molecular machine capable of sequence-specific ß-homo (ß3) peptide synthesis. The machine utilizes nonproteinogenic ß3-amino acids, a class of amino acids not generally accepted by the ribosome, particularly consecutively. Successful operation of the machine via native chemical ligation (NCL) demonstrates that even challenging 15- and 19-membered ligation transition states are suitable for information translation using this artificial molecular machine. The peptide-bond-forming catalyst region can be removed from the transcribed peptide by peptidases, artificial and biomachines working in concert to generate a product that cannot be made by either machine alone.

Efficient assembly of threaded molecular machines for sequence-specific synthesis.

We report on an improved strategy for the preparation of artificial molecular machines that can pick up and assemble reactive groups in sequence by traveling along a track. In the new approach a preformed rotaxane synthon is attached to the end of an otherwise fully formed strand of building blocks. This "rotaxane-capping" protocol is significantly more efficient than the "final-step-threading" method employed previously and enables the synthesis of threaded molecular machines that operate on extended oligomer, and potentially polymer, tracks. The methodology is exemplified through the preparation of a machine that adds four amino acid building blocks from a strand in sequence, featuring up to 20-membered ring native chemical ligation transition states.

Sequence-specific peptide synthesis by an artificial small-molecule machine.

The ribosome builds proteins by joining together amino acids in an order determined by messenger RNA. Here, we report on the design, synthesis, and operation of an artificial small-molecule machine that travels along a molecular strand, picking up amino acids that block its path, to synthesize a peptide in a sequence-specific manner. The chemical structure is based on a rotaxane, a molecular ring threaded onto a molecular axle. The ring carries a thiolate group that iteratively removes amino acids in order from the strand and transfers them to a peptide-elongation site through native chemical ligation. The synthesis is demonstrated with ~10(18) molecular machines acting in parallel; this process generates milligram quantities of a peptide with a single sequence confirmed by tandem mass spectrometry.

Polymer-supported organotin reagent for prosthetic group labeling of biological macromolecules with radioiodine.

In this study, we investigated the use of poly-mer-bound precursor for generating a radiolabeled prosthetic group to be used for conjugate labeling of biological macromolecules. For the approach, a trialkyltin chloride in which the tin was bound to a hydrophilic PEG-based resin support via one of the alkyl groups was synthesized. This resin was then used to prepare a resin-bound trialkyltin benzoic acid, which in some cases was further derivatized on-resin by converting it to a succinimidyl ester. Exposure of the resin-bound compounds to electrophilic radioiodine (¹²5I) in either an aqueous or methanol solvent liberated either free radiolabeled [¹²5I]iodobenzoic acid or its succinimidyl ester without co-release of the resin-bound precursors. Radiochemical yield was between 35% and 75%, depending on the solvent system and precursor. As example applications for the released compounds, the amine-reactive N-succinimidyl-[¹²5I]iodobenzoate prosthetic group was used for conjugate radiolabeling of a peptide, tomato plant systemin, and two proteins, albumin and IgG antibody. These results demonstrate that resin-bound organotin precursors in which the compound to be labeled is tethered to the support via the tin group to be substituted can be used to produce radioiodine-labeled aromatic prosthetic groups in good specific activity without the need for HPLC purification. This solid-phase approach is potentially adaptable to kit-formulation for performing conjugate radiolabeling of biological macromolecules.