Effect of gate length and dielectric thickness on ion and fluid transport in a fluidic nanochannel

We have simulated the effect of gate length and dielectric thickness on ion and fluid transport in a fluidic nanochannel with negative surface charge on its walls. A short gate is unable to induce significant cation enrichment in the nanochannel and ion current is controlled mostly by cation depletion at positive gate potentials. The cation enrichment increases with increasing gate length and/or decreasing dielectric thickness due to higher changes induced in the surface charge density and zeta-potential. Thus, long gates and thin dielectric layers are more effective in controlling ion current. The model without Navier-Stokes equations is unable to correctly predict phenomena such as cation enrichment, increase in channel conductivity, and decreasing electric field. Body force and induced fluid velocity decrease slowly and then rapidly with gate potentials. The effectiveness of ion current control by a gate reduces with increasing surface charge density due to reduced fractional change in zeta-potential.     

The stereoselective synthesis of cis-/trans-fused hexahydropyrano[4,3-b]chromenes via Prins cyclization trapping by a tethered nucleophile

A novel intramolecular Prins cyclization of (Z)-2-(5-hydroxypent-2-enyl)phenol with various aldehydes has been achieved using 10 mol% In(OTf)(3) and 30 mol% TsOH to produce the cis-fused hexahydropyrano[4,3-b]chromene derivatives in good yields, while the coupling of (E)-2-(5-hydroxypent-2-enyl)phenol with aldehydes under similar conditions affords the corresponding trans-fused hexahydropyrano[4,3-b]chromene derivatives.     

Synthesis and Structure of α/δ‐Hybrid Peptides—Access to Novel Helix Patterns in Foldamers

Origami peptides : A novel class of foldamers consisting of α/δ‐hybrid peptides has been investigated theoretically and experimentally by exploiting the rigidity of the side chain of a new δ‐amino acid prepared from D ‐glucose and D ‐xylose with a furanose side chain (see figure).

     

Nucleation of beta-hairpin structures with cis amide bonds in E-vinylogous proline-containing peptides

Synthesis and conformational studies of peptides containing the E-vinylogous prolines 1 (VPro1) and 2 (VPro2), Boc-Ala-Val-VPro1-Xaa-Leu-OMe (3, Xaa = Gly; 4, Xaa = Phe), Boc-Ala-Val-VPro2-Xaa-Leu-OMe (5, Xaa = Gly; 6, Xaa = Phe), Boc-Leu-Ile-Val-VPro1-Xaa-Leu-OMe (7, Xaa = Gly; 8, Xaa = Phe), and Boc-Leu-Ile-Val-VPro2-Xaa-Leu-OMe (9, Xaa = Gly; 10, Xaa = Phe), were carried out. It has been shown that both VPro1 and VPro2 lead to the formation of 12-membered intramolecularly hydrogen bonded structures very similar to type VI beta-turns with a cis Xaa-VPro amide bond in the major conformers in all the peptides 3-10, resulting in the nucleation of beta-hairpin type structures in these molecules in CDCl(3).     

Studies on novel peptidomimetics having bi-directional dispositions of hydroxylated D-Pro-Gly motifs anchored on a C(2)-symmetric iminosugar-based foundation

A rigid pyrrolidine based scaffold comprising of 2,5-dideoxy-2,5-imino-D-idaric acid (1) is developed. Attachment of peptide strands to the carboxylic groups at both ends of this novel template led to the peptidomimetics 2 and 3. Conformational analysis by NMR studies revealed that compounds 2b, 3b and 2c, 3c take interesting turn structures (C(2) symmetric for 2c and 3c) in DMSO-d(6) consisting of identical intramolecular hydrogen bonds at two ends between LeuNH --> sugar-OH as depicted in structure A, whereas 2a and 3a display structures with regular beta-turns with hydrogen bonds between LeuNH --> Boc-C=O in one-half of their molecular frameworks (structure B), characteristic of the turn structures commonly observed in "D-Pro-Gly"-containing peptides. These results suggest that a cis hydroxyl group at the 3-position of the proline residue favors a pseudo beta-turn-like nine-membered ring structure in hydroxyproline-containing peptides involving an intramolecular hydrogen bond between the hydroxyl and the i + 2 backbone amide.     

Norbornene-constrained cyclic peptides with hairpin architecture: design, synthesis, conformation, and membrane ion transport

A novel family of hairpin cyclic peptides has been designed on the basis of the use of norbornene units as the bridging ligands. The design is flexible with respect to the choice of an amino acid, the ring size, and the nature of the second bridging ligand as illustrated here with the preparation of a large number of norborneno cyclic peptides containing a variety of amino acids in ring sizes varying from 12- to 29-membered, with the choice of the second bridging ligand being a rigid norbornene (11, 13a,b), an adamantane unit (7a,b and 8), or a flexible cystine residue (4a,b and 10). The presence of built-in handles (as protected COOH groups) permits the attachment of a variety of subunits as shown here with the ligation of Leu-Leu, Val-Val, or Aib-Aib pendants in 4b, 7b, and 13b, respectively. This novel class of constrained cyclic peptides are demonstrated to adopt beta-sheet- or hairpin-like conformation as shown by (1)H NMR and CD spectra. Membrane ion-transport studies have shown that the norborneno cyclic peptides 4b and 7b containing Leu-Leu or Val-Val pendants symmetrically placed on the exterior of the ring show high efficiency and selectivity in the transport of specifically monovalent cations. This property can be attributed to the hairpin-like architecture induced by the norbornene unit since the bis-adamantano peptide 15 containing two pairs of Leu-Leu pendants on the exterior is able to transport both monovalent (Na(+), K(+)) and divalent (Mg(2+)/Ca(2+)) cations.     

A Sakurai-Prins-Ritter reaction sequence for the diastereoselective synthesis of 4-amidotetrahydropyrans catalyzed by bismuth triflate

The synthesis of 4-amidotetrahydropyrans has been achieved by a single-step Sakurai-Prins-Ritter reaction sequence in a domino fashion by the reaction of an aldehyde and allyltrimethylsilane in acetonitrile using Bi(OTf)₃ as catalyst. The present synthesis is highly efficient and diastereoselective.     

Asymmetric synthesis of (+)-allo-quercitol and (+)-talo-quercitol via free radical cycloisomerization of an enantiomerically pure alkyne-tethered aldehyde derived from a carbohydrate.

We describe for the first time the free radical cyclization of enantiomerically pure alkyne-tethered aldehydes obtained from a carbohydrate (6, 7). The synthesis of compounds 6 and 7 obtained from a derivative of D-ribose is reported. These radical precursors have been submitted to cyclization with tributyltin hydride plus azobisisobutyronitrile to yield, after ring closure, two carbocycles, respectively. These carbocycles have been obtained as mixtures of E and Z vinyltin isomers, but with excellent diastereoselection at the new stereocenter formed during the ring closure. After protodestannylation, only one diastereomer was detected and isolated. The absolute configuration at the new stereocenter formed during the carbocyclization has been established by detailed (1)H NMR analysis. The specific transformation of 7-methoxymethoxy-2,2-dimethyl-4-methylene-5-tert-butyldimethylsilyloxy-(3aR,5S,7S,7aS)-perhydrobenzo[d][1,3]dioxole into optically pure (+)-allo-quercitol and (+)-talo-quercitol is described. From these results, we conclude that under an appropriate choice of radical precursors and conditions, the synthesis of highly functionalized cyclohexane derivatives of biological interest is now available.