260
metric backbone conformation l 2 for cyclo(G1y-ProGly)2in DMSO-& solution Glyi -0 O NO0
9
i
Pro 120“ 300 O
Glyz 270 180”
Nmr spectra in aqueous solution suggest that the same skeletal conformation exists in this solvent. The spectra of the enriched sample (Figure 2) permitted assignment of the highest field carbonyl resonance to the Gly, carbonyl carbon-the one involved in H bonding. This result is somewhat surprising, since shifts to lower field of 5-10 ppm have been reported for intermolecularly H-bonded carbonyl carbons, although shielding due to magnetic anisotropy of the end peptide group of the p turn provides a possible explanation in this case.14 Further, in spectra of the deuterated sample, both in DMSO-ds and in D 2 0 , it was noted that the central carbonyl resonance had markedly reduced intensity relative to the other carbonyls, while in the enriched sample (undeuterated) this was not the case.’: Since the dominant 3C spin-lattice relaxation mechanism is through neighboring protons, deuteration of the C, of Glyp should lead to a less efficient relaxation (longer T I ) and, hence, to the observed reduced peak size. Thus, the Gly2 C=O may be assigned to the central resonance, and the Pro C=O to the remaining downfield peak (Figure 2a and b). Although the observed coupling constants are interpreted in terms of a single conformation for cycZo(G1yPro-Gly)2, described by the 4,$ angles indicated, the Jx, values are equally consistent with a weighted average of similar conformational states related b y rotations (of perhaps h30”) around the C, bonds of the four glycyl residues. In contradistinction to the serine-containing cyclic hexapeptides, 3 , 4 cyclo(Gly-Pro-Gly)2 showed no asymmetric conformations or structures containing cis-X-Pro peptide bonds in the solvents examined. The results with cyclo(Gly-Pro-Gly)2L6 establish a general conformational preference for this class of proline-containing cyclic hexapeptides, which favors a trans’ orientation of the proline residue, and intramolecular H bonding of the residue preceding the proline. Acknowledgments. We thank Dr. Eric T. Fossel for the 13Cnmr spectra. We also thank Dr. Aksel Bothnerby and Dr. Joseph Dadok of the Carnegie-Mellon University for making their 250-MHz nmr facility available to us. We acknowledge with thanks the use of the Varian XL-100-15 nmr instrument (provided by the National Science Foundation) at the Department of Chemistry. This work has been supported, in part, (12) The estimated uncertainty in all (+,+) angles obtained from 10”. (13) F. A. Bovey in “Nuclear Magnetic Resonance Spectroscopy,” Academic Press, New York, N. Y., 1969, p 233, and references therein. (14) D. W. Urry and M. Ohnishi in “Approaches to Biomolecular Conformation,” 13. W. Urry, Ed., American Medical Association, Chicago, Ill., 1970, pp 263-299, and references therein. (15) spectra of the deuterated sample taken with a longer pulse interval (acquisition time plus pulse delay = 8.5 sec) show recovery of the amplitude of the central resonance relative to the other carbonyl resonances. This result confirms that the observed effects are due to a change i n T I , (16) NOTE ADDEDI N PROOF. Professor Robert Schwyzer of the Eidg. Technische Hochschule, Zurich-Honggerberg, has informed us that related investigations on this compound have recently been carried out in his laboratory (Help. Chim. A c t a , in press).
JN, data is
*
Journal o f t h e American Chemical Society
95:l
by U. S. Public Health Service Grants AM-07300 and AM-10794. One of us (L. G. P.) holds a National Science Foundation Predoctoral Fellowship. Lila G . Pease, Charles M. Deber, Elkan R. Blout* Department of Biological Chemistry Harvard Medical School Boston, Massachusetts 02115 Received October 18, 1972
Ferromagnetic Behavior of Phenanthrolineiron(I1) Dichloride and Related Compounds
Sir: Examples of inorganic ferromagnetic order among transition metal organometallic compounds are rare. The only such compound known to us is the five coordinate, spin 3/2 bis(diethy1 dithiocarbamat0)iron(111) chloride studied by Wickman and coworkers. This complex is not polymeric and thus the magnetic exchange is probably due to a weak (T, = 2.5 K) interaction2 between discrete molecules. In this communication we present Mossbauer and magnetic data for Fe(phen)C12, in which a cooperative ferromagnetic behavior is observed, and for some similar systems. Powder susceptibility measurements for analytically pure samples of Fe(phen)Clz over the temperature range 4.2-300 K and at various fields (Figure 1) have been made. Fits with a Curie-Weiss law yield a paramagnetic Curie temperature o = +12 * 4 K. The Curie-Weiss constant C = 1.24 X emu/g determined at high temperatures ( T > 100) corresponds to an effective magnetic moment of 5.4 pB, consistent with quintet iron(I1). This is also suggested by the iron-57 Mossbauer isomer shift [6(300 K) = 1.04 mmisec, A E = 1.45 mmjsec] relative to iron metal. The temperature dependence of dc magnetization at low fields (Figure 2) and ac susceptibility indicates a Curie temperature, To = 8 -f 2 K. In addition, the field dependence of magnetization for high fields and T < 4.2 K clearly shows that Fe(phen)Clz is not completely saturated for fields up to 50 kG (Figure 3), and hysteresis is detected at low fields as expected for a ferromagnetic material. The low-field data in Figure 3 show a rapid rise of magnetic moment, U, us. applied field largely reflecting the demagnetizing field of the particular sample shape. The dashed initial portion indicates the region where hysteresis is observed and the region above -10 kG shows a gradual almost linear increase of u us. field. The moment u = 55.6 emuig at 48 kG corresponds to 3.07 pB/Fe atom in Fe(phen)C12 and magnetic saturation is clearly not achieved. Similar data down to 1.4 K show the same general features with slightly ( 0), but negative for the para position ( u p < 0). From the intensity of the CIDNP spectra it appears that > U F >~ la&. Because of possible relaxation effects we did not attempt to determine the exact ratios of these coupling constants.6 We also studied the photodecomposition of the symmetrical di(fluorobenzoy1) peroxides in tetrachloromethane. The polarized products observed in these reactions indicated that the fluorine hyperfine interactions of both o- and p-fluorophenyl radicals are UFO
(3) (a) G. L. Closs and D . R. Paulson, ibid., 92, 7229 (1970); (b) M. L. Kaplan and H. D . Roth, Chem. Commun., 970 (1972); (c) R. Kaptein, Ph.D. Thesis, Leiden, The Netherlands, 1971. (4) The nmr spectra were recorded on a Jeolco C-60 HL spectrometer at 56 MHz. This instrument contained a quartz probe and was fitted wlth two mirrors to permit uv irradiation (200-W high-pressure mercury lamp) of the samples in the area of the receiver coil. All samples were deaerated with argon. Products 2 and 3 were identified by comparison of their chemical shifts with those of authentic materials. ( 5 ) Cf. (a) E. Lippmaa, T. Pehk, A. L. Buchachenko, and S. V. Rykov, Dokl. Akad. Nauk S S S R , 195, 632 (1970); Chem. Phys. Lerf., 5, 521 (1970); (b) R. Kaptein, J. A. Den Hollander, D. Antheunis, and L. J. Oosterhoff, Chem. Commurt., 1687 (1970); (c) S . R. Fahrenholtz and A. M. Trozzolo, J . Amer. Chem. Soc., 93,251 (1971). (6) To our knowledge, no esr study of the pentafluorophenyl radical has as yet been published. Upon photolysis of 1 at - 196” in a benzene matrix, an esr spectrum was observed with g = 2.0015 and an apparent 1 :2:1 triplet splitting of -20 G. We are indebted to Mr. J. H . Marshall for informing us of these results.
95:I / January I O , I973
