Solution conformation study of substance P methyl ester and [Nle10]-neurokinin A (4-10) by NMR spectroscopy

High-resolution proton spectra at 500 MHz of two tachykinin peptides, substance P methyl ester (SPOMe) and [Nle10]-neurokinin A (4-10), have been obtained in dimethylsulfoxide (DMSO), and for SPOMe, also in 2,2,2-trifluoroethanol (TFE)/water mixtures. Complete chemical shift assignments for these peptides were made based on two-dimensional (2D) nmr techniques, correlated spectroscopy and total COSY. J coupling measurement and nuclear Overhauser effect spectroscopy (NOESY) were then used to determine the conformation of these peptides in the various solvents. Based on the J coupling, NOE correlations, and temperature coefficients of the NH resonances, it is concluded that these two peptides exist in DMSO at room temperature as a mixture of conformers that are primarily extended. For SPOMe in TFE/water with high TFE content, however, helical structures are found to be present, and they become quite clear at temperatures between 270 and 280 K. The variation of the 13C chemical shifts of the C alpha (the secondary shift) with TFE contents corroborates this conclusion. The NOE and C alpha shifts show that the main helical region for SPOMe lies between 4P and 9G. The C-terminus segment L-M-NH2 is found to be quite flexible, which appears to be quite common for neurokinin-1 selective peptides.