Search results for "Conformational isomerism"
showing 10 items of 163 documents
Pentapeptides containing two dehydrophenylalanine residues - synthesis, structural studies and evaluation of their activity towards cathepsin C
2008
Synthesis, structural and biological studies of pentapeptides containing two Delta Phe residues (Z and E isomers) in position 2 and 4 in peptide chain were performed. All the investigated peptides adopted bent conformation and majority of them could exist as two different. conformers in solution. Only pentapeptides. containing free N-termini appeared to act as weak inhibitors of cathepsin C with the slow-binding, competitive mechanism of inhibition. free acids being bound slightly better than their methyl esters. Results of Molecular modeling suggested significant difference between peptides, depending of the type of amino acid residue in position 5 in peptide chain. Dehydropeptides contain…
Conformational investigation of α,β-dehydropeptides. XVI. β-turn tendency in Ac-Pro-ΔXaa-NHMe: crystallographic and theoretical studies
2006
The crystal structures of two diastereomeric alpha,beta-dehydrobutyrine peptides Ac-Pro-(Z)-DeltaAbu-NHMe (I) and Ac-Pro-(E)-DeltaAbu-NHMe (II) have been determined. Both dehydropeptides adopt betaI-turn conformation characterized by the pairs of (phi(i+1), psi(i+1)) and (phi(i+2), psi(i+2)) angles as -66, -19, -97, 11 degrees for I and -59, -27, -119, 29 degrees for II. In each peptide, the betaI turn is stabilized by (i + 3) --> i intramolecular hydrogen bonds with N...O distance of 3.12 A for I and 2.93 A for II. These structures have been compared to the crystal structures of homologous peptides Ac-Pro-DeltaVal-NHMe and Ac-Pro-DeltaAla-NHMe. Theoretical analyses by DFT/B3LYP/6-31 + G** …
The effect of β-methylation on the conformation of α, β-dehydrophenylalanine: a DFT study
2009
Dehydroamino acids are non-coded amino acids that offer unique conformational properties. Dehydrophenylalanine (ΔPhe) is most commonly used to modify bioactive peptides to constrain the topography of the phenyl ring in the side chain, which commonly serves as a pharmacophore. The Ramachandran maps (in the gas phase and in CHCl3 mimicking environments) of ΔPhe analogues with methyl groups at the β position of the side chain as well as at the C-terminal amide were calculated using the B3LYP/6-31 + G** method. Unexpectedly, β-methylation alone results in an increase of conformational freedom of the affected ΔPhe residue. However, further modification by introducing an additional methyl group a…
X-ray Crystal Structure of Woodinine and Conformational Analysis by Semiempirical and 1H-NMR Methods
1997
The mol. structure of (-)-woodinine (I), a carboline-based alkaloid with antibacterial and antimycobacterial activities, was investigated by X-ray crystallog., NMR, and semiempirical quantum chem. methods. The X-ray crystal structure of I showed the indole ring in the expected planar conformation, the pyrrolidine ring in an envelope conformation, and a weak intramol. hydrogen bond between the pyrrolidine nitrogen and the proton of the indole nitrogen. NMR expts. indicated that this hydrogen bond is not present in soln. and that further differences exist between the crystal and the soln. structures of I. By semiempirical quantum chem. methods, different local energy min. conformations of I, …
Light‐Induced On/Off Switching of the Surfactant Character of the o ‐Cobaltabis(dicarbollide) Anion with No Covalent Bond Alteration
2021
Cobaltabis(dicarbollide) anion ([o-COSAN]- ) is a well-known metallacarborane with multiple applications in a variety of fields. In aqueous solution, the cisoid rotamer is the most stable disposition in the ground state. The present work provides theoretical evidence on the possibility to photoinduce the rotation from the cisoid to the transoid rotamer, a conversion that can be reverted when the ground state is repopulated. The non-radiative decay mechanisms proposed in this work are coherent with the lack of fluorescence observed in 3D fluorescence mapping experiments performed on [o-COSAN]- and its derivatives. This phenomenon induced by light has the potential to destruct the vesicles an…
Anharmonic Vibrational Spectrum and Experimental Matrix Isolation Study of Thioformic Acid Conformers -- Potential Candidates for Molecular Cloud and…
2021
Thioformic acid (TFA) is the sulfur analog of formic acid, the simplest organic acid. It has three analogues HCOSH, HCSOH, and HCSSH, each of them having two rotational isomeric (rotameric) forms: trans and cis where the trans form is energetically more stable. In this article, we study computational energetics and anharmonic vibrational spectrum of TFA including overtone and combination vibrations. We also studied experimental photoisomerization and photodecomposition channels of HCOSH molecules with different wavelengths. We suggest that TFA is a potential sulfur containing candidate molecule for interstellar and planetary observations and discuss these in a light of different radiation e…
Optimal arrangements of 1,3-diphenylisobenzofuran molecule pairs for fast singlet fission.
2019
A simplified version of the frontier orbital model has been applied to pairs of C2, C2v, Cs, and C1 symmetry 1,3-diphenylisobenzofuran rotamers to determine their best packing for fast singlet fission (SF). For each rotamer the square of the electronic matrix element for SF was calculated at 2.2 × 109 pair geometries and a few thousand most significant physically accessible local maxima were identified in the six-dimensional space of mutual arrangements. At these pair geometries, SF energy balance was evaluated, relative SF rate constants were approximated using Marcus theory, and the SF rate constant kSF was maximized by further optimization of the geometry of the molecular pair. The proce…
Taming conformational heterogeneity in and with vibrational circular dichroism spectroscopy
2019
Chemical science 10, 7680 -7689 (2019). doi:10.1039/C9SC02866H
The rotational gas-phase spectrum of trans- and cis-HSSOH at 100GHz
2009
Abstract We present the first pure rotational spectra of the two most stable conformers of oxatrisulfane, trans- and cis-HSSOH, in their vibrational ground state. For both conformers a-, b-, and c-type transitions have been recorded in the range from 75 to 120 GHz using an all solid-state spectrometer. More than 200 lines have been assigned for each conformer, most of them belonging to the r Q 2 - and the r Q 3 -branch of the perpendicular spectra. The least-squares fit analysis using a semirigid rotor Hamiltonian in S-reduction yields precise values for the ground-state rotational constants A 0 = 21 145.93422 ( 66 ) , B 0 = 4479.92301 ( 21 ) , C 0 = 3828.34269 ( 21 ) for cis-HSSOH, and A 0…
The Closed/Open Model for Lipase Activation. Addressing Intermediate Active Forms of Fungal Enzymes by Trapping of Conformers in Water-Restricted Env…
2001
The behavior of prototypic fungal lipases in a water-restricted environment has been investigated by exploiting the reported experimental strategy that allows the trapping (freeze-drying) of the enzyme in the conformation present in aqueous solution and to subsequently assay it in nonaqueous media [Mingarro, I., Abad, C., and Braco, L. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 3308-3312]. We now report, using simple esterification as well as acidolysis (triglycerides as substrates) as nonaqueous model reactions, that the presence of a detergent (n-octyl-beta-glucopyranoside) in the freeze-drying buffer, at concentrations below the critical micellar concentration, generates different catalyti…