Amary Cesar

Membrane structure and conformational changes of the antibiotic heterodimeric peptide distinctin by solid-state NMR spectroscopy

The heterodimeric antimicrobial peptide distinctin is composed of 2 linear peptide chains of 22- and 25-aa residues that are connected by a single intermolecular S-S bond. This heterodimer has been considered to be a unique example of a previously unrecorded class of bioactive peptides. Here the 2 distinctin chains were prepared by chemical peptide synthesis in quantitative amounts and labeled with 15N, as well as 15N and 2H, at selected residues, respectively, and the heterodimer was formed by oxidation. CD spectroscopy indicates a high content of helical secondary structures when associated with POPC/POPG 3:1 vesicles or in membrane-mimetic environments. The propensity for helix formation follows the order heterodimer >chain 2 >chain 1, suggesting that peptide-peptide and peptide-lipid interactions both help in stabilizing this secondary structure. In a subsequent step the peptides were reconstituted into oriented phospholipid bilayers and investigated by 2H and proton-decoupled 15N solid-state NMR spectroscopy. Whereas chain 2 stably inserts into the membrane at orientations close to perfectly parallel to the membrane surface in the presence or absence of chain 1, the latter adopts a more tilted alignment, which further increases in the heterodimer. The data suggest that membrane interactions result in considerable conformational rearrangements of the heterodimer. Therefore, chain 2 stably anchors the heterodimer in the membrane, whereas chain 1 interacts more loosely with the bilayer. These structural observations are consistent with the antimicrobial activities when the individual chains are compared to the dimer.

Solution NMR structures of the antimicrobial peptides phylloseptin-1, -2, and -3 and biological activity: The role of charges and hydrogen bonding interactions in stabilizing helix conformations

Phylloseptins are antimicrobial peptides of 19-20 residues which are found in the skin secretions of the Phyllomedusa frogs that inhabit the tropical forests of South and Central Americas. The peptide sequences of PS-1, -2, and -3 carry an amidated C-terminus and they exhibit 74% sequence homology with major variations of only four residues close to the C-terminus. Here we investigated and compared the structures of the three phylloseptins in detail by CD- and two-dimensional NMR spectroscopies in the presence of phospholipid vesicles or in membrane-mimetic environments. Both CD and NMR spectroscopies reveal a high degree of helicity in the order PS-2> or =PS-1>PS-3, where the differences accumulate at the C-terminus. The conformational variations can be explained by taking into consideration electrostatic interactions of the negative ends of the helix dipoles with potentially cationic residues at positions 17 and 18. Whereas two are present in the sequence of PS-1 and -2 only one is present in PS-3. In conclusion, the antimicrobial phylloseptin peptides adopt alpha-helical conformations in membrane environments which are stabilized by electrostatic interactions of the helix dipole as well as other contributions such hydrophobic and capping interactions.

Structures, semi-empirical calculations and thermolyses of some five- and six-membered chelate organotin mercaptide complexes

Abstract Characterization of Ph 2 Sn(Cl)(MBT) ( 1 ) (HMBT=2-mercaptobenzothiazole) has been carried out by IR, Mossbauer, 1 H, 13 C and 119 Sn spectroscopies and by X-ray crystallography for 1 together with that of Ph 2 Sn(SCH 2 CH 2 S) ( 2 ). Compound 1 , unexpectedly obtained from the reaction between Ph 2 SnCl 2 and KMBT in a 1:2 mole ratio, has a trigonal-bipyramidal geometry, due to intramolecular SnN interactions, both in the solid state and in solution; the axial sites are occupied by N and Cl, N(24)SnCl(4)=155.27(17)°, the chelate bite angle, N(24)SNS(15), is 64.65(17)°. Compound 2 is essentially monomeric in the solid state and has a distorted tetrahedral structure; the bond angles at tin vary from 92.50(16)° [S(2)SnS(5)] to 116.4(6)° [C(6)SnC(12)]. The shortest intermolecular Sn⋯S contact in 2 is 3.885 A, just within the sum of the van der Waals’ radii for Sn and S (4.0 A). PM3 semi-empirical calculations for 2 indicated that the geometry at the tin center can be accounted for by a high degree of p-character in the tin bonding orbitals to sulfur; PM3 semi-empirical calculations on Ph 2 Sn(SCH 2 CH 2 CH 2 S) ( 3 ) indicated the geometry at tin to be less distorted from tetrahedral, with a SSnS angle of 99°; the calculations further indicated that the only stable conformation of the six-membered ring in 3 is the chair form.

Membrane interactions of phylloseptin-1, -2, and -3 peptides by oriented solid-state NMR spectroscopy.

Phylloseptin-1, -2, and -3 are three members of the family of linear cationic antimicrobial peptides found in tree frogs. The highly homologous peptides encompass 19 amino acids, and only differ in the amino acid composition and charge at the six most carboxy-terminal residues. Here, we investigated how such subtle changes are reflected in their membrane interactions and how these can be correlated to their biological activities. To this end, the three peptides were labeled with stable isotopes, reconstituted into oriented phospholipid bilayers, and their detailed topology determined by a combined approach using (2)H and (15)N solid-state NMR spectroscopy. Although phylloseptin-2 and -3 adopt perfect in-plane alignments, the tilt angle of phylloseptin-1 deviates by 8° probably to assure a more water exposed localization of the lysine-17 side chain. Furthermore, different azimuthal angles are observed, positioning the amphipathic helices of all three peptides with the charged residues well exposed to the water phase. Interestingly, our studies also reveal that two orientation-dependent (2)H quadrupolar splittings from methyl-deuterated alanines and one (15)N amide chemical shift are sufficient to unambiguously determine the topology of phylloseptin-1, where quadrupolar splittings close to the maximum impose the most stringent angular restraints. As a result of these studies, a strategy is proposed where the topology of a peptide structure can be determined accurately from the labeling with (15)N and (2)H isotopes of only a few amino acid residues.

The principles of infrared-x-ray pump-probe spectroscopy. Applications on proton transfer in core-ionized water dimers

In this paper we derive the basic physics underlying infrared-x-ray pump-probe spectroscopy (IR, infrared). Particular features of the spectroscopy are highlighted and discussed, such as dependence on phase of the infrared pulse, duration and delay time of the x-ray pulse, and molecular orientation. Numerical applications are carried out for the water dimer using wave packet techniques. It is shown that core ionization of the donor oxygen of the water dimer results in a drastic change of the potential with the global minimum placed in the proton transfer region. The results of the modeling indicate that IR-x-ray pump-probe spectroscopy can be used to study the dynamics of proton transfer in this core-ionized state, and that, contrary to conventional core level photoelectron spectroscopy, x-ray core-ionization driven by an IR field is a proper method to explore the proton transfer in a system like the water dimer. We observe that the trajectory of the nuclear wave packet in the ground state potential well is strongly affected by the absolute phase of the IR pulse.

Theoretical study of specific solvent effects on the optical and magnetic properties of copper(II) acetylacetonate.

Specific and basicity solvent effects on the visible near-infrared electronic transitions and the electron paramagnetic resonance (EPR) parameters of the copper(II) acetylacetonate complex, Cu(acac)2, have been investigated at the density functional theory level. The computed absorption transitions as well as the EPR parameters show a strong dependence on the direct coordination environment around the Cu(II) complex. High solvatocromic shifts are observed for 3d-3d transitions, with the highest effect observed for the dz(2)→dxy transition, which is red-shifted by 6000 cm(-1) and 9000 cm(-1) in water and pyridine solvent models, respectively. Compared to the electronic g-tensors, the hyperfine coupling constants of the Cu(acac)2 complex show a more pronounced dependence on the effect of base strength of solvent. Overall, the present methodology satisfactorily models the solvent effect on the optical and magnetic properties of the Cu(acac)2 complex, and theory and experiment agree sufficiently well to warrant the use of the computed optical and EPR parameters to elucidate the coordination environment of the Cu(II) systems in basic solutions.

X-ray structure of [Nb(η5-C5H5)2(SCH2CH2CH2S].: ECP quantum chemistry of niobium five and six metallacycle complexes

Abstract [Nb(η5-C5H5)2PDT] (1) (PDT=propane-1,3-dithiolate), η5-C5H5=Cp, in the chair form, was characterized by spectroscopic methods as well as by X-ray crystallography, that indicated a tetrahedral geometry around niobium. Quantum chemistry calculations were performed for 1, for the hypothetical complex containing the five member ring metallacycle [NbCp2EDT], (EDT=ethane-1,2-dithiolate), (2), as well for the cationic complex analog of 2, [NbCp2EDT]+, 2+. The data obtained indicated also a tetrahedral geometry around niobium in 2, with a SNbS angle of 82.1°. The five-membered ring adopts a twisted conformation, with the carbon atoms lying ±12.5° above and below the SNbS plane, resulting in the conformers λ and Δ, that are separated by an ROHF/ECP barrier of 8.6 kcal mol−1. For 2+, the SNbS angle was found to increase to 85.6°, in agreement with the fact that the single occupied SOMO orbital is expected to keep the LML′ angle to values close to 80°. For complex 1 the obtained theoretical results are in good agreement with parameters obtained from the X-ray crystallographic studies.

Síntese de 2-iodobenzamidas e 3-(iodoacetamido)benzamidas ligadas à D-galactose e suas reações de carbociclização radicalar mediadas por hidreto de tri-n-butilestanho

Starting from methyl 6-O-allyl-4-azido-2,3-di-O-benzyl-4-deoxy-α-D-galactopyranoside, four new derivatives containing 2-iodobenzamido and 3-(iodoacetamido)benzamido groups were synthesized. These four compounds were submitted to tri-n-butyltin hydride mediated radical cyclization reactions, resulting in two macrolactams from 11- and 15-endo aryl radical cyclization. The corresponding four hydrogenolysis products were also obtained. The structures of the new compounds were elucidated by 1H and 13C NMR spectroscopy, DEPT, COSY, HMQC and HMBC experiments.

tri-n-butyltin hydride-mediated radical reaction of a 2-iodobenzamide: formation of an unexpected carbon-tin bond

The tri-n-butyltin hydride-mediated reaction of methyl 2,3-di-O-benzyl-4-O-trans-cinnamyl -6-deoxy-6-(2-iodobenzoylamino)-a-D-galactopyranoside afforded an unexpected aryltributyltin compound. The structure of this new tetraorganotin(IV) product has been elucidated by 1H, 13C NMR spectroscopy, COSY and HMQC experiments and electrospray ionization mass spectrometry (ESI-MS). The formation of this new compound via a radical coupling reaction and a radical addition-elimination process is discussed.