To determine the structural perturbations induced by the CαH→Nα exchange in aza‐peptides, we have examined by H NMR and IR spectroscopy various derivatives of the aza‐analogues of alanine, aspartic acid and asparagine in different organic solvents with increasing polarity. Their general formulas are: R'‐AzXaa‐NR2R3, R'‐Pro‐AzXaa‐NR2R3 and R‐AzXaa‐Pro‐NR2R3 (where AzXaa denotes the aza‐analogue of the amino acid residue Xaa = Ala, Asp, Asn; R = Boc, Z; R2, R3= H, Me, iPr). The aza‐analogue of an amino acid residue appears to be a strong p‐turn‐inducing motif, and the AzAsn carboxamide side‐chain is capable of interacting, as a proton donor, with the preceding peptide carbonyl group.
The homologous RCO-Pro-Xaa-NHR‘ model pseudodipeptides containing the reduced peptide (CαCH2NHCα), reduced azapeptide (CαCH2NHNα), methyleneoxy (CαCH2OCα), and iminoazapeptide (CαCHNNα) surrogate of the middle amide group have been prepared. Their structural analysis has been carried out
in solution by 1H NMR and IR spectroscopy and in the solid state by X-ray diffraction. The last three fragments,
not protonated in the pH range 2−12, and the reduced fragment in its neutral amine form induce quite similar
molecular structures characterized by a hydrogen bond between NHR‘ and the N/O atom replacing the amide
NH group and closing a five-membered cycle. The neutral amine or protonated ammonium state of the reduced
amide fragment, with a pK
a value of about 7, depends on the environment. Protonation induces a conformational
transition due to the strong proton donating properties of the ammonium group which interacts with the RCO
carbonyl.
Azaproline (AzPro) is an analogue of proline containing a nitrogen atom in place of the CαH group. AzPro has been introduced in various model peptides, and especially in the Boc‐Ala‐AzPro‐Ala‐NHiPr tripeptide. The structural consequence of that modification has been investigated in solution by using IR and 1H NMR, with reference to the cognate proline‐containing peptide. Contrary to proline, which induces β‐folding of the Pro‐Ala sequence, azaproline apparently favors βVI‐folding of the Ala‐AzPro one with high occurrence. Opening of the AzPro pyrazolidine ring to get N‐methylazaalanine fundamentally does not change the structural properties of the azatripeptide, but allows the existence of open conformers to an extent depending on the solvent.
SYNOPSISEtOH/ETBE azeotropic mixture was separated by pervaporation through films of polyurethaneimides (PUI), i.e., alternating block copolymers varying in their structure only by their oligomeric soft blocks. Pervaporation performances strongly depend on the flexible segments: fluxes on the molecular weight and selectivity towards ethanol on the chemical structure in the decreasing order: PEG, PCL, PCD, PTMG, PPG. In order to correlate selectivity with segment polarity, a new solvatochromic polarity probe that was well soluble in PUI was synthesized. Using 13 solvents covering a wide polarity range, Vis A""" of the photochromic indicator open form was linearly correlated with the ET(30) polarity scale. By illumination of the dissolved dye, PUI polarity was investigated in relation to soft segment nature and size. The very high PUI polarity values and their splitting for highest size segments were assigned to preferential probe solvation by interblock urethane junctions combined with phase segregation. Polarity values consistent with the chemical structure of the flexible segments were provided by similar measurements on suitably end-capped precursors of these segments and were then linearly correlated with the related PUI pervaporation selectivity. 0 1995 John Wiley & Sons, Inc.
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