220 Mc Nuclear Magnetic Resonance Spectra of Gramicidin S in Solution

Abstract

IN a previous communication¹ we reported the nuclear magnetic resonance (NMR) spectra at 100 Mc of gramicidin S in dimethylsulphoxide (DMSO) solution and made a preliminary tentative assignment of the resonance peaks. We have done further experiments using an HA/100 spectrometer with double and triple resonance apparatus and a high resolution HR/220 spectrometer. The results show that: (a) the peak at 9.1 p.p.m., which had been attributed to an impurity, corresponds to the NH protons of phenylalanine residues (Fig. 1); (b) the peak at 8.05 p.p.m. corresponds to the NH₂ protons of ornithine, which had been overlooked in the previous assignment; (c) a new peak at 7.13 p.p.m. can be seen in the spectrum carried out at 70° C with an HR/220 spectrometer (Fig. 1B); in the spectrum at lower resolution this peak was masked by the ring CH protons of phenylalanine; it can now be assigned to the NH protons of the valine residues; this peak is also clearly observable in DMSO + CDCl₃ (Fig. 2) and in DMSO + CD₃OD (Fig. 3); (d) the peak at 4.80 p.p.m., previously assigned to the αCH protons of proline, corresponds to the αCH protons of ornithine, as shown by double resonance experiments; (e) the peak at 4.50 p.p.m. corresponds to the αCH protons of leucine (double resonance experiments).