The free electron model of Pauling is used to calculate the magnetic field around a benzene ring which is rotating rapidly about all axes in an external magnetic field. It is assumed that the π electrons precess in two circular paths, one on each side of the ring, equal in radius to the C–C distance in the benzene ring. The separation of these rings is taken as 1.28 A, which gives a calculated value for the nuclear magnetic resonance shielding value for benzene protons equal to the observed value. The field thus calculated is employed to predict shielding values for other aromatic compounds. Agreement with experiment is in general good, but there are some exceptions.

The carbon-13 magnetic resonance spectra oí a selection of simple proline derivatives are compared and discussed. The general features of such spectra are identified, with especial importance given to the problem of differentiation of cis and trans X-proline amide bonds. Spectra of the proline moiety in more complex oligopeptides are discussed in terms of the conclusions drawn from the spectra of simple compounds.Because of the conformational requirements inherent in the pyrrolidine ring, proline has a unique place among the naturally occurring amino acids. Its occurrence in biologically active oligopeptides and proteins has important consequences upon the three-dimensional structures of these compounds.1 As a result, proline and polypeptides containing proline have received extensive and detailed study by a wide variety of physical methods, including proton magnetic resonance (pmr) spectroscopy.2Unlike the pmr spectrum of the proline residue, the fully proton-decoupled carbon-13 magnetic resonance (cmr) spectrum would be expected to be relatively simple. Furthermore, because of the importance of steric3 and other proximity4-6 effects in carbon-13 chemical shifts, one might expect that cmr spectroscopy would provide useful information regarding the conformation within and around the proline unit.We have therefore surveyed the cmr spectra of a number of proline derivatives and proline-containing oligopeptides. The purpose of the present paper is to identify and discuss the major features of the cmr spectrum of the proline unit. Experimental SectionThe cmr spectra of the proline derivatives, all of which were commercially available, were measured as 10% (w/v) aqueous solutions, using a Varían XL-100 spectrometer adapted for Fourier transform spectroscopy.7 Chemical shifts were measured relative to internal 1,4-dioxane, then referred to external carbon disulfide on the basis of the chemical shift of 1,4-dioxane relative to the reference (126.2 ppm). ResultsThe cmr spectra of the proline derivatives studied are presented in Table I. The problem of peak assignments in such simple compounds is reduced to a minimum and can be based entirely upon the wellknown correlations between carbon chemical shift and substitution.8-10 These assignments are supported by comparisons within the series of proline deriviatives (1) L. Mandelkern in "Poly-a-amino Acids," G.

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