A topographically and conformationally constrained, spin-labeled, alpha-amino acid: crystallographic characterization in peptides

2,2,6,6-Tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC) is a topographically and conformationally restricted, nitroxide containing, C(alpha)-tetrasubstituted alpha-amino acid. Here, we describe the molecular and crystal structures, as determined by X-ray diffraction analyses, of a TOAC terminally protected derivative, the cyclic dipeptide c(TOAC)(2).1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) solvate, and five TOAC-containing, terminally protected, linear peptides ranging in length from tetra- to hepta-peptides. Incipient and fully developed, regular or distorted 3(10)-helical structures are formed by the linear peptides. A detailed discussion on the average geometry and preferred conformation for the TOAC piperidine ring is also reported. The X-ray diffraction structure of an intramolecularly cyclized side product resulting from a C-activated TOAC residue has also been determined.

On the orange color of Z-Trp-ONPo

Out of all nitrophenyl esters of N(alpha)-protected alpha-amino acids Z-Trp-ONP(o) is the only one which is deep orange colored in the crystalline state. Any change in N(alpha)-protection, nature of amino acid, spatial separation between Trp and the ester-group or position of the nitro-substitutent in the aromatic ring of the ester function results in a loss of this characteristic property. We solved the molecular and crystal structure of Z-l-Trp-ONP(o) by X-ray diffraction analysis and investigated its color changes and visible (vis) and infrared (IR) absorption spectra in the solid state as a function of the amino acid derivative/KBr (w/w) ratio in the pellets. This investigation was extended to toluene solutions of different Z-Trp-ONP(o) concentrations by use of vis absorption and proton magnetic resonance spectroscopic techniques. The onset of the orange color correlates closely with the appearance of a concentration-dependent absorption band near 500 nm and concentration-dependent shifts of the urethane and indole NH proton resonances. Our observations can be explained by the formation of an intermolecular charge transfer complex involving the Trp indole and the -ONP(o) nitrophenyl as the donor and the acceptor moieties, respectively.

New tools for the control of peptide conformation: the helicogenic Cα-methyl, Cα-cyclohexylglycine*

The novel Calpha-tetrasubstituted alpha-amino acid Calpha-methyl, Calpha-cyclohexylglycine was prepared by hydrogenation of its Calpha-methyl, Calpha-phenylglycine precursor. Terminally protected homodi-, homotri-, and homotetrapeptides from Calpha-methyl, Calpha-cyclohexylglycine and co-oligopeptides to the pentamer level in combination with Gly or alpha-aminoisobutyric acid residues were prepared by solution methods and fully characterized. The results of a conformational analysis, performed by use of Fourier transform infrared (FT-IR) spectrophotometer absorption, 1H NMR, and X-ray diffraction techniques, support the contention that this Calpha-methylated, Cbeta-trisubstituted aliphatic alpha-amino acid is an effective beta-turn and 3(10)-helix inducer in tri- and longer peptides as its Calpha-methyl valine parent compound, but partially divergent from the corresponding aromatic Calpha-methyl, Calpha-diphenylmethylglycine residue, known to promote folded and fully extended structures to a significant extent in these oligomers.

Self-assembling and membrane modifying properties of a lipopeptaibol studied by CW-ESR and PELDOR spectroscopies

Trichogin GA IV is a short lipopeptaibol antibiotic that is capable of enhancing the transport of small cations through the phospholipid double layer of the membrane. The antibiotic activity of the undecapeptide is thought to be based on either its self-assembling or membrane-modifying property. The chemical equilibrium between self-aggregated and non-aggregated molecular states was studied by CW-ESR spectroscopy using solutions of TOAC nitroxide spin-labelled trichogin analogues in an apolar solvent to mimic the membrane bound state. At room temperature the two different sets of signals observed in the spectrum were attributed to the presence of both monomers and aggregates in the sample. The ESR spectra of the monomeric and aggregated forms were separated and the dependence of the fraction of monomeric peptide molecules on concentration was obtained over the range 5 x 10(-6) to 7 x 10(-4) M. A two-step aggregation mechanism is proposed: dimerization of peptide molecules followed by aggregation of dimers to assemblies of four peptide molecules per aggregate. The equilibrium constants were estimated for both steps. In addition, the lower lifetime limit was determined for dimers and tetramers. It is shown that when the peptide concentration exceeds 10(-5) M. the major part of the peptide molecules in solution has the form of tetrameric aggregates. Independently, the PELDOR technique was used to investigate the concentration dependence of the parameters of the dipole-dipole interaction between spin labels in frozen (77 K] glassy solutions of aggregates of mono-labelled TOAC analogues. The number of molecules in aggregates as well as the frequency and amplitude of PELDOR signal oscillations were found to be concentration independent in the range 5 x 10(-4) to 8 x 10(-3) M. In the frozen glassy solution state, the number of peptide molecules per aggregate was determined to be close to four, which is in agreement with the value obtained for spin-labelled trichogin at room temperature. The present data provide experimental evidence in favour of a self-assembling rather than a membrane-modifying ion conduction mechanism.

Control of peptide conformation by the Thorpe-Ingold effect (C alpha-tetrasubstitution)

The preferred conformations of peptides heavily based on the currently extensively exploited achiral and chiral alpha-amino acids with a quaternary alpha-carbon atom, as determined by conformational energy computations, crystal-state (x-ray diffraction) analyses, and solution ((1)H-NMR and spectroscopic) investigations, are reviewed. It is concluded that 3(10)/alpha-helical structures and the fully extended (C(5)) conformation are preferentially adopted by peptide sequences characterized by this family of amino acids, depending upon overall bulkiness and nature (e.g., whether acyclic or C(alpha) (i) <--> C(alpha) (i) cyclized) of their side chains. The intriguing relationship between alpha-carbon chirality and bend/helix handedness is also illustrated. gamma-Bends and semiextended conformations are rarely observed. Formation of beta-sheet structures is prevented.

C(alpha)-hydroxymethyl methionine: synthesis, optical resolution and crystal structure of its (+)-N(alpha)-benzoyl derivative

(R, S)-Methionine was transformed into C(alpha)-hydroxymethyl methionine by a route involving C(alpha)-hydroxymethylation of 2-phenyl-4-methylthioethyl-5-oxo-4,5-dihydro-1,3-oxazole. The absolute configuration of (-)-C(alpha)-hydroxymethyl methionine was elucidated to be (S) by chemical correlation with (S) (-)-C(alpha)-ethyl serine. Absolute structure determination (by single crystal X-ray diffraction) on N(alpha)-benzoyl-C(alpha)-hydroxymethyl methionine confirmed the (R)-configuration for the (+)-enantiomer. In addition, the X-ray diffraction analysis showed that the C(alpha,alpha)-disubstituted glycyl residue adopts the fully extended (C5) conformation.

An azacrown-functionalized peptide as a metal ion based catalyst for the cleavage of a RNA-model substrate

The previously synthesized, terminally blocked heptapeptide Ac-Aib-ATANP-Aib-Aib-ATANP-Aib-Aib-OMe (1a), where ATANP is (S)-2-amino-3-[1-(1,4,7-triazacyclononane)]propanoic acid and Aib is alpha-aminoisobutyric acid, which is soluble in neutral water where it largely adopts a 3(10)-helical conformation, has been studied, as bimetallic complex [metal ions: Cu(II), Ni(II), Zn(II)], for the transphosphorylation catalysis of the RNA-model substrate 2-(hydroxypropyl)-p-nitrophenyl phosphate (HPNP). A detailed analysis was carried out with the Zn(II) dinuclear complex. Comparison with the mononuclear Zn(II) complex with 1,4,7-triazacyclononane (3) points to cooperativity between the two Zn(II) ions in the process catalyzed by 1a-2Zn(II). On the contrary, the dinuclear Zn(II) complex of dipeptide Ac-(ATANP)(2)-OMe (2), lacking any ordered conformation, is less active than 3-Zn(II). The kinetic analysis suggests the following: (a) the peptide is conformationally very robust and does not loose activity up to 50 degrees C; (b) the substrate binds to the peptide-Zn(II) complex, although not all modes of complexation allow us to take advantage of the cooperativity between the two metal centers. The maximum rate acceleration estimated at pH 7 for the fully bound substrate is ca. 200-fold compared with the uncatalyzed process.

Interaction between TOAC free radical and photoexcited triplet chromophores linked to peptide templates

The intramolecular quenching of photoexcited triplet states by free radicals linked to peptide templates was studied by time-resolved electron paramagnetic resonance (EPR) with pulsed laser excitation. The systems investigated are 3(10)-helix forming peptides, having in the amino acid sequence the free radical 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC) and a triplet precursor, such as Bin, Bpa, or Trp, incorporated at different relative positions. Upon interaction with the excited triplet the TOAC radical spin sublevel populations assume values that differ from the Boltzmann equilibrium values. This spin polarization effect produces EPR lines in emission whose time evolution reflects the triplet quenching rate. In particular, in a series of peptides labeled with Bpa and TOAC at successive positions in the 3(10)-helix, radical-triplet interaction was observed in all cases. However, for the peptide where Bpa and TOAC are at positions 2 and 4 the rate of triplet quenching is lower than for the other peptides in the series. In addition, the radical-excited triplet complex in the quartet spin state was observed in a peptide containing fullerene (C(60)) as a triplet precursor and TOAC.

Electron spin resonance of TOAC labeled peptides: folding transitions and high frequency spectroscopy

The unnatural, conformationally constrained nitroxide amino acid TOAC (2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) stabilizes helical structure and provides a means for studying rigidly spin labeled peptides by electron spin resonance (ESR). Two new directions in TOAC research are described. The first investigates intermediates formed during alpha-helix unfolding. Double TOAC labeled alpha-helical peptides were unfolded at low temperature in aqueous solution with increasing concentrations of guanidine hydrochloride. Comparison of ESR spectra from two doubly labeled peptides suggests that 3(10)-helix emerges as an intermediate. The second research direction involves the use of high frequency ESR (140 GHz) at low temperature to assess dipolar couplings and, hence, distances between TOAC pairs in a series of 3(10)-helical peptides. Preliminary simulations suggest that high frequency ESR is able to extract correct distances between 6 and 11 A. In addition, the spectra appear to be very sensitive to the relative orientation of the TOAC labels.

Partial [alphaMe]Aun scan of [l-Leu11-OMe]-trichogin GA IV, a membrane active synthetic precursor of the natural lipopeptaibol

We synthesized using solution-phase methods three analogs of [l-Leu11-OMe] trichogin GA IV, a membrane active synthetic precursor of the lipopeptaibol antibiotic in which the N-terminal n-octanoyl group and each of the three Aib residues in positions 1, 4 and 8 are replaced by an acetyl group and the lipophilic Calpha,alpha-disubstituted glycine l-(alphaMe)Aun, respectively [partial (alphaMe)Aun scan]. FT-IR absorption and CD analyses unequivocally show that the main three-dimensional structural features of [l-Leu11-OMe] trichogin GA IV are preserved in the analogs. Also, [l-Leu11-OMe] trichogin GA IV and the three Nalpha-acetylated l-(alphaMe)Aun analogs exhibit strictly comparable membrane-modifying properties. Taken together, these results strongly favor the conclusion that a shift of the long hydrocarbon moiety from the Nalpha-blocking group to the side-chain of the 1, 4 or 8 residue does not have any significant effect on the conformational properties or the membrane activity of [l-Leu11-OMe] trichogin GA IV and, by extension, of the natural lipopeptaibol.

Lipopeptaibols, a novel family of membrane active, antimicrobial peptides

Lipopeptaibols are members of a novel group of naturally occurring, short peptides with antimicrobial activity, characterized by a lipophilic acyl chain at the N-terminus, a high content of the turn/helix forming alpha-aminoisobutyric acid and a 1,2-amino alcohol at the C-terminus. The amino acid sequences range from 6 to 10 residues and the fatty acyl moieties from 8 to 15 carbon atoms. The peptide portion of lipopeptaibols can be shorter than those of the nonlipidated peptaibols that range from 10 to 19 amino acid residues. The longest peptides fold into a mixed 3(10)/alpha helix, whereas the shortest peptides tend to adopt a beta-turn/sheet structure. Using solution methodologies, a series of analogues of trichogin GA IV was synthesized which allowed determination of the minimal lipid chain and peptide main-chain lengths for the onset of membrane activity and exploitation of a number of spectroscopic techniques aimed at determining its preferred conformation under a variety of conditions and investigating in detail its mode of interaction with, and its effect on, the phospholipid membranes.

Solution structure, dimerization, and dynamics of a lipophilic alpha/3(10)-helical, C alpha-methylated peptide. Implications for folding of membrane proteins

The solution structure and the dimerization behavior of the lipophilic, highly C(alpha)-methylated model peptide, mBrBz-Iva(1)-Val(2)-Iva(3)-(alphaMe)Val(4)-(alphaMe)Phe(5)-(alphaMe)Val(6)-Iva(7)-NHMe, was studied by NMR spectroscopy and molecular dynamics simulations. The conformational analysis resulted in a right-handed 3(10)/alpha-helical equilibrium fast on the NMR time scale with a slight preference for the alpha-helical conformation. The NOESY spectrum showed intermolecular NOEs due to an aggregation of the heptapeptide. In addition, temperature-dependent diffusion measurements were performed to calculate the hydrodynamic radius. All these findings are consistent with an antiparallel side-by-side dimerization. The structure of the dimeric peptide was calculated with a simulated annealing strategy. The lipophilic dimer is held together by favorable van der Waals interactions in the sense of a bulge fitting into a groove. The flexibility of the helical conformations concerning an alpha/3(10)-helical equilibrium is shown in a 3 ns molecular dynamics simulation of the resulting dimeric structure. Both overall helical structures of each monomer and the antiparallel mode of dimerization are stable. However, transitions were seen of several residues from a 3(10)-helical into an alpha-helical conformation and vice versa. Hence, this peptide represents a good model in which two often-discussed aspects of hierarchical transmembrane protein folding are present: i <-- i + 3 and i <-- i + 4 local H-bonding interactions cause a specific molecular shape which is then recognized as attractive by other surrounding structures.

The crystal structure of the 1:1 inclusion complex of beta-cyclodextrin with squaric acid

The crystal and molecular structure of the 1:1 inclusion complex of beta-cyclodextrin (cyclomaltoheptaose) with squaric acid (3,4-dihydroxycyclobutene-1,2-dione) was determined by X-ray diffraction. The complex crystallizes in the monoclinic P2(1) space group and belongs to the monomeric cage-type, characterized by a herringbone-like packing motif. Co-crystallized water molecules are present on seven sites, of which six are fully occupied. The guest molecule is placed inside the beta-cyclodextrin cavity, perpendicular to the plane defined by the glycosidic O-4n atoms, and held in place by direct and water-mediated hydrogen bonds mainly involving symmetry-related beta-cyclodextrin molecules. The accommodation of the planar guest molecule into the beta-cyclodextrin cavity determines a significant distortion of the latter from the sevenfold symmetry.

Crystal structure of a synthetic cyclodecapeptide for template-assembled synthetic protein design

The structural prototype of a new generation of regioselectively addressable functionalized templates (RAFTs) for use in protein de novo design has been synthesized and crystallized. The structure of the aromatically substituted cyclodecapeptide was determined by X-ray diffraction; it consists of an antiparallel beta sheet spanned by heterochirally induced type IIprime prime or minute beta turns, similar to that observed in gramicidin S. The three-dimensional structure of the artificial template was also examined by an NMR spectroscopic analysis in solution and shown to be compatible with a beta-sheet plane suitable for accommodating secondary functional peptide fragments for the synthesis of template-assembled synthetic proteins (TASPs).

The secondary structure of a membrane-modifying peptide in a supramolecular assembly studied by PELDOR and CW-ESR spectroscopies

The new technique of pulsed electron-electron double resonance in electron spin-echo (PELDOR) in combination with the CW-ESR method has been used to investigate the secondary structure of a double spin-labeled peptide (the [TOAC-1,8]-analogue of the peptaibol antibiotic trichogin GA IV) that is hidden into a tetrameric supramolecular assembly of unlabeled peptide molecules. The magnetic dipole-dipole relaxation of spin labels has been experimentally studied in glassy solutions of the double-labeled peptide frozen to 77 K in a mixture of chloroform-toluene with an excess of unlabeled peptide. The PELDOR signal oscillations have been observed at high degrees of dilution with unlabeled peptide. The intramolecular distance between the spin labels of the peptide molecule in the aggregate has been determined from the oscillation frequency to be 15.7 A which is close to the value of (approximately equal to) 14 A calculated for a 3(10)-helical structure. Estimation of the fraction of this ordered secondary structure shows that about 19% of the peptide molecules in aggregates are folded in the 3(10)-helical conformation. The present experimental results are consistent with our molecular model presented in J. Am. Chem. Soc. 2000, 122, 3843-3848, wherein four amphiphilic 3(10)-helical peptide molecules form a vesicular system with the polar amino acid side chains pointing to the interior, and the apolar side chains, to the exterior of the cluster. The experimental data were compared with the results obtained with other techniques.

Ac10c: a medium-ring, cycloaliphatic Calpha,alpha-disubstituted glycine. Incorporation into model peptides and preferred conformation

Two complete series of N-protected oligopeptide esters to the pentamer level from 1-amino-cyclodecane-1-carboxylic acid (Ac10c), an alpha-amino acid conformationally constrained through a medium-ring Calphai <--> Calphai cyclization, and either the L-Ala or Aib residue, along with the N-protected Ac10c monomer and homo-dimer alkylamides, were synthesized using solution methods and fully characterized. The preferred conformation of these model peptides was assessed in deuterochloroform solution using FT-IR absorption and 1H NMR techniques. Furthermore, the molecular structures of two derivatives (Z-Ac10c-OH and Fmoc-Ac10c-OH) and two peptides (the dipeptide ester Z-Ac10c-L-Phe-OMe and the tripeptide ester Z-Aib-Ac10c-Aib-OtBu) were determined in the crystal state using X-ray diffraction. The experimental results support the view that beta-bends and 3(10)-helices are preferentially adopted by peptides rich in Ac10c, the third largest cycloaliphatic C(alpha,alpha)-disubstituted glycine known. This investigation allowed us to complete a detailed conformational analysis of the whole 1-amino-cycloalkane-1-carboxylic acid (Ac(n)c, with n = 3-12) series, which represents the prerequisite for our recent proposal of the 'Ac(n)c scan' concept.

Analogs of the antimicrobial peptide trichogin having opposite membrane properties

Four analogs of the antimicrobial peptide trichogin GA IV were studied. Their sequences are as follows: GT, n-octanoyl-Aib-Gly-Leu-Aib-Gly-Gly-Leu-Aib-Gly-Ile-Leu-OMe; ST, n-octanoyl-Aib-Ser-Leu-Aib-Ser-Ser-Leu-Aib-Ser-Ile-Leu-OMe; BT, n-octanoyl-Aib-Ser(tBu)-Leu-Aib-Ser(tBu)-Ser(tBu)-Leu-Aib-Ser(tBu)-Ile-Leu-OMe; and DT, n-octanoyl-Aib-Ser(tBu)-Leu-Aib-Ser(tBu)-Ser(tBu)-Leu-Aib-Ser(tBu)-Ile-Leu-Aib-Ser(tBu)-Leu-Aib-Ser(tBu)-Ser(tBu)-Leu-Aib-Ser(tBu)-Ile-Leu-OMe. The trichogin GA IV differs from GT only in the nature of the C-terminal residue, being a 1,2 aminoalcohol (leucinol) in the case of the parent peptide. Compared with GT, ST has an increased amphiphilicity. In contrast, BT has little amphiphilicity being composed only of hydrophobic amino acids. DT is an octanoylated head-to-tail dimer of BT. We show that BT and DT lower the bilayer-to-hexagonal phase transition temperature (T(H)) of dipalmitoleoylphosphatidylethanolamine, indicating that the peptides promote negative curvature. These two peptides, composed of only hydrophobic amino acids, have their bulkier groups on one face of the helix, suggesting that they may penetrate membranes at an oblique angle. In contrast, GT and ST, like trichogin itself, increase TH, promoting positive curvature. These peptides have contrasting membrane lytic activities. Whereas DT and BT did not produce leakage of aqueous contents, GT and ST, like trichogin, did cause rapid leakage. The leakage activity with liposomes also correlates with the greater potency of GT and ST, compared with the hydrophobic analogs, in their hemolytic and bacteriostatic action. ST has greater lytic ability than GT in liposomal leakage as well as hemolysis. We also measured the rate of peptide-promoted lipid mixing as an indication of membrane fusion. BT produced lipid mixing only with large unilamellar vesicles enriched with dioleoylphosphatidylethanolamine; ST did not produce lipid mixing, as its apparent reduction of energy transfer proved to be artifactual. Quasi-elastic light scattering of large unilamellar vesicles was also carried out after adding ST and BT. Peptide BT, but not ST, was able to aggregate large unilamellar vesicles. Thus, one of the properties of BT that leads to the induction of lipid mixing is that it is able to aggregate vesicles, placing the bilayers in juxtaposition. Thus, the two pairs of peptides, BT and DT vs GT and ST, exhibit contrasting behaviour with respect to a number of membrane biophysical properties. This occurs despite the fact that the chemical structures of the peptides are rather similar. Such distinct behavior is also reflected in their hemolytic and bacteriostatic actions.

Disaccharide modulation of the mitochondrial membrane fluidity changes induced by the membrane potential

The influence of the medium composition on the dynamic properties of mitochondrial membranes on depolarization was studied by following the fluorescence anisotropy changes of mitochondria-bound 1,6-diphenyl-1,3,5-hexatriene (DPH) and hematoporphyrin (HP) as reporters, respectively, of lipid and protein regions. On collapse of the potential, the membrane fluidity increased in NaCl-, KCl-, and monosaccharide-based media and decreased in disaccharides. Infrared spectroscopy experiments suggested that disaccharides likely change water's structure and association on the membrane surface. These results indicate that disaccharides induce membrane perturbation, which may interfere in the study of structure-function correlation in biological membranes.

The first water-soluble 3(10)-helical peptides

Two water-soluble 3(10)-helical peptides are synthesized and fully characterized for the first time. The sequence of these terminally blocked heptamers comprises two residues of the Calpha-trisubstituted alpha-amino acid 2-amino-3-[1-(1,4,7-triazacyclononyl)]propanoic acid and five residues of a Calpha-tetrasubstituted alpha-amino acid (either alpha-aminoisobutyric acid or isovaline). Using CD and NMR techniques we were able to show that both heptapeptides are well structured in water, and that the type of conformation adopted is indeed the ternary 3(10)-helix.

Preferred conformation of peptides based on cycloaliphatic C(alpha,alpha)-disubstituted glycines: 1-amino-cycloundecane-1-carboxylic acid (Ac11c)

Two complete series of N-protected oligopeptide esters to the pentamer level from 1-amino-cycloundecane-1-carboxylic acid (Ac11c), an alpha-amino acid conformationally constrained through a medium-ring C(i)alpha<-->C(i)alpha cyclization, and either the L-Ala or Aib residue, along with the N-protected Ac11c monomer and homo-dimer alkylamides, have been synthesized by solution methods and fully characterized. The preferred conformation of these model peptides has been assessed in deuterochloroform solution by FT-IR absorption and 1H-NMR techniques. Furthermore, the molecular structures of one derivative (Z-Ac11c-OH) and two peptides (the tripeptide ester Z-Aib-Ac11c-Aib-OtBu and the pentapeptide ester Z-Ac11c-(Aib)2-Ac11c-Aib-OtBu) have been determined in the crystal state by X-ray diffraction. The experimental results support the view that beta-bends and 3(10)-helices are preferentially adopted by peptides rich in Ac11c, the second largest cycloaliphatic C(alpha,alpha)-disubstituted glycine known. This investigation has allowed the authors to approach the completion of a detailed conformational analysis of the whole 1-amino-cycloalkane-1-carboxylic acid (Ac(n)c, with n = 3-12) series, which represents the prerequisite for their recent proposal of the 'Ac(n)c scan' concept.

(alphaMe)Nva: stereoselective syntheses and preferred conformations of selected model peptides

Using different stereoselective chemical and chemoenzymatic approaches we synthesized the chiral, Calpha-methylated alpha-amino acid L-(alphaMe)Nva with a short, linear side-chain. A set of terminally protected model peptides to the pentamer level containing either (alphaMe)Nva or Nva in combination with Ala and/or Aib was prepared using solution methods and characterized fully. Two (alphaMe)Nva peptides were also synthesized using side-chain hydrogenation of the corresponding Calpha-methyl, Calpha-allylglycine (Mag) peptides. A detailed solution and crystal-state conformational analysis based on FT-IR absorption, 1H NMR and X-ray diffraction techniques allowed us to define that: (i) (alphaMe)Nva is an effective beta-turn and 3(10)-helix former; and (ii) the relationship between (alphaMe)Nva chirality and the screw sense of the turn/helix formed is that typical of protein amino acids, i.e. L-(alphaMe)Nva induces the preferential formation of right-handed folded structures. In more general terms, this study reinforced previous conclusions that peptides based on alpha-amino acids with a Calpha-methyl substituent and a Calpha-linear alkyl substituent are characterized by a strong tendency to fold into turn and helical structures.

CIDEP effects of intramolecular quenching of singlet and triplet excited states by nitroxide radicals in oligopeptides: a potentially useful new method for investigating peptide secondary structures in solution

Two hexapeptides, each bearing one photoactive alpha-amino acid (Bin or Bpa) and one nitroxide-containing TOAC residue, have been synthesized and fully characterized. FT-IR absorption measurements indicate that a 3(10)-helical conformation is adopted by these peptides in solution. As two amino acid units separate the photoactive residue from TOAC in the peptide sequences, the two moieties face each other at a distance of about 6 A after one complete turn of the ternary helix. Irradiation by a light pulse from an excimer laser populates the excited states localized on the chromophores. An intramolecular interaction between the singlet (Bin) or triplet (Bin and Bpa) excited states and the doublet state of the TOAC nitroxide makes a spin-selective decay pathway possible, that produces transient spin polarization. In addition, in order to determine whether the intramolecular exchange interaction occurs through-bond or through-space, we have prepared linear and cyclic TOAC-Bin dipeptide units. A CIDEP study revealed that a through-space intramolecular interaction is operative. The observation of spin polarization makes the two helical hexapeptides suitable models to test the possibility of application of this novel technique to conformational studies of peptides in solution.

(alphaMe)Aun: a highly lipophilic, chiral, Calpha-tetrasubstituted alpha-amino acid. Incorporation into model peptides and preferred conformation

Using a chemo-enzymatic approach we prepared the highly lipophilic, chiral, Calpha-methylated alpha-amino acid (alphaMe)Aun. Two series of terminally protected model peptides containing either D-(alphaMe)Aun in combination with Aib or L-(alphaMe)Aun in combination with Gly were synthesized using solution methods and fully characterized. A detailed solution conformational analysis, based on FT-IR absorption, 1H NMR and CD techniques, allowed us to determine the preferred conformation of this amino acid and the relationship between chirality at its alpha-carbon atom and screw sense of the helix that is formed. The results obtained strongly support the view that D-(alphaMe)Aun favors the formation of the left-handed 3(10)-helical conformation.

Conformational restriction through C alpha i <--> C alpha i cyclization: Ac12c, the largest cycloaliphatic C alpha,alpha- disubstituted glycine known

Two complete series of N-protected, monodispersed oligopeptide esters to the pentamer level from 1-aminocyclododecane-1-carboxylic acid (Ac(12)c), an alpha-amino acid conformationally constrained through C(alpha)(i) <--> C(alpha)(i) cyclization, and either L-Ala or Aib residues, along with the N-protected Ac(12)c homopeptide alkylamide series from monomer to trimer, have been synthesized by solution methods and fully characterized. The solution-preferred conformations of these peptides have been assessed by Fourier transform ir absorption and (1)H-nmr techniques. Moreover, the molecular structures of one derivative (Z-Ac(12)c-OH) and three peptides [the tripeptide ester Z-L-Ala-Ac(12)c-L-Ala-OMe, the tripeptide alkylamide Z-(Ac(12)c)(3)-NHiPr, and the tetrapeptide ester Z-(Aib)(2)-Ac(12)c-Aib-OtBu (Aib, alpha-aminoisobutyric acid)] have been determined in the crystal state by x-ray diffraction. The results obtained point to the conclusion that beta-bends and 3(10)-helices are preferentially adopted by peptides based on Ac(12)c, the largest cycloaliphatic C-disubstituted glycine known. A comparison with the structural tendencies extracted from published works on peptides from Aib, the prototype of C-disubstituted glycines, and the other extensively studied members of the class of 1-aminocycloalkane-1-carboxylic acids (Ac(n) c, with n = 3-9), is made and the implications for the use of the Ac(12)c residue in the Ac(n) c scan approach of conformationally restricted analogues of bioactive peptides are briefly discussed.

Preferred solution conformation of peptides rich in the lipophilic, chiral, C(alpha)-methylated alpha-amino acid (alpha Me)Aoc

The lipophilic, chiral, C(alpha)-methylated alpha-amino acid L-(alphaMe)Aoc (2-methyl-2-amino-octanoic acid) was prepared using a chemo-enzymatic approach. Two series of terminally protected model peptides, from dimer through to hexamer, containing L-(alphaMe)Aoc in combination with either Gly or Aib, were synthesized by solution methods and were fully characterized. A solution conformational analysis, based on FT-IR absorption, 1H-NMR and circular dichroism (CD) techniques, was performed with the aim at determining the preferred conformation of this novel amino acid and the relationship between chirality at its alpha-carbon atom and screw sense of the helix that is formed. The results obtained strongly support the view that L-(alphaMe)Aoc favours the formation of the right-handed 3(10)-helical conformation.

The antimicrobial peptide trichogin and its interaction with phospholipid membranes

The interaction of the antimicrobial peptide trichogin GA IV with phospholipid bilayers has been studied. A series of analogs of trichogin was synthesized in which the nitroxide spin label, 4-amino-4-carboxy-2,2,6,6-tetramethylpiperidino-1-oxyl (TOAC), replaced one of the three alpha-aminoisobutyric acid (Aib) residues in the sequence. These modified peptides were used to assess the location of different residues of the peptide in a phospholipid bilayer composed of egg phosphatidylcholine containing 0.4 mol% of a fluorescently labelled phospholipid. We demonstrate that the substitution of Aib residues with TOAC does not alter the manner in which the peptide affects membrane curvature or induces vesicle leakage. The proximity of the nitroxide group on the peptide to the 4,4-difluoro-4-bora-3a,4a-diaza-S-indacene (BODIPY) fluorophore attached to the phospholipid was estimated from the extent of quenching of the fluorescence. By this criterion it was concluded that the peptide penetrates into the bilayer and that Aib4 is the most deeply inserted of the Aib residues. The results suggest that the helix axis of the peptide is oriented along the plane of the membrane. All of the peptides were shown to raise the bilayer to the hexagonal phase transition temperature of dipalmitoleoylphosphatidylethanolamine, indicating that they promote positive membrane curvature. This is a property observed with peptides that do not penetrate deeply into the bilayer or are oriented along the bilayer normal. We also demonstrate trichogin-promoted leakage of the aqueous contents of liposomes. These results indicate that the peptides cause bilayer destabilization. The extent of leakage induced by trichogin is very sensitive to the peptide to lipid ratio over a narrow range.

Orientation and immersion depth of a helical lipopeptaibol in membranes using TOAC as an ESR probe

Trichogin GA IV is a lipopeptaibol antibiotic characterized by the sequence nOct-Aib1-Gly-Leu-Aib4-Gly-Gly-Leu-Aib8-Gly-Ile- Lol (nOct: n-octanoyl; Aib: alpha-aminoisobutyric acid; Lol, leucinol), which exhibits membrane-modifying properties. We synthesized step-by-step by solution methods three trichogin analogues, each with a single Aib --> 2,2,6,6-tetramethylpiperidin-1-oxyl-4-amino-4-carboxylic acid (TOAC) substitution. The similarity in the conformational propensities of the Calpha-tetrasubstituted alpha-amino acids Aib and TOAC allowed us to exploit these analogues to investigate the orientation and therefore the mechanism of action of trichogin in the membranes by the electron spin resonance (ESR) technique. A conformational analysis by Fourier transform ir absorption and CD in different organic solvents and in a membrane-mimetic environment indicated that the conformation of the natural lipopeptaibol remains almost unchanged in the three analogues. Moreover, for all of the analogues permeability measurements revealed membrane-modifying properties comparable to those of trichogin. Our ESR investigation demonstrated that, in liposomes based on phosphatidylcholine, trichogin lays parallel to the membrane surface with its hydrophobic face oriented toward the membrane interior. These results suggest that trichogin might modify membrane permeability via a carpet-like mechanism, at least in liposomes and in the absence of a transmembrane potential.

Synthesis, conformational study, and spectroscopic characterization of the cyclic C alpha, alpha-disubstituted glycine 9-amino-9-fluorenecarboxylic acid

A series of terminally blocked peptides (to the pentamer level) from L-Ala and the cyclic C alpha, alpha-disubstituted Gly residue Afc and one Gly/Afc dipeptide have been synthesized by solution method and fully characterized. The molecular structure of the amino acid derivative Boc-Afc-OMe and the dipeptide Boc-Afc-Gly-OMe were determined in the crystal state by X-ray diffraction. In addition, the preferred conformation of all of the model peptides was assessed in deuterochloroform solution by FT-IR absorption and 1H-NMR. The experimental data favour the conclusion that the Afc residue tends to adopt either the fully-extended (C5) or a folded/helical structure. In particular, the former conformation is highly populated in solution and is also that found in the crystal state in the two compounds investigated. A comparison with the structural propensities of the strictly related C alpha, alpha-disubstituted Gly residues Ac5c and D phi g is made and the implications for the use of the Afc residue in conformationally constrained analogues of bioactive peptides are briefly examined. A spectroscopic (UV absorption, fluorescence, CD) characterization of this novel aromatic C alpha, alpha-disubstituted Gly residue is also reported.

Total synthesis and membrane modifying properties of the lipopeptaibol trikoningin KB II and its analogues with acyl chains of different length at the N- and C-termini

Trikoningin KB II, a ten-amino acid residue lipopeptaibol blocked at the N-terminus by the n-octanoyl group and at the C-terminus by the 1,2-amino alcohol L-leucinol, and extracted from the fungus Trichoderma koningii, exhibits membrane-modifying properties. We have synthesized by solution-phase methods trikoningin KB II and several analogues with acyl chains of different length at the N- and C-termini. Permeability measurements showed that an appropriate length of the linear acyl chain is a more important characteristic for the onset of significant membrane-modifying activity than its position in the peptide chain.

Determining the occurrence of a 3(10)-helix and an alpha-helix in two different segments of a lipopeptaibol antibiotic using TOAC, a nitroxide spin-labeled C(alpha)-tetrasubstituted alpha-aminoacid

Trichogin GA IV is a 11-residue lipopeptaibol antibiotic exhibiting membrane modifying properties. We synthesized step-by-step by solution methods three trichogin analogues, each with a double Aib (alpha-aminoisobutyric acid)-->TOAC (2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) replacement. The strict similarity in the conformational propensities of Aib and TOAC allowed us to exploit these analogues in a detailed investigation of the conformation of this lipopeptaibol in different organic solvents and in a membrane-mimetic environment using in particular the double spin labeling ESR technique. We conclude that the secondary structure in solution remains essentially unchanged if compared to that previously found in the crystal state for trichogin. More specifically, the N-terminal region of the peptide folds in a 3(10)-helix, while the central and C-terminal regions are mainly alpha-helical. An additional, significant proof for the modest plasticity of the trichogin structure was obtained by an X-ray diffraction analysis of the nOct-[TOAC4,8, Leu-OMe11] analogue. For the three analogues permeability measurements revealed membrane-modifying properties comparable to those of natural trichogin.

Solution conformational analysis of amphiphilic helical, synthetic analogs of the lipopeptaibol trichogin GA IV

The step-by-step synthesis by solution methods of the [Ser2,5,6,9, Leu-OMe11] analog of trichogin GA IV is described. The four Ser residues have been incorporated into the sequence as replacements of the naturally occurring Gly residues to increase the amphiphilicity of the 3D-structure of the lipopeptaibol. A detailed solution conformational analysis has been performed on this undecapeptide and its prototypical [Leu-OMe11] trichogin GA IV analog using FTIR absorption and CD spectroscopies, and two-dimensional NMR under a variety of experimental conditions, including a membrane-mimetic environment. Both peptides adopt a mixed 3(10)/alpha-helical structure, which in the micellar system was found to be less flexible for the Ser-containing analog. For both analogs permeability measurements revealed membrane-modifying properties comparable to those of the natural lipopeptaibol.

Conformation and membrane activity of an analogue of the peptaibol antibiotic trichogin GA IV with a lipophilic amino acid at the N-terminus

We have synthesized by solution-phase methods two analogues of the 11-residue lipopeptaibol antibiotic trichogin GA IV in which the N-terminal n-octanoyl group is replaced either by an N-acetylated 2-amino-2-methyl-L-undecanoic acid or by an N-acetylated alpha-aminoisobutyric acid. CD, FTIR absorption. and NMR analyses unequivocally show that the main structural features of trichogin GA IV are preserved in these analogues. Since only the peptide containing the lipophilic chain exhibits membrane-modifying properties, these results strongly support the view that moving the long acyl moiety from the Nalpha-blocking group to the side chain of the N-terminal extra-residue does not affect the conformational properties or the membrane activity of trichogin GA IV.