Glycosylation is a common post-translational modification of proteins implicated in cellular recognition processes and controlling protein conformation. 1 Typically, carbohydrates are O-linked to serine (Ser) and threonine (Thr) or N-linked to asparagine (Asn). Glycosylation of (2S,4R)-4-hydroxyproline (Hyp) is widespread in the plant kingdom and occurs in Hyp-rich glycoproteins (HRGPs) that are associated with the cell walls of algae and flowering plants. 2 HRGPs are characterized by extensively glycosylated Hyp sequences that contain O-glycosidic linkages to the pyranose Dgalactose or the furanose L-arabinose. 2 Although HRGPs are broadly implicated in many aspects of plant growth, development 3 and cell wall stability, 2 no information is available about the structural and conformational implications of Hyp-glycosylation on peptide backbone conformation.Hyp and proline (Pro) are unique among the proteinogenic amino acids since they are characterized by limited rotation of the ø dihedral angle (Figure 1) as their side chain is fused to the peptide backbone. As a consequence, there is a reduction in the energy difference between the prolyl amide cis and trans isomers, making them nearly isoenergetic; this leads to higher cis N-terminal amide content relative to the other amino acids. Moreover, the isomerization of the prolyl amide bond has been shown to be the ratedetermining step in the folding pathways of many peptides and proteins. 4 Herein we describe the effects of galactosylation of Hyp on the conformation as well as the thermodynamics and kinetics of prolyl N-terminal amide isomerization. Compounds 4a AcHyp(R-D-Gal)-NHMe and 4b AcHyp( -D-Gal)NHMe were selected as glycopeptide mimics, while AcProNHMe 1, AcHypNHMe 2, and AcHyp(Otert-butyl)NHMe 3 served as non-glycosylated reference compounds (Figure 1). The trans rotamers in compounds 1-4b were assigned on the basis of higher C δ atom NMR chemical shifts relative to the cis rotamer 5 and nOe transfer between H-δ of proline and the N-acyl protons in selective 1D GOESY experiments. 6 The relative amounts of cis and trans isomers were determined by integrating and averaging as many distinct proton signals as possible for both the major and minor isomers in the 1 H NMR spectra. 7 In D 2 O at 37°C, the trans/cis isomer ratio equilibrium constant (K t/c ) for 4a (3.41 ( 0.30) and 4b (3.37 ( 0.28) are nearly identical to those of 2 (3.52 ( 0.05) and 3 (3.34 ( 0.15), and the observed differences are within the experimental errors (Table 3).The kinetics of cis/trans isomerization for compounds 1-4b were determined by 1 H NMR spectroscopy inversion transfer experiments 8 in D 2 O at elevated temperature. At 67°C, the cis-to-trans rate constant of isomerization (k ct ) of the R-glycosylated Hyp model peptide 4a (k ct ) 0.83 s -1 ) is very similar compared to the hydroxyproline model peptide 2 (k ct of 0.73 s -1 ) and 3 (k ct ) 0.77 s -1 ), while the -anomer 4b gave slightly lower rates (k ct ) 0.61 s -1 ). A similar trend was observed for the trans-to-cis rate const...
Investigation of the constituents of Pycnanthus angolensis roots has resulted in the isolation of four new cyclolignene derivatives, named pycnanthulignene A (1), pycnanthulignene B (2), pycnanthulignene C (3), and pycnanthulignene D (4), and six known compounds, 4,5-dimethoxy-3',4'-methylenedioxy-2,7'-cycloligna-7,7'-diene, 2,7-dimethoxy-3,6-dimethylnaphthalene, 4'-methoxy-4,5-methylenedioxy-2,7'-cyclolign-7-ene, genkwainin, 8-hydroxykanzakiflavone-2, and formononetin. The structures of these compounds were established using spectroscopic methods. Compounds 1 and 3 showed significant antimicrobial activities against a panel of drug-resistant pathogens.
MARAT. Can. J. Chem. 62, 1592 (1984. Long-range spin-spin coupling constants over six bonds between I3C nuclei in the methyl group and ring protons or "F nuclei in the para position are reported for 4-fluoroanisole, 2,3,5,6-tetrafluoroanisole, and pentafluoroanisole in solution. The couplings are a-.rr electron mediated, as indicated by INDO MO FPT computations and by measurements on anisole, 2,6-dibromoanisole, 2,6-dichloroanisole, 2,6-dibromo-4-fluoroanisole, and 2,6-dibromo-4-methylanisole. On the basis of the measured coupling magnitudes and a hindered rotor model, it is concluded that the barrier to internal rotation about the C,,,2-0 bond in 4-fluoroanisole lies near 6 kcal/mol, is nearly zero in the tetrafluoroanisole, and is somewhat less than L kcal/mol in the pentafluoroanisole. In the latter, the preferred conformation is that in which the rnethoxy group lies in a plane perpendicular to the pentafluorophenyl plane. Some inconclusive dynamic nmr experiments on anisole, including T I , measurements, are briefly discussed.
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