Cyclic Tetrapeptide Cyclo Research Articles

Alpha Helix Nucleation by a Simple Cyclic Tetrapeptide

The simple cyclic tetrapeptide cyclo-(1,4)-[Ala-Arg-Ala-homoGlu]-NH2 (3) is shown to adopt an unusual α-turn structure, which is not α-helical but can nucleate α-helicity when attached to the N-terminus of either model peptides or two biologically relevant peptides. This new N-terminal helix-capping macrocycle provides very simple and rapid synthetic access to α-helical peptide structures.

HIV testing and sexual risk reduction counseling in office-based buprenorphine/naloxone treatment

antagonist activity would prevent both stress and drug-induced reinstatement of extinguished cocaine-seeking behavior. Methods: In mice, we tested two putative mixed opioid agonist/antagonists, (−)pentazocine and the novel cyclic tetrapeptide cyclo[Ala-d-Pro-Phe-d-Trp] (Cmpd 1), for opioid efficacy and selectivity in the 55 ◦C warm-water tail-withdrawal assay, and then determined both compounds effects on the reinstatement of cocaine in a conditioned place preference (CPP) assay. Results: (−)Pentazocine and Cmpd 1 demonstrated equivalent efficacy in the tail-withdrawal assaywith ED50 (and 95% CI) values of 2.51(1.66–4.27) and 3.03(2.16–4.58) nmol, i.c.v., respectively. However, whereas (−)pentazocine antinociception was mediated by all three opioid receptors, the antinociception of Cmpd 1 appeared to be primarily mediated by KOR, with a small contribution by MOR. Pretreatment (10 nmol i.c.v., −3h) with Cmpd 1 selectively antagonized KOR, whereas (−)pentazocine primarily antagonized DOR. Surprisingly, while pretreatment (−3h/d, i.c.v.) with either compound dose-dependently prevented a 2-day stressinduced reinstatement, acute administration of Cmpd 1 but not (−)pentazocine blocked cocaine-induced reinstatement. Conclusions: Overall, these data suggest that with its distinct activity profile, compound 1 is a promising lead compound for potential development, particularly as a therapeutic to prevent relapse to drug seeking behavior in abstinent subjects. Financial support: Funding provided by NIDA (DA023924 and DA032928) and the State of Florida, Executive Office of the Governor’s Office of Tourism, Trade, and Economic Development.

A lead (II) 3D coordination polymer based on a marine cyclic peptide motif.

The crystal structure of a naturally occurring cyclic tetrapeptide cyclo(Gly-l-Ser-l-Pro-l-Glu) [cyclo(GSPE)] was obtained. The conformation of synthesized cyclo(GSPE) fixes the coordination to lead ion in a 1:1 ratio. This cyclo(GSPE)-Pb complex was constructed as an asymmetric 3D network in the crystalline state. The polymerization of a heavy metal ion with a rigid asymmetric cyclic tetrapeptide represents the first example of a new class of macrocyclic complexes.

Conformation and Dynamics of a Cyclic Disulfide-Bridged Peptide: Effects of Temperature and Solvent

The cyclic disulfide-bridged tetrapeptide cyclo(Boc-Cys-Pro-Gly-Cys-OMe) (1) was designed as a model for the study of solvent-driven conformational changes in peptides. The three-dimensional structure and dynamics of 1 were studied using a variety of experimental and computational techniques. The crystal structure of 1 reveals a β-turn stabilized by a hydrogen bond between the two cysteine residues. In solution, the UV-CD and NMR analysis of 1 suggest a β-turn II conformation, stable up to 60 °C. The characteristic NMR (13)C shifts of the Cβ and Cγ atoms of proline show that the peptide adopts exclusively the energetically favored trans conformation of the peptidyl-prolyl bond. The combination of IR spectroscopy with Car-Parrinello MD simulations and DFT calculations allowed us to assign the absorptions in the amide I region to the individual amino acids. The NH group of Gly, which as hydrogen bond donor competes with the NH group of Cys4 for the carbonyl oxygen atom of Cys1 as hydrogen bond acceptor, plays a relevant role for the structure and spectroscopic properties of the peptide. Since Gly is more exposed to the solvent, its hydrogen-bonding capability can be partially blocked by external solvent molecules in solution or by a second peptide molecule in the crystal. Furthermore, the presence of only one molecule of acetonitrile is sufficient to change the preferred conformation of 1, and even in acetonitrile solution the simulations suggest that on average only one solvent molecule strongly interacts with the cyclic core of the peptide.

Exploring Solution-Phase Cyclization and Sulfamyl Safety-Catch Resin Strategies for the Total Synthesis of the Marine Antimicrobial Cyclic Tetrapeptide Cyclo(GSPE)

Head-to-tail cyclization of 12-membered tetrapeptides has been reported to be synthetically challenging due to their highly strained ring systems. Previously, we reported the total synthesis of the marine antimicrobial cyclic tetrapeptide cyclo(GSPE) using the glutamic acid side-chain Wang resin tethering strategy with a 5 % yield under microwave irradiation (Lim et al., Int J Pept Res Ther 16:145–152, 2010). Besides the low yield, this strategy was also constrained to glutamic acid-containing peptides. In this communication, we report the outcomes of two alternate strategies to synthesize cyclo(GSPE): (A) solution-phase cyclization and (B) solid-phase synthesis using the sulfamylbutyryl safety-catch resin. Experimental results using all four possible linear precursors revealed that the former strategy failed to produce the target cyclic peptide while the latter approach afforded an improved overall yield of 8–9 % using the linear precursor H-Ser(tBu)-Pro-Glu(OtBu)-Gly-resin. Interestingly, we discovered that the placement of proline at the N- or C-termini of the linear peptide precursors failed to produce the target cyclic peptide. We also concluded that strategy B could provide the versatility needed to make a cyclic tetrapeptide library for bioactivity screening.

Synthesis of strained cyclic peptides via an aza-Michael–acyl-transfer reaction cascade

A new method is presented to prepare strained lactams. Esterification of the C-terminus of a dipeptide with β-nitrostyrene or quinoline-type auxiliaries is followed by lactam formation by an intramolecular aza-Michael-acyl-transfer reaction cascade. Ultimately, the cyclic tetrapeptide cyclo[Phe-Tyr-Ala-Gly] has been prepared.

Solid-Phase Synthesis and NMR Structural Studies of the Marine Antibacterial Cyclic Tetrapeptide: Cyclo[GSPE

Twelve-membered head-to-tail cyclic tetrapeptides (CTPs) are rigid molecules found in nature and possess a diverse range of biological activities. A possible reason may be due to their ability to adopt rigid conformations in solution mimicking reverse-turns. Reverse-turns are common structural motifs which serve as molecular recognition sites in many protein-receptor interactions. In this paper, we describe the solid-phase synthesis of the antibacterial cyclic tetrapeptide cyclo[Gly-Ser-Pro-Glu] (cyclo[GSPE]), first isolated from the Ruegeria strain of marine bacteria by Mitova et al. (J Nat Prod 67:1178–1181, 2004). Our NMR experiments in H2O:D2O:DMSO (18:1:1) revealed that it possessed three conformations in an approximate ratio of 4:2:1 based on NMR amide peak intensities. 2D NMR studies and computer calculations revealed that the major conformer adopted a reverse-turn conformation and have ω torsion angles twisted by up to 2°, with two transoid amide bonds between Gly-Ser, Pro-Glu and two cisoid amide bonds between Ser-Pro, Glu-Gly in a cis–trans-cis–trans (ctct) pattern. This supports previous reports that majority of CTPs adopt a ctct pattern when dissolved in hydrogen-bond disrupting solvents (Che and Marshall in J Med Chem 49:111–124, 2006 and references cited therein). An ensemble of ten lowest-energy-minimised 3D structures generated using XPLOR-NIH software revealed that cyclo[GSPE] possessed a rigid backbone ring scaffold. The remaining two minor conformers were present in quantities too low for NMR structural studies.

Difficult macrocyclizations: new strategies for synthesizing highly strained cyclic tetrapeptides.

[reaction: see text] Cyclic tetrapeptides are an intriguing class of natural products. To synthesize highly strained cyclic tetrapeptides we developed a macrocyclization strategy that involves the inclusion of 2-hydroxy-6-nitrobenzyl (HnB) group at the N-terminus and in the "middle" of the sequence. The N-terminal auxiliary performs a ring closure/ring contraction role, and the backbone auxiliary promotes cis amide bonds to facilitate the otherwise difficult ring contraction. Following this route, the all-L cyclic tetrapeptide cyclo-[Tyr-Arg-Phe-Ala] was successfully prepared.

Formation of several stable complexes between the minor components of the cyclic tetrapeptide cyclo-(-Pro1-Ala2-D-Phe3-Leu4-) and some specific Boc-amino acids.

The cyclic tetrapeptide cyclo (-Pro1-Ala2-D-Phe3-Leu4-) was modeled and synthesized to be used for molecular interactions and chiral discrimination studies in CDCl3. Total correlation spectroscopy and nuclear Overhauser effect spectroscopy spectra of the cyclic tetrapeptide showed the presence of one dominant stereoisomer-1- and three minor ones--2a, 2b, and 2c--in a relationship of 92:6:1:1. They formed three- to five-hydrogen bond complexes with Boc-D-Ser, Boc-L-Ser and Boc-L-Thr (Boc: t-butyloxylcarbonyl). These three Boc-amino acids interact more strongly with 2a, 2b, and 2c than with 1, altering their relative concentrations to 48:40:6:6. In the complex between the cyclic tetrapeptide and Boc-D-Ser, the stereoisomer 2a exchanged chemically with 1, 2b, and 2c, while 1 did not exchange with either 2b or 2c. This chemical exchange is due to cis-trans isomerization of the peptide bonds. The chiral discrimination of 2a, 2b, and 2c was stronger than that of 1. No complexation occurred with Boc-L-Ala or Boc-L-Trp.

Beta-Alanine containing peptides: gamma-turns in cyclotetrapeptides.

In the present paper we describe the synthesis, purification, single-crystal x-ray analysis, solution conformational characterization, and conformational energy calculations of the cyclic tetrapeptide cyclo-(beta-Ala-L-Pro-beta-Ala-L-Val). The peptide was synthesized by classical solution methods and the cyclization of the free tetrapeptide was accomplished in good yields in diluted methylene chloride solution using N,N-dicyclohexyl-carbodiimide. The compound crystallizes in the monoclinic space group P2(1) from ethanol with two independent molecules in the unit cell. All peptide bonds are trans. The nmr molecular conformation in the acetonitrile solution as well as that derived from the molecular dynamic simulation in vacuo is quite different from those observed in the solid state and is very similar to that previously observed for the parent compound cyclo- (beta-Ala-L-Pro-beta-Ala-L-Pro).

Beta-alanine containing peptides: a novel molecular tool for the design of gamma-turns.

In the present paper we describe the synthesis, purification, single crystal x-ray analysis, and solution conformational characterization of the cyclic tetrapeptide cyclo-(L-Pro-beta-Ala-L-Pro-beta-Ala). This peptide was synthesized by classical solution methods and the cyclization of the free tetrapeptide was accomplished in good yields in diluted methylene chloride solution using N,N-dicyclohexyl-carbodiimide (DCCI). The compound crystallizes in the orthorombic space group P2(1)2(1)2(1) from ethyl acetate. All peptide bonds are trans. The molecular conformation is stabilized by two intramolecular hydrogen bonds between the CO and NH groups of the two beta-alanine residues. These hydrogen bonds take place in a C7 structure in which both proline residues occupy the 2 position of an inverse gamma-turn. The two beta-alanine residues have a typical folded conformation (around the C alpha-C beta bond) observed in other cyclic peptides containing this residue. A detailed 1H-nmr analysis in CD3CN solution has been carried out. The molecule assumes a twofold symmetry in solution with a molecular conformation consistent with that observed in the solid state.

Cyclic β‐alanyl‐β‐alanine containing peptides: A new molecular tool for β‐turned peptides

AbstractIn the present paper we describe the synthesis, purification, and single‐crystal x‐ray analysis of the cyclic tetrapeptide cyclo‐(L‐Pro‐L‐Phe‐β‐Ala‐β‐Ala). This compound contains the β‐alanyl‐β‐alanine dipeptide as putative cyclization arm to force the remaining dipeptide‐L‐Pro‐L‐Phe‐ in a β‐turned conformation. Thepeptide was synthesized by classical methods and cyclization of the free linear tetrapeptide was accomplished in reasonable yields in diluted methylene chloride solution. The compound crystallizes in space group P21 from hot water with two independent tetrapeptide molecules and seven solvent molecules in the unit cell. This compound shows in the solid state an intramolecular hydrogen bond between the CO group of the β‐Ala4 and the NH group of the β‐Ala3 residues stabilizing a type I β‐turn conformation in which Pro1 and Phe2 occupy the relative position 2 and 3 of the turn, respectively. A rather complex network of 18 hydrogen bonds involving all the remaining CO and NH groups and the water molecules is present in the crystal.

13C‐ and 1H‐nmr evidence for all‐cis peptide bonds in cyclic tetrapeptide cyclo(L‐Pro‐Sar)2

AbstractConformations of the cyclic tetrapeptide cyclo(L‐Pro‐Sar)2 in solution were studied by 1H‐ and 13C‐nmr spectrometry and model building. The nmr data provide definite evidence that this cyclic peptide exists chiefly in two conformations, namely, a C2‐symmetric conformation and an asymmetric structure. The former was demonstrated to be predominant in polar solvents (100% in Me2SO‐d6). This structure contains all cis‐peptide bond linkages and all trans′ Pro CαCO bonds. It represents the first cyclic tetrapeptide in which all four peptide bonds have been found in the cis‐conformation. As the polarity of the solvent decreases, the population of C2‐symmetric conformers decreases (88% in CD3CN and 65% in CDCl3). At the same time, a minor asymmetric conformer, characterized by cis‐cis‐cis‐trans peptide bond sequences (two cis Sar‐Pro bonds, one cis Pro‐Sar bond, and one trans Pro‐Sar bond), is seen to increase (9% in CD3CN and 30% in CDCl3). A proposed predominant conformation in solution for cyclo(L‐Pro‐Sar)2 was compared with a crystal structure, as reported in an accompanying paper. Both structures show striking overall similarities.

Cation binding cyclic peptides composed of imino acid residues

AbstractCyclo(L‐Pro‐Sar)n (n = 2–4) with moderate flexibility and hydrophobicity of molecular structure was synthesized, and the characteristics of these cyclic peptides and their metal complexes in acetonitrile were investigated in connection with the residual properties using 13C‐nmr measurements. The cyclic tetrapeptide cyclo(L‐Pro‐Sar)2 showed a sterically hindered phenomenon in acetonitrile in which the amide backbone adopted a cis‐trans‐cis‐trans sequence. The cyclic hexapeptide cyclo(L‐Pro‐Sar)3 existed as a mixture of several conformers whose interconversion is slow on the nmr time scale, including cis‐cis‐trans and/or cis‐trans‐trans arrangement of the Sar‐Pro bond. Finally, it was demonstrated that the cyclic octapeptide cyclo(L‐Pro‐Sar)4 behaved as a mixture of multiple conformers which allowed for cis‐trans isomerism about the Pro‐Sar peptide bond, of which 20–30% had the all‐cis Sar‐Pro bond isomer and the remaining 70–80% had one (or more) cis Sar‐Pro bond isomer. 13C‐nmr spectra also demonstrated that cyclo(L‐Pro‐Sar)n (n = 3,4) formed a 1:1 ion complex whose conformation was characterized by an all‐trans peptide bond in the presence of excess metal salt. Cation binding studies, using CD measurements, established that the ion selectivity of cyclo(L‐Pro‐Sar)4 in acetonitrile decreased in the order, Ba2+ > Ca2+ > Na+ > Mg2+ > Li+.

Conformation of the cyclic tetrapeptide cyclo(-L-leucyl-L-tyrosyl-δ-amino valeryl-δ-aminovaleryl-) crystallized from acetone/H2O and comparison with the crystal from acetone/DMSO

Conformation of the cyclic tetrapeptide <i>cyclo</i>(-<scp>L</scp>-leucyl-<scp>L</scp>-tyrosyl-δ-amino valeryl-δ-aminovaleryl-) crystallized from acetone/H₂O and comparison with the crystal from acetone/DMSO