Carbon-13 magnetic resonance spectroscopy. Spectrum of proline in oligopeptides

Abstract

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.

NMR conformational study of iturin A, an antibiotic from Bacillus subtilis

“…The value is in accordance with [ 121. As could be expected, the chemical shifts of the proline carbons Cp and C, are consistent with a trans-peptide bond [20].…”

Koshikamide A2, a Cytotoxic Linear Undecapeptide Isolated from a Marine Sponge ofTheonellasp.

“…Although the carbon chemical shifts of Pro-10 hardly fluctuated between the two conformers, those of Pro-9 differed significantly. The difference in chemical shifts of the -and -carbons of Pro-9 were 4.0 ppm for the trans isomer and and 8.0 ppm for the cis isomer, 7,8) in agreement with the cis/trans isomerization of the Phe-8/Pro-9 amide bond. Interestingly, the amide bond between Pro-9 and Pro-10 adopted only the trans geometry.…”

Structure of the (1/1) complex cis-3-(2,5-dihydroxyphenyl)-5-hydroxy-3-methyl-2-(N-methylpropionamido)-2,3-dihydrobenzo[b]furan–(2H6)acetone, C19H21NO5.C3D6O

“…This was confirmed also by nuclear Overhauser enhancement (NOE) studies (Anet & Bourn, 1965;Lewin & Frucht, 1975). The same conclusion with respect to the direction of the anisochrony is derived from ~3C NMR studies on simple amides (Piccini-Leopardi & Reisse, 1981;McFarlane, 1970;Levy & Nelson, 1972;Dorman & Bovey, 1973a) and amino acids (Dorman & Bovey, 1973b;Voelter & Oster, 1973). According to this general consensus the prevailing (60%) trans rotamer in solution was assigned based on the fact that the C(9) and C(18) atoms give high-frequency shifts in this form while the N-methyl group absorbs at lower frequencies as shown by the present ~H and ~3C NMR results.…”