Co-oligopeptides of aromatic amino acids and glycine with a variable distance between the aromatic residues. VI. Circular dichroism studies of Co-oligopeptides of l-phenylalanine, L-tryptophan, and glycine

High-performance liquid chromatographic enantiomeric separation of an enzyme inhibitor which possesses both a chiral center and tautomeric moieties

The enantiomeric separation using high-performance liquid chromatography (HPLC) on chiral stationary phases (CSPs) of a chiral compound which exists in solution in several tautomeric forms is described. 2,4-Dioxo-5-acetamido-6-phenylhexanoic acid is the most potent inhibitor known for peptidylamidoglycolate lyase (PGL, EC 4.3.2.5), an enzyme which plays an essential role in carboxyl-terminal amidation of many biological peptides. Synthesis of this inhibitor entails an alkaline hydrolysis step, under which condition the compound is racemized; thus, HPLC with a CSP was employed to obtain the individual enantiomers of this inhibitor. Since 2,4-dioxo-5-acetamido-6-phenylhexanoic acid exists in solution in several tautomeric forms, the strategy of first converting this compound from its multiple enol forms into a single diketo tautomer, which was then applied to various CSPs, was employed. Successful preparative scale enantiomeric separation of this compound was achieved using a Chiralpak AD CSP. Enantiomeric separation was also accomplished on a D-penicillamine column, but this CSP was found to be less satisfactory for preparative purposes.

The spectroscopic properties of the lipodepsipeptide, syringomycin E

The spectroscopic properties of syringomycin E, an antibiotic lipodepsinonapeptide associated with pathological states in plants, have been investigated by uv absorbance and CD spectroscopies, and by the synthesis of relevant model compounds. Initial studies [E. Vaillo, A. Ballio, P. L. Luisi, and R. M. Thomas (1990) in Peptides 1990, Giralt, E. & Andreu, D., Eds., Escom Scientific, Leiden, Netherlands] suggested that a significant contribution to the spectra was due to the presence of a zdehydroaminobutyric acid residue in the amino acid sequence. The model peptides N-Boc-L-Phe-delta zAbu-OMe and its analogue, N-Boc-L-Phe-L-Thr-OMe, lacking the unsaturated bond, were synthesized using standard solution chemistry, and a detailed investigation was made in which the spectra of the models and that of syringotoxin (an antibiotic closely related to syringomycin E but without a Phe residue) were compared with those of syringomycin E under a variety of solvent conditions. The uv absorbance spectra of both N-Boc-L-Phe-delta zAbu-OMe and syringomycin E clearly showed the presence of the unsaturated residue while the CD spectra were complex, environmentally sensitive, and contained contributions from both the delta zAbu and Phe residues. In the course of these studies extinction coefficients were obtained for syringomycin E and its dipeptide model. The origins of the uv and CD spectra are discussed in detail, and a comparison is made with the spectra of other, similar lipopeptide antibiotics. Finally, a structural model for syringomycin is proposed in which the changes induced in the spectrum by alterations in the solvent environment are accommodated.

Conformational study of the chromosomal protein MC1 from the archaebacterium Methanosarcina barkeri

Methanogen chromosomal protein MC1 is a polypeptide of 93 amino acid residues (Mr 10,757) which represents the major protein associated with the DNA of the archaebacterium Methanosarcina barkeri and can protect DNA against thermal denaturation. The conformation of protein MC1 has been investigated by means of predictive methods, infrared spectroscopy, circular dichroism and tryptophan fluorescence studies. Protein MC1 has a low amount of alpha-helix but contains antiparallel beta-sheet strands. The larger hydrophobic cluster which contains tryptophan at position 61 appears buried in the protein. Addition of salts induces the unfolding of the protein and makes the tryptophan indole ring more rigid. With respect to its primary structure and its conformation, protein MC1 appears radically different from the chromosomal DNA-binding protein II (also called HU-type protein) in eubacteria.

Denaturation of concanavalin A by urea at acid pH

The denaturation of dimeric concanavalin A induced by urea at pH 3 has been studied using optical activity and sedimentation velocity. Under the conditions employed Mn+2 and Ca+2 are dissociated from the protein, but the basic structural elements are little changed from those prevailing in the functional lectin at pH 5.5 [H.E. Auer and T. Schilz, preceding paper in this issue]. The protein passes through three stages as the urea concentration is varied from 0 to 10 M. Below 4 M urea the only effect observed is the loss of optical activity of the aromatic amino acid residues. At 4 M, a conformational change occurs producing extensive aggregation, which persists to 7 M. At 8-10 M urea a disordered monomeric protein molecule prevails. The protein could be reactivated provided that dilution to native conditions was very rapid and the protein concentration remained very low. Kinetics of denaturation were monitored by optical activity at 218, 225 and 283 nm. Transients with one, two or three components were observed, which were resolved by nonlinear regression according to sequential first-order decay laws. First order character was confirmed by independence of the kinetic parameters from protein concentration over a two- to four-fold range. Enthalpies and entropies of activation for the various steps were also determined. The transients at the three wavelengths monitor changes in beta structure, beta turns and aromatic groups, respectively. The urea dependence of the rate constants is unique in most cases. It is concluded that different structural elements of the concanavalin A molecule unfold independently from one another.