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Isoleucine, Asparagine, Glutamine, Proline, Leucine, and Glycine Which Bears a Carboxamide Group

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Isoleucine, Asparagine, Glutamine, Proline, Leucine, and Glycine Which Bears a Carboxamide Group OXYTOCIN AND NEUROHYPOPHYSEAL PEPTIDES: SPECTRAL ASSIGNMENT AND CONFORMA TIONAL ANAL YSIS BY 220 M1Hz NUCLEAR MAGNETIC RESONANCE*,t BY LEROY F. JOHNSON, I. L. SCHWARTZ, AND RODERICH WALTERt VARIAN ASSOCIATES, ANALYTICAL INSTRUMENT DIVISION, PALO ALTO, CALIF.; DEPARTMENT OF PHYSIOLOGY, MOUNT SINAI MEDICAL AND GRADUATE SCHOOLS OF THE CITY UNIVERSITY OF NEW YORK; AND MEDICAL RESEARCH CENTER, BROOKHAVEN NATIONAL LABORATORY, UPTON, NEW YORK Communicated by Maurice Goldhaber, May 26, 1969 Abstract. 1\Jagnetic resonance peaks have been assigned to individual protons of the constituent amino acids in the neurohypophyseal hormone, oxytocin, and in related peptides. The assignments were made possible by operation at 220 M\Hz with the use of variable temperature studies, proton homonuclear spin- decoupling, and comparison of spectra of oxytocin analogs. Some of the observed chemical shifts, and NH-CHa coupling constants were studied in relation to the conformation of the hormone. Earlier we investigated the conformation of neurohypophyseal hormones by means of partition chromatography1 and circular dichroism.2 In continuation of these studies we turned to 220 1\IHz proton nuclear magnetic resonance (NM'\1R)- a technique which offers great promise in revealing information about helix-- coil transitions and mobility of side chains as well as intra- and intermolecular interactions in peptides and proteins. In this paper' we wish to report on the analysis of NM'R spectra, in deuterated dimethylsulfoxide, of oxytocin a cyclic peptide hormone composed of eight different amino acids, viz., cystine, tyrosine, isoleucine, asparagine, glutamine, proline, leucine, and glycine which bears a carboxamide group. Materials and Methods.-Spectra were recorded using a Varian Associates HR- 220 spectrometer. The sample concentrations were 4 to 6 weight per cent and the internal reference was tetramethylsilane. The temperature in the sample zone was controlled and known within + 20. Proton spin-decoupling was done by the field sweep method using a side band generated by an HP 4204-A oscillator. Results and Discussion.-One prerequisite for a successful conformational in- vestigation of a peptide or protein by NM\IR is the identification of the resonance pattern of individual protons of the constituent amino acids. To assign reso- nance peaks to specific residues in the neurohypophyseal peptides we used: (a) examination of NMIR spectra at different temperature levels; (b) comparison of spectra of intermediates of oxytocin and of analogs with selected structural modi- fications; and (c) homonuclear proton spin decoupling. In the course of the assignment, the variable temperature studies were particularly helpful in re- solving overlapping resonance signals, while the spectra of intermediates and analogs give an indication of the chemical shifts and the resonance pattern to be expected for the individual amino acid residues in the hormone molecule. The decoupling experiments were vital for deciphering which NH, C', and C" protons belong to a particular amino acid residue. When maximal fingerprinting 1269 Downloaded by guest on October 1, 2021 1270 BIOCHEMISTRY: JOHNSON ET AL. PROC. N. A. S. was required, i.e., for determining which amide proton -peaks were coupled with which Ca-proton peaks, the samples were dissolved in deuterated dimethyl- sulfoxide in order to prevent proton exchange of the NH moiety; when we were interested in determining which C' and Ca protons were coupled, it was occasion- ally advantageous to record the spectra in a solvent system which promotes exchange (deuterated dimethylsulfoxide containing 5 per cent deuterium oxide). Considering the structure of oxytocin4 or deamino-oxytocin,5 it is evident that two amino acid residues, glycine and proline, will exhibit resonance patterns qualitatively different from those of the other residues in the molecule: (a) the amide proton of the glycine residue ought to be split by the two Ca-protons into a triplet, while all the optically active amino acid residues should ideally exhibit NH doublets. IThe one-proton triplet of the glycine residue is readily recognized in the spectrum of oxytocin at 7.97 ppm (Fig. 1). Upon irradiation 988 Hz upfield the triplet collapses indicating the resonances ofthe glycine methyl- ene group to be located at 3.47 ppm;6 (b) the proline residue lacks the amide pro- ton, and hence the C' proton ought only be coupled with protons appearing further upfield. This criterion is met in the spectrum of oxytocin by the doublet -actually two unresolved, closely spaced doublets-centered at 4.40 ppm which collapses to a broad singlet when irradiated 515 Hz upfield. By virtue of their special proton patterns, the glycine and proline residue were readily detected. More difficulties may be expected in the assignment of the remaining amino acid residues, because even after decoupling experiments reveal which NH, C' and Ca protons are linked, a criterion must still be found for associating these proton resonances with a particular amino acid residue in the hormonal molecule. Since the NH doublets per se never reveal the particular 9 8 7 6 5 ppm FiG. l.-Oxytocin. Bridges indicate resonance patterns connected by decoupling. Downloaded by guest on October 1, 2021 VOL. 64, 1969 BIOCHEMISTRY: JOHNSON ET AL. 1271 optically active amino acid residue with which they are associated and since the complex Ca-proton patterns will yield such information only in rare in- stances, it is the Ca-proton resonance pattern which carries the major burden for providing the clue to a more specific assignment. For example, the observed unsymmetrical two-proton triplet at 1.47 ppm in the spectrum of deamino- oxytocin (see Fig. 3) can only be attributed to leucine, which possesses a C- methylene moiety flanked by two single protons (decoupling experiment of the leucine residue is shown in expansion of Fig. 2). The correctness of the assignment was attested with deamino-8-alanine-oxytocin, an analog in which the leucine residue is replaced by an alanine residue.8 The spectrum of this analog lacks the signals assigned to the C"-methylene moiety of leucine and, although the Ca- proton resonances of the alanine residue exhibit approximately the same chemical shift as those of the replaced leucine residue the splitting pattern differs. To complete the determination of the NMR pattern of the C-terminal tri- peptide sequence of oxytocin, we examined the spectrum of Z-Pro-Leu-GlyNH2. The two amide protons of the glycine amide moiety are centered at 7.00 ppm, therefore, they are totally hidden in the spectrum of oxytocin (Fig. 1) and par- tially buried in the spectrum of deamino-oxytocin (Fig. 2) by the low field doublet of the tyrosine residue. From this spectrum we also obtained the chemical shifts and splitting patterns of the isopropyl group of the leucine and the fl-, ay-, and 5-methylene groups of the proline residue, respectively; these data were directly applicable to the resonance assignment of these groups in the oxytocin and deamino-oxytocin spectra. The next target was the assignment of the amino acid residues which com- ... , . DEAMINO-OXYTOCIN tWo:) 220 MHz A a.- o._ IN DMSO-da AT 30 C IM AT &C 1125HHi.2 Ir U) _J _J 5~~~~~05z - 4- OH 836 H I~~~~~~~ 730 Hz ~ ~ IMlXI X XU 95Hz r_ Ii[Ii II Ii 865 Hz FIG.~~~~~~~~9Demn-xtcn>- (~Exanio onteletilsresdcuinofheL11ppmppm poonf theprolin reides11111111exasoH~~on th86rihH lutae tedculn f h mdC n C~~~~~~~~~3proonof th leucin reides I ~~~~~~AA!XXTYR 4HzI P z ---- -- --I AI - 10 9 8 7 6 5 4 3 2 0 ppm FIG. 2.-Deamino-oxytocin. Expansion on the left illustrates decoupling of the C" proton of the proline residues; expansion on the right illustrates the decoupling of the amide, C' and Co protons of the leucine residues. Downloaded by guest on October 1, 2021 1272 BIOCHEMISTRY: JOHNSON ET AL. PROC. N. A. S. IA t) v636A , I -_ -, -, -, t A. I i '- -, i 9 8 7 6 5 4 3 2 0 PPM FIG. 3.-Protected C-terminal pentapeptide of oxytocin. Expansion shows the decoupling of the NH proton of the cysteine, leucine and glycine residues. prise the ring component of the hormonal molecule. Although the spectrum of the C-terminal pentapeptide of oxytocin (Fig. 3) exhibits the characteristic pro- ton resonance associated with the proline, leucine, and glycine residues, the prob- lem of assigning the corresponding resonances for the cysteine and asparagine residues persists. Therefore, we studied the spectrum of a pentapeptide deriva- tive in which the asparagine residue had been replaced by a valine residue (Fig. 4) -for our purpose a particularly advantageous replacement. In the course of this study, we had noted that the chemical shift of the Ca-proton resonances of the proline residue remained remarkably constant when the peptide chain was lengthened, when the hydrogen ion concentration or the temperature of the sam- ple was changed, and, hence, that the Ca-proton resonances of this residue may serve in first approximation as a reference, at least in the case of neurohypo- physeal hormones and their peptide derivatives. The Ca-proton resonances of amino acid residues which bear an electron withdrawing group on the fl-carbon such as SH, CONH2, or p-hydroxyphenyl appear downfield, while the Ca-proton resonances of amino acid residues bearing hydrogen, methylene, or methyl groups on the ,8-carbon appear upfield from those of the C' proton of proline. However, this rule has to be applied with caution because any significant change in the magnetic anisotropies (e.g., as a result of change in conformation or solva- tion) of the three groups attached to a CH' group would of course change its chemical shift. Returning to Figure 4, it can be seen that the leucyl and valyl CH' resonances appear further upfield when compared with those of prolyl, while the signal of the cysteine CH' residue is downfield at 4.68 ppm.
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