A Noncovalent Approach to Antiparallel β-Sheet Formation
摘要:
Four tripeptide chains, when attached to the same end of a hydrogen-bonded duplex (1.2) with the unsymmetrical, complementary sequences of ADAA/DADD, have been brought into proximity, leading to the formation of four hybrid duplexes, 1a.2a, 1a.2b, 1b.2a, and 1b.2b, each of which contains a two-stranded beta-sheet segment. The extended conformations of the peptide chains were confirmed by 1D and 2D NMR. The peptide strands stay registered through hydrogen bonding and the beta-sheets are stabilized by side chain interactions. Two-dimensional NMR data also indicate that the duplex template prevents further aggregation in the peptide segment. When the peptide chains are attached to the two different termini of the duplex template, NMR studies show the presence of a mixture with no clearly defined conformations. In the absence of the duplex template, the tripeptides are found to associate randomly. Finally, isothermal titration calorimetry studies revealed that the hybrid duplex 1a.2a was more stable than either the duplex template or the peptides alone.
Triphenylantimony dicarboxylates (Ph3Sb(O2CR)2, where R=Me, CF3, Ph and CH2NH-Z) readily reacted with amines (R′NH2, where R′=n-C6H13, s-Bu, C6H11, and Ph) to afford corresponding amides and triphenylstibine oxide in fairly good yields. The amidation of RCO2H with R′NH2 was also catalyzed by the orgnoantimony compounds.
A simple NaOMe catalyst provides superior accessibility to a wide variety of functionalized amides including peptides through direct amination of esters in an atom-economical and environmentally benign way.
An unconventional approach to peptidecyclization involving the use of acyl ammonium species was developed. Rapid and epimerization/dimerization-free cyclization of synthetically challenging peptides was possible, including a difficult cyclization reaction involving N-methyl amide bond formation. The approach is characterized by ease of purification of the products, high productivity, and high reaction