Zs. Majer

Mixed intramolecular h-bonds of secondary thioamides

Abstract IR and NMR studies on thioamide models containing one or more Cue5fbO acceptor groups revealed the existence of three types of -Cue5fbO...H-N-CS- mixed intramolecular H-bonds (MIHs) and the formation of thioamide turn conformations in nonpolar solvents. Thioacetyl alanine methyl ester ( 2 ) was found to adopt an extended or C5t conformation fixed with a 2t → 2 MIH. The dominant conformation of the N-protected dipeptide N-methyl thioamide models 7a and 8 is a βt -turn featuring a 1←-4t MIH. 1H NOE data suggest that its φ and φ torsion angles are close to those found in β-turns of peptides. Mixed 1←13t intramolecular H-bonded conformations (γt or C7t) seem to be less favoured as a result of the repulsion between the thiocarbonyl sulphur and the Cβ group at ψ angles required for these conformations.

Chiroptical properties and solution conformations of protected endothiodipeptide esters

Abstract The syntheses and CD spectra in different solvents are described for four series of N-benzyloxycarbonyl-endothiodipeptide esters of types a - d with Ala, Val, Phe and Pro residues. Difference 1 H NOE and IR measurements were also performed on selected models. The spectroscopic evidence pointed to the existence of two main conformers in solution. Conformation A, predominating in nonpolar solvents, can be characterized as a folded C 7 C 5 form with a bifurcated system of intramolecular H-bonds formed by the thioamide N-H and the Cue5fbO groups of the N-terminal urethane and the C-terminal ester moieties. Conformation B, the major conformation in H-bond-accepting solvents such as DMSO, is more extended and flexible in the N-terminal region and assumes a “proline-like” rotamer state (φ 2 ~ -60°) at the C-terminus. The sign and magnitude of the Cotton effect due to the n →π * transition (near 350 nm) can be rationalized with an octant-like sector rule. The optical activity of the π→π * transition is determined in part by exciton interaction of the thioamide with the urethane and/or ester chromophores. Through application of the sector rule, finer details of the correlation between the chiroptical properties and the conformation can be explained.

Solid phase synthesis of a GHRP analog containing C-terminal thioamide group

[Lyst6]GHRP, the C-terminally thionated analog of the highly potent growth hormone releasing hexapeptide His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 was prepared by using solid support. The success of the synthesis showed that Lawessons reagent can be used for selective thionation of an amide group not only in solution but also on the surface of a resin. The C-terminal thioamide group proved to be stable under the conditions of the solid phase synthesis.

Interactions of Al(III) with a neurofilament heptapeptide fragment: AcLysSerProValValGluGly

Abstract The solution state of the neurotoxic Al(III) in biological systems is discussed briefly, and the importance of the Al(III)–peptide and Al(III)–protein interactions in the various neurodegeneration processes is emphasised and evaluated. The possible involvement of Al(III) in the formation of Alzheimers disease marker senile plaques and neurofilament tangles is discussed in light of the solution speciation of the Al(III)–AcLysSerProValValGluGly system and the structural features of the complexes formed.

ON-LINE CD DETECTION IN CHIRAL SEPARATION OF SPIRO-λ4-SULFANES

Enantiomers of chiral spiro-λ4-sulfanes with equatorial diaryl and axial bis(acyloxy), (alkoxy)-(acyloxy) or (acylamino)-(acyloxy) S-substituents were separated on a Kromasil-based chiral sorbent, namely on O,O′-bis(3,5-dimethylbenzoyl)-N,N′- diallyl-L-tartardiamide silica CSP, by using n-hexane (or n-heptane) – dioxane (or 2-propanol) isocratically mixed mobile phases. Direct chiral separation was monitored by a home-made HPLC-CD system consisting of a circular dichroism (CD) spectrophotometer (Jobin-Yvon Model III Dicrograph) and a high performance liquid chromatograph. The system was used in conjunction with HPLC-UV to study the chemical and stereochemical purity of the samples. By the stopped-flow technique CD spectra were measured in the 350–230 nm spectral range. Monitoring at a selected wavelength enabled high-sensitivity detection. Racemic spiro-λ4-sulfanes were also separated into pure enantiomers on preparative scale. Regardless of the structural type of diaryl-spiro-λ4-sulfanes, the first-elute...

Conformational study of linear and cyclic peptides corresponding to the 276–284 epitope region of HSV gD-1

The results of conformational analysis of linear and cyclic peptides from the 276SALLEDPVG(284) sequence of glycoprotein D of Herpes simplex virus are presented. The epitope peptides were synthesized by SPPS and on resin cyclization was applied for preparation of cyclic compounds. Circular dichroism spectroscopy, Fourier-transform infrared spectroscopy and nuclear magnetic resonance (NMR) were used to determine of the solution structure of both linear and cyclic peptides. The results indicated that the cyclopeptides containing the core of the epitope (DPVG) as a part of the cycle have more stable beta-turn structure than the linear peptides or the cyclic analogues, where the core motif is not a part of the cycle. NMR study of H-SALLc(EDPVGK)-NH(2) confirm presence of a type I beta-turn structure which includes the DPVG epitope core.

Vibrational spectroscopic detection of H-bonded β- and γ-turns in cyclic peptides and glycopeptides

Abstract The conformation of N-glycoproteins and N-glycopeptides has been the subject of many spectroscopic studies over the past decades. However, except for some preliminary data, no detailed study on the vibrational spectroscopy of glycosylated peptides has been published until recently. This paper reports FTIR spectroscopic properties in DMSO and TFE of the N-glycosylated cyclic peptides cyclo[Gly-Pro-Xxx(GlcNAc)-Gly-δ-Ava] 3a and 3b in comparison with data on the non-glycosylated parent peptides cyclo(Gly-Pro-Xxx-Gly-δ-Ava) 2a and 2b [a, Xxx ue5fb Asn; b, Xxx ue5fb Gln; δ-Ava ue5fb NH-(CH2)4-CO] and N-acetyl 2-acetamido-2-deoxy-β- d -gluco pyranosylamine (GlcNAc-NHAc, 4). The assignment of amide I band frequencies to conformation is based on ROESY experiments and determination of the temperature coefficients in DMSO-d6 solution. (For the synthesis and NMR characterization of 2a and 3a see Ref. [19].) Cyclic peptides are expected to adopt folded (β- and/or γ-turn) conformations which may be fixed by intramolecular H-bonding(s). A comparison of the temperature coefficients of the NH protons and amide I band frequencies and intensities suggests that in DMSO there is no significant difference in the backbone conformation and H-bond system of the N-glycosylated models and their parent cyclic peptides. The common feature of the backbone conformation of models 2 and 3 is the predominance of a 1 ← 4 (C10) H-bonded type II β-turn encompassing Pro-Xxx or Pro-Xxx(GlcNAc), respectively. The ROESY connectivities in the Asn(GlcNAc) model (3a) have not been found to reflect intramolecular H-bondings between the peptide and the sugar. The unique feature of the FTIR spectra in DMSO of the cyclic models is the lack or weakness of low-frequency ( Because of its destabilizing effect upon γ-turns and other weakly H-bonded structures, DMSO decreases the number of backbone conformers. DMSO also destroys side-chain-backbone H-bondings of type C7, C6 or C8. Possible ‘glyco’ C7 H-bondings in GlcNAc-NHAc (4) or in glycopeptides 3a and 3b cannot resist the effect of DMSO either. The FTIR data in TFE of models 2–4 suggest that the acceptor amide group of strong C7 H-bondings in peptides and glycopeptides absorbs at 1630 ± 5 cm−1 and that of bifurcated H-bondings between 1600–1620 cm−1.

FTIR and CD spectroscopic detection of H-bonded folded polypeptide structures

Abstract The FTIR spectra of a selection of cyclic and linear peptides were measured in DMSO and TFE. The backbone conformation in DMSO of the cyclic models has been inferred from ROESY-based interproton connectivities and the temperature coefficients of the NH protons. The FTIR measurements give support to the following assignment of low-frequency amide I bands: > 1645cm −1 , open (weakly H-bonded) β- and γ-turns; ≈ 1640 cm −1 , β-turns (1 ← 4 H-bonded); ← 1625 cm − , γ-turns (1 ← 3 H-bonded); −1 , β- and γ-turns with bifurcated H-bondings. Solvent-dependence studies on the diamide models Ac-Xxx-NHCH 3 (8, Xxx = Pro, Ala and Gly) suggest that it is the inverse γ-turn (C eq 7 ) structure which is capable of forming a strong 1 ← 3 intramolecular H-bond (band at ≈ 1625 cm −1 ). DMSO destroys 1 ← 3 IHBs of γ-turns and bifurcated turn systems even in cyclic peptides but does not affect 1 ← 4 IHBs. TFE has a stabilizing effect on both 1 ← 4 and 1 ← 3 H-bondings which gives rise to mixtures of β- and γ-turn conformers. Contrary to vibrational spectroscopy, circular dichroism can differentiate between type I(III) and type II β-turns showing class C and class B CD spectra, respectively. Based on the above findings, a class C CD spectrum measured in TFE can reflect the predominance of type I(III) β-turn conformation, but it may also be a composite of subspectra of H-bonded β-turn and γ-turn as well as open conformers.

Secondary solvent effects on the CD bands of thioamides

Abstract The CD spectra of simple thiolactams with fixed geometry, ( R )-3-methyl- and ( R )-5-methyl-2-pyrrolidinethione, reveal a pronounced solvent dependence. The changes in the spectral position of the n →π* CD band have been shown to arise from dimerization of the solute or from H-bond donor or acceptor interactions of the solvent. The spectral data due to secondary solvent effects can be fitted into Tafts solvatochromic equation and are explained by stabilization or destabilization brought about by H-bonding of the HOMO and LUMO of the thioamide chromophore.

Chromatographic studies on the racemization of thiopeptides

Abstract It was found by chromatographic, CD and NMR methods, that the thionation of piperazine-2,5-diones [cyclo(Aaa1-Aaa2) → cyclo (Aaat 1-Aaat 2 (Aaa=-NH-CHR-CO-; Aaat=-NH-CHR-CS-)] or piperazine-2,5-onthiones [cyclo(Aaat 1-Aaa2) → cyclo (Aaat-Aaat)] and, occasionally, even the spontaneous cyclization of endo-thiodipeptide esters [H-Aaat-Aaa-OR] result in enantiomeric (Aaa or Aaa=Gly) or diastereomeric mixtures of piperazine monothiones or dithiones. The diastereoisomers were separated by semipreparative HPLC and their quantitative product distribution was determined by an optimized HPLC method on Hypersil-silica column with CH2Cl2-EtOAc eluent mixtures. Isocratic RP-HPLC on ODS-Hypersil column and pre-column derivatization with 1-flu-oro-2,4-dinitrophenyl-5-L-alanine amide (Marfeys reagent) were used to monitor the racemization of Ala and Pro residues and to determine the ratio of enantiomers. Thionation of urethane protected dipeptide esters or dethionation of the corresponding endoth-iodipeptide de...