The cyclic pentapeptide, oWo-(glycyl-L-prolylglycyl-D-alanyl-L-prolyl) [cyc7o-(Glyi-Pro-Gly2-D-Ala-Pro)], has been synthesized and studied by 'H and 13C nuclear magnetic resonance (NMR) and circular dichroism (CD). We find that the predominant conformation in chloroform, acetonitrile, dimethyl sulfoxide, and water contains all trans peptide bonds and displays two N-H's (those of Gly-1 and D-Ala) which by the usual criteria (solvent, concentration, and temperature independence of 'H chemical shifts) do not participate in any intermolecular interactions. Infrared spectra (IR) in chloroform support the interpretation that the conformation contains N-H's which are intramolecularly hydrogen bonded. I3C NMR chemical shifts and CD spectra suggest strongly that one of the hydrogen bonds is in a y turn involving one of the Pro residues, A model containing one /3 turn (1<-4 hydrogen bond) and one y turn (1<-3 hydrogen bond) is proposed for the solution conformation based on all of the spectral data. Dr. I. L. Karle has completed a crystal-structure determination, reported in an accompanying paper, on cyc7o-(Gly-Pro-Gly-D-Ala-Pro), which reveals that the same /3,-y-turn conformer is present in the solid state. Ion binding studies with cyc/o-fGly-Pro-Gly-D-Ala-Pro) establish a new class of synthetic cyclic peptide complexing agents, cyclic pentapeptides. The peptide has been found to bind the divalent cations Mg2+, Mn2+, Ca2+, and Ba2+, and the alkali metal Li+, strongly. Titration curves from CD data demonstrate at least two complexes of probable stoichiometry 1 peptide: 1 cation (PC) and 1 peptide:2 cations (PC2). NMR data suggest that the cyclic pentapeptide adopts a conformation with one cis X-Pro bond in a PC complex, and that this complex may be in equilibrium with an all-trans PC species. A model for the specific mechanism of binding is discussed.Cyclic peptides are well-established models for conformational features which are important in protein structure.2-9 Recent investigations of the details of protein tertiary struc-ture10-12 and of the preferred conformations of cyclic2-9 and linear13-15 peptides, both natural and synthetic, have revealed the prevalence and importance of hydrogen-bonding arrangements in which the peptide chain reverses direction. Two

A range of ortho-disubstituted C-aryl aldimines has been synthesized. N.m.r. spectroscopic analysis revealed a significant proportion of the Z-isomer a t equilibrium in solution. The E-Z-isomer distribution is critically examined in terms of electronic, steric, and solvent effects. The effect of trace amounts of carboxylic acid on imine stereomutation is discussed.THE barrier to E-Z-isomerization about a carbonnitrogen double bond is very sensitive to the nature of the s u b s t i t u e n t ~. ~-~The barrier to interconversion in simple N-alkylimines is sufficiently large (> 80 k J mol-l)for the latter process to be slow on the n.m.r. time-scale at ambient temperature. N.m.r. techniques have previously been applied in the detection and estimation of E-Z-ketimine isomer ratios at equilibrium.2,6 HOWever, other studies 7-9 using similar methods led to the conclusion that aldimines exist completely (>99%) in the E-configuration.

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The migration and retention of deuterium which occurs during aromatic hydroxylation of several substrates (NIH Shift) with a range of fungi is compared to the migration and retention obtained with liver microsomes and a chemical model system; enzymatic mechanisms are discussed in terms of arene oxide intermediates.