The solution conformations of the all l‐α‐peptide 1 and the corresponding retro‐all d‐α‐peptide 2, two 20‐meric peptides which generate antibodies that cross‐react with the gp120 envelop protein of human immunodeficiency virus‐1 (HIV‐1), have been investigated by high‐field H NMR spectroscopy. Complete sequential and inter‐residue interaction assignments were made from the 2D NMR spectra acquired at 500 MHz and 600 MHz in 40% deuterotrifluoroethanol (d3‐TFE)/H2O at pH 2.3, and in 300 mm sodium dodecyl sulphate (SDS) in 100% D2O or 90% H2O/10% D2O at pH 2.6. Based on analysis of the nuclear Overhauser effect (NOE) and amide exchange data, peptide 1 and its retro‐inverso isomer 2 in the polar solvent environment of 40% d3‐TFE/H2O at pH 2.3 show very similar topological features. However, in the relatively non‐polar 300 mM SDS micellar environment, peptides 1 and 2 exhibit differences in their solution structures in terms of the amide backbone and side‐chain orientations. In particular, under the SDS micellar condition, peptide 1 maintains much of the secondary structure observed for this 20‐mer peptide in 40% d3‐TFE/H2O, pH 2.3, whereas peptide 2 adopts a more extended structure. These NMR results provide the first confirmation that the secondary structure of the all l‐α‐peptide 1 is maintained in both polar and non‐polar environments, whereas the secondary structure and topology of the notionally equivalent retro‐inverso isomer depends more on the solvent conditions. These results with the all l‐α‐peptide 1 and its retro‐inverso isomer 2 provide important insight into the conformational influences of the C‐ and N‐end group with l‐α‐ and retro‐l‐α‐isomer pairs in non‐polar environments, and thus have general relevance to the design of bioactive retro‐inverso peptidomimetic analogues related to immunogenic or hormonal peptides.
Natural abundance 75 MHz I3C NMR spectral assignments are reported for bovine and porcine zinc insulin in solution. A large number of protein resonances are well resolved, and approximately 80% of these have been assigned to either residue types or to specific sites within the protein. Assignment techniques included consideration of free amino acid or peptide shifts, pH studies and comparison of sequence and spectral differences between bovine and porcine insulin, in addition to the use of NMR relaxation times. The DEPT spectral editing technique was also found to be particularly valuable as an assignment aid. This technique allows subspectra containing only CH, CH, or CH, carbon types to be generated. The method also produces signal enhancement relative to broad band decoupled I3C NMR spectra of large proteins which generally have reduced nuclear Overhauser enhancements.
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