Crystal structure of [Leu5]enkephalin

Exploring Conformational Preferences of Leu-enkephalin Using the Conformational Search and Double-Hybrid DFT Energy Calculations

The conformational preferences of Leu-enkephalin (Leu-Enk) were explored by the conformational search and density functional theory (DFT) calculations. By a combination of low-energy conformers of each residue, the initial structures of the neutral Leu-Enk were generated and optimized using the ECEPP3 force field in the gas phase. These structures were reoptimized at the HF/3-21G(d) and M06-2X levels of theory with 6-31G(d) and 6-31+G(d) basis functions. We finally located the 139 structures with the relative energy <10 kcal mol-1 in the gas phase, from which the structures of the corresponding zwitterionic Leu-Enk were generated and reoptimized at the M06-2X/6-31+G(d) level of theory using the implicit solvation model based on density (SMD) in water. The conformational preferences of Leu-Enk were analyzed using Gibbs free energies corrected by single-point energies calculated at the double-hybrid DSD-PBEP86-D3BJ/def2-TZVP level of theory in the gas phase and in water. The neutral Leu-Enk dominantly adopted a folded structure in the gas phase stabilized by three H-bonds with a βII'-bend-like motif at the Gly3-Phe4 sequence and a close contact between the side chains of Phe4 and Leu5. The zwitterionic Leu-Enk exhibited a folded structure in water stabilized by three H-bonds with double β-bends such as a βII' bend at the Gly2-Gly3 sequence and a βI bend at the Gly3-Phe4 sequence. The calculated ensemble-averaged distance between CGly2 α and CLeu5 α of the zwitterionic Leu-Enk in water is consistent with the value estimated from the simulated annealing using the distance constraints derived from nuclear Overhauser effect spectroscopy (NOESY) spectra in water. Interestingly, the preferred conformations of the neutral and zwitterionic Leu-Enk are new folded structures not predicted by earlier computational studies. According to the refined model of the zwitterionic Leu-Enk bound to δ-opioid receptor (δOR), there were favorable interactions of the terminal charged groups of Leu-Enk with the side chains of charged residues of δOR as well as a favorable CAryl···H interaction of the Phe4 residue of Leu-Enk with Trp284 of δOR. Hence, these favorable interactions would induce the folded structure of the zwitterionic Leu-Enk with double β-bends isolated in water into the "bioactive conformation" like an extended structure when binding to δOR.

Selective MOR activity of DAPEA and Endomorphin-2 analogues containing a (R)-γ-Freidinger lactam in position two

The use of α-amino-γ lactam of Freidinger (Agl) may serve as an impressive method to increase the biological stability of peptides and an appropriate tool to elucidate their structure-activity relationships. The endomorphin-2 (EM-2) and [D-Ala², des-Leu⁵] enkephalin amide (DAPEA) are two linear opioid tetrapeptides agonists of MOR and MOR/DOR respectively. Herein, we investigated the influence of the incorporation of (R/S)-Agl in position 2 and 3 on the biological profile of the aforementioned products in vitro and in vivo. Receptor radiolabeled displacement and functional assays were used to measure in vitro the binding affinity and receptors activation of the novel analogues. The mouse tail flick and formalin tests allowed to observe their antinociceptive effect in vivo. Data revealed that peptide A2D was able to selectively bind and activate MOR with a potent antinociceptive effect after intracerebroventricular (i.c.v.) administration, performing better than the parent compounds EM-2 and DAPEA. Molecular docking calculations helped us to understand the key role exerted by the Freidinger Agl moiety in A2D for the interaction with the MOR binding pocket.

Fine refinement of solid-state molecular structures of Leu- and Met-enkephalins by NMR crystallography

This paper presents a methodology that allows the fine refinement of the crystal and molecular structure for compounds for which the data deposited in the crystallographic bases are of poor quality. Such species belong to the group of samples with molecular disorder. In the Cambridge Crystallographic Data Center (CCDC), there are approximately 22,000 deposited structures with an R-factor over 10. The powerful methodology we present employs crystal data for Leu-enkephalin (two crystallographic forms) with R-factor values of 14.0 and 8.9 and for Met-enkephalin (one form) with an R-factor of 10.5. NMR crystallography was employed in testing the X-ray data and the quality of the structure refinement. The GIPAW (gauge invariant projector augmented wave) method was used to optimize the coordinates of the enkephalins and to compute NMR parameters. As we reveal, this complementary approach makes it possible to generate a reasonable set of new coordinates that better correlate to real samples. This methodology is general and can be employed in the study of each compound possessing magnetically active nuclei.

Crystal structures of peptide enantiomers and racemates: probing conformational diversity in heterochiral Pro-Pro sequences

Multiple conformational states in heterochiral diproline sequences have been characterized in the solid state by the determination of the crystal structures of seven tripeptides in enantiomeric and racemic forms. The sequences of the type Piv-DPro-LPro-DXxx-NHMe (D-L-D) [DXxx=DVal 1, DLeu 3, and DPhe 5] and their corresponding enatiomeric L-D-L sequences [LXxx=LVal 2, LLeu 4, and LPhe 6] have been investigated. Single crystals have been obtained for the pure enantiomers 1, 2, 3, 4 and for the racemates 1/2, 3/4, and 5/6. For Xxx=Leu, mirror image conformations (type II/II' beta-turns) at Pro-Leu segment are obtained. For Xxx=Val, a LPro-DPro type II beta-turn in 2 and an open/extended structure is obtained in the solvated form of the enantiomer 1. For Xxx=Phe, suitable crystals could not be obtained for enatiomeric peptides. The racemate 5/6 revealed a cis peptide bond between the diproline segment with the absence of any intramolecular hydrogen bonds. Crystal structures of enantiomers and racemates prove useful in characterizing the multiple conformational states that are accessible to Pro-Pro segments.

A multidimensional 1H NMR investigation of the conformation of methionine-enkephalin in fast-tumbling bicelles

Enkephalins are pentapeptides found in the central nervous system. It is believed that these neuropeptides interact with the nerve cell membrane to adopt a conformation suitable for their binding to an opiate receptor. In this work, we have determined the three-dimensional structure of methionine-enkephalin (Menk) in fast-tumbling bicelles using multidimensional (1)H NMR. Bicelles were selected as model membranes because both their bilayer organization and composition resemble those of natural biomembranes. The effect of the membrane composition on the peptide conformation was explored using both zwitterionic (PC bicelles) and negatively charged bicelles (Bic/PG). Pulsed field gradient experiments allowed the determination of the proportion of Menk bound to the model membranes. Approximately 60% of the water-soluble enkephalin was found to associate to the bicellar systems. Structure calculations from torsion angle and NOE-based distance constraints suggest the presence of both micro - and delta-selective conformers of Menk in each system and slightly different conformers in PC bicelles and Bic/PG. As opposed to previous studies of enkephalins in membrane mimetic systems, our results show that these opiate peptides could adopt several conformations in a membrane environment, which is consistent with the flexibility and poor selectivity of enkephalins.

Enkephalins: Raman spectral analysis and comparison as function of pH 1-13

Raman spectral studies are carried out on Leu- and Met-enkephalin as a function of the pH value in the range of 1-13. The molecules are dissolved in KCl solvent and the pH is controlled at each value. Spectral analyses reveal the dependence of the structural conformation on the pH, and a comparison of the two molecules is made in three frequency regions: the tyrosine Fermi doublet (850-830 cm(-1)), aromatic side chains (1650-1550 cm(-1)), and carboxylate (1430-1400 cm(-1)). All regions and frequencies are presented, discussed, and compared for the two molecules.

Assemblies of Alzheimer's peptides A beta 25-35 and A beta 31-35: reverse-turn conformation and side-chain interactions revealed by X-ray diffraction

Alzheimer's beta amyloid protein (A beta) is a 39 to 43 amino acid peptide that is a major component in the neuritic plaques of Alzheimer's disease (AD). The assemblies constituted from residues 25-35 (A beta 25-35), which is a sequence homologous to the tachykinin or neurokinin class of neuropeptides, are neurotoxic. We used X-ray diffraction and electron microscopy to investigate the structure of the assemblies formed by A beta 25-35 peptides and of various length sequences therein, and of tachykinin-like analogues. Most solubilized peptides after subsequent drying produced diffraction patterns characteristic of beta-sheet structure. Moreover, the peptides A beta 31-35 (Ile-Ile-Gly-Leu-Met) and tachykinin analogue A beta(Phe(31))31-35 (Phe-Ile-Gly-Leu-Met) gave powder diffraction patterns to 2.8A Bragg spacing. The observed reflections were indexed by an orthogonal unit cell having dimensions of a=9.36 A, b=15.83 A, and c=20.10 A for the native A beta 31-35 peptide, and a=9.46 A, b=16.22 A, and c=11.06 A for the peptide having the Ile31Phe substitution. The initial model was a beta strand where the hydrogen bonding, chain, and intersheet directions were placed along the a, b, and c axes. An atomic model was fit to the electron density distribution, and subsequent refinement resulted in R factors of 0.27 and 0.26, respectively. Both peptides showed a reverse turn at Gly33 which results in intramolecular hydrogen bonding between the antiparallel chains. Based on previous reports that antagonists for the tachykinin substance P require a reverse turn, and that A beta is cytotoxic when it is oligomeric or fibrillar, we propose that the tachykinin-like A beta 31-35 domain is a turn exposed at the A beta oligomer surface where it could interact with the ligand-binding site of the tachykinin G-protein-coupled receptor.

The structure of an endomorphin analogue incorporating 1-aminocyclohexane-1-carboxlylic acid for proline is similar to the beta-turn of Leu-enkephalin

Endomorphin (EM2, Tyr-Pro-Phe-Phe-NH(2)) can assume various conformations related to cis/trans-rotamers of the amide linkage of Tyr-Pro. To control isomerization, restricted or flexible components have been introduced at the Pro position. We focused on [Chx(2)]EM2, an EM2 analogue substituting 1-aminocyclohexane-1-carboxlylic acid (Chx) for Pro. X-ray diffraction analysis revealed that [Chx(2)]EM2 is folded into the trans-form of Tyr-Chx. The manner of folding resembled that seen in D-TIPP, an EM analogue incorporating tetrahydroisoquinoline carboxylic acid, as well as the beta-turn of Leu-enkephalin. Selectivity for the opioid mu-receptor was fairly well conserved by [Chx(2)]EM, suggesting that the folded form is important for mu-selectivity.

Crystal structure of biphalin sulfate: a multireceptor opioid peptide

Biphalin is a dimeric opioid peptide, composed of two tetrapeptides connected 'tail-to-tail', that exhibits a high affinity for all three opioid receptor types (i.e. mu, delta and kappa). This study presents the X-ray crystal structure of biphalin sulfate and compares it to other opioids that interact with the same biological targets. Both halves of the molecule have a folded backbone conformation but differ significantly from one another. Residues 1-4 in biphalin, which compare well with the delta selective opioid peptide DADLE, fold into a random coil. Residues 5-8, which can be fit to the mu selective peptide D-TIPP-NH2, exhibit a fairly normal type III' beta bend. Biphalin also exhibits structural similarities with two naltrexone analogs, naltrexonazine and norbinaltorphamine, that are specific to mu and kappa receptor sites.

Crystal structures of peptides and modified peptides

The X-ray diffraction experiments on peptides and related molecules which have been carried out in Western Europe, except Italy, in the last eight years are reviewed. The crystal structures of some bioactive peptides such as Leu-enkephalin (a neurotransmitter), cyclosporin A (an immunomodulator in both the free and protein-bound state), balhimycin (an antibiotic) and octreotide (a somatostatin analogue) are briefly presented. Crystallized N- and C-protected model peptides have given an insight into the folding tendency and folding modes depending on the peptide sequences. The crystal structures of various pseudopeptide molecules reveal how the three-dimensional structure of peptide analogues can be modulated by substituting non-peptide groups for the peptide bond. A few examples of structural mimetics of the beta- and gamma-turns, and of templates for alpha-helix induction are also presented.

A nonhelical, multiple beta-turn conformation in a glycine-rich heptapeptide fragment of trichogin A IV containing a single central alpha-aminoisobutyric acid residue

The conformational properties of the protected seven-residue C-terminal fragment of the lipopeptaibol antibiotic Trichogin A IV (Boc-Gly-Gly-Leu-Aib-Gly-Ile-Leu-OMe) has been examined in CDCl3 and (CD3)2SO by 1H-nmr. Evidence for a multiple beta-turn conformation [type I' at Gly(1)-Gly(2), type II at Leu(3)-Aib(4), and a type I' at Aib(4)-Gly(5)] suggests that Leu(3) has preferred an extended or semiextended conformation over a helical conformation in CDCl3. This structure is thus in contrast to earlier observations of seven-residue peptides containing a single central Aib preferring helical conformations in both solution and crystalline states. A structural transition to a frayed right-handed helix is observed in (CD3)2SO. These results suggest that nonhelical conformations may be important in Gly-rich peptides containing Aib. Further, the presence of amino acids with contradictory influences on backbone conformational freedom can lead to well-defined conformational transitions even in small peptides.

Investigation of conformational equilibrium of polypeptides by internal coordinate stochastic dynamics. Met5-enkephalin

The equilibrium population of different conformational states of a polypeptide can in principle be obtained by a very long molecular dynamics simulation. The method of internal coordinate molecular dynamics earlier developed in this laboratory (A.K. Mazur and R.A. Abagyan J. Biomol. Struct. Dyn. 6,833 (1989)) allows one to use time steps much larger than usual for computing molecular trajectories. It is shown here that the sampling of the conformational space can be additionally enhanced by adding a random component to the set of forces applied to atoms. We describe the algorithms by which the random force is introduced and also a special method which excludes the fast rotation of polar hydrogens from equations of motion but keeps them movable. As a result the task stated in the title becomes realistic. Internal coordinate stochastic dynamics is applied for scanning the conformational space of the pentapeptide Met5-enkephalin which is a common test example widely used in theoretical studies. A large number of conformational transitions is observed during the 20 ns simulation starting from the global energy minimum thus allowing us to arrive at a nearly Boltzmann distribution of populations of conformational states. A few states are found which are distinguished by high apparent configurational entropy which turn out to correspond well to experimentally observed conformations of enkephalins.

Hydrogen bond connectivity patterns and hydrophobic interactions in crystal structures of small, acyclic peptides

Crystal structures of all available unblocked linear peptides with two to five residues were retrieved from the Cambridge Structural Database and their intermolecular contacts and packing modes studied using molecular graphics. This survey reveals that interactions between hydrophobic portions of the molecules are critically important in determining the overall features of their crystal packing patterns. Distinct hydrophobic columns or layers are observed in almost all crystal structures. Analyses of the relationships between these interactions and crystal growth properties of small peptides are given. It is suggested that needle growth is promoted by hydrophobic packing, usually along a short crystallographic axis (4.6-6.0 angstroms). Also contributing to these morphologic characteristics are entropic factors associated with hydrophobic aggregation as well as tightly bound water molecules on hydrophobic faces. The paper also provides a comprehensive overview of hydrogen bond patterns in acyclic peptide crystals. It is demonstrated that one of their primary roles is to provide a scaffolding within which hydrophobic groups can aggregate. Even though there is a high density of hydrogen bonds in the crystals, often with complex patterns and networks, certain motifs are found to recur in a number of structures indicating specific hydrogen bond preferences. Water, for example, is an integral part of the hydrogen bond networks in these crystals, usually acting as the primary donor for main-chain carboxylate groups in peptide hydrates.

Conformational preferences of [Leu5]enkephalin in biomimetic media. Investigation by 1H NMR

The conformation of [Leu5]enkephalin has been studied by 1H-NMR spectroscopy in media more like the actual environment in which the agonist-receptor interaction takes place than water, i.e. in three cryoprotective mixtures (dimethylformamide/water, methanol/water and ethylene glycol/water), in aqueous SDS and in two neat solvents, dimethylformamide and acetonitrile, whose dielectric constants (36.7 and 37.5) are intermediate between that of water and that of the lipid phase. In all cases examined, contrary to the studies in water or dimethylsulfoxide, we were able to detect numerous nuclear Overhauser effects, indicating that the media employed favour well-defined structures and/or reduce the internal motions of the peptide. Data from both organic solvents and cryoprotective mixtures suggest a 4----1 beta turn as the most probable structure of [Leu5]enkephalin in solution, whereas in SDS/H2O micelles the structural picture appears completely different, suggesting the presence of a 5----2 beta turn. The existence of two different preferred conformations of enkephalins may possibly be related to their ability to be effective towards both mu and delta opioid receptors.

Transferred nuclear Overhauser effect analyses of membrane-bound enkephalin analogues by 1H nuclear magnetic resonance: correlation between activities and membrane-bound conformations

Leu-enkephalin, [D-Ala2]Leu-enkephalin, and [D-Ala2]Leu-enkephalinamide (agonists) and [L-Ala2]Leu-enkephalin (inactive analogue) bind to lipid bilayer consisting of phosphatidylcholine and phosphatidylserine. The conformations that these compounds assume, once bound to perdeuterated phospholipid bilayer, have been shown to be unique, as shown by the transferred nuclear Overhauser effect (TRNOE) of 1H NMR spectroscopy. In addition, their location in the bilayer was analyzed by TRNOE in the presence of spin-labeled phospholipids. These analyses showed a clear relationship between the activity and the peptide-membrane interaction. The three active peptides, when bound to membranes, adopt the same conformation, characterized by a type II' beta-turn around Gly3-Phe4 and a gamma-turn around Gly2 (or D-Ala2). The inactive analogue, [L-Ala2]Leu-enkephalin, displayed a completely different TRNOE pattern corresponding to a different conformation in the membrane-bound state. The tyrosine residue of the active compounds is not inserted into the interior of membrane, but it is inserted into the bilayer for the L-Ala2 analogue. According to these results, [L-Ala2]Leu-enkephalin may be explained to be inactive because the mode of binding to the membranes is different from that of active compounds.

A crystal molecular conformation of leucine-enkephalin related to the morphine molecule

Leucine-enkephalin (Try1-Gly2-Gly3-Phe4-Leu5) has been crystallized as a trihydrate from water solution. X-ray diffraction reveals a tightly folded molecular conformation with two fused beta III- (Gly2-Gly3) and beta I- (Gly3-Phe4) turns. The Tyr1 and Phe4 aromatic rings have a close orthogonal arrangement analogous to the tyramine and cyclohexenyl rings in morphine. This suggests that the conformation found in the trihydrate crystal structure could be required for recognition by mu-receptor sites.

Pseudopeptides and beta folding: x-ray structures compared with structures in solution

In order to restrain the flexibility of the peptide molecules and reduce their biodegradation, modifications of the main chain are now introduced in pseudopeptide analogues. Surprisingly, there is very little data on the conformational properties of these derivatives. We have examined pseudopeptide analogues of RCO-X-Y-NHR' model dipeptides in the depsi, N-methylated, reduced, retro, alpha, beta-dehydro, beta-amino acid, and hydrazino series, in the solid state by x-ray diffraction, and in solution by ir and 1H-nmr spectroscopy. This study provides us with accurate dimensions of the peptide surrogates, and gives some information on the conformational tendencies induced by these substitutions, with reference to those of the related dipeptide sequences.

Calculations on crystal packing of a flexible molecule, Leu-enkephalin

Crystal packing calculations have been carried out on a substantial number of conformations of Leu-enkephalin; namely, those obtained both from crystal structures and from energy minimizations on isolated molecules, and with and without waters of crystallization. The known crystal structures represent the most energetically stable packings found. The conformations of the enkephalin molecules in the crystal are not the most stable for an isolated molecule; i.e. intermolecular interactions force the isolated molecule to change conformation in order to achieve a small packing volume and an optimal packing energy in the crystal. It is found that the packing energy of an enkephalin molecule is a reasonably smooth function of its molecular volume in the unit cell, if structures with intermolecular hydrogen bonding are excluded, and is substantially independent of other details of the molecular conformation or of the crystal packing. Hydrogen bonding provides additional stabilization of the crystal structure, and would likely permit crystallization of the system if it is sufficiently dense. Solvent molecules further stabilize the structure when they can also provide intermolecular hydrogen bonds.

Stereochemistry of peptides containing a thioacyl group

Endothiopeptides are oligopeptides in which one or more oxoacyl moieties in peptide groups are replaced with thioacyl moieties. The stereochemical consequences of this replacement are discussed and compared with results from X-ray crystallographic determinations of small oligopeptides containing one or two thiopeptide groups. The analysis shows that the conformations normally observed around C alpha carbon atoms in proteins are also stereochemically favourable for peptides in which a thiopeptide group has been introduced. Limitations are only present in cases where the thiopeptide is part of structural elements like helices and beta-sheets. This opens the possibility of using one or more thiopeptide groups as functional modifiers of biologically active oligopeptide and protein molecules.

The binding of opioid peptides to the Mcg light chain dimer: flexible keys and adjustable locks

Enkephalins and beta-casomorphins (opioid peptides) were found to bind in a variety of conformations to a human light chain (Bence-Jones) dimer from a patient (Mcg) with amyloidosis. The peptides were diffused into crystals of the protein and their positions, relative occupancies and modes of binding were determined at 2.7 A resolution by difference Fourier analyses. Collectively, the opioid peptides occupied practically all of the available space in the concave, internal parts of the binding region, as well as flat or convex external surfaces around the rim of the binding cavity. Suitable ligands ranged in size from four to seven residues. As many as five residues could be accommodated inside the binding region, and there was space for at least four residues on the external surfaces. External binding was influenced by solvent effects and local packing interactions among adjacent protein molecules in the crystal lattice. In the enkephalin series the presence of amino-terminal tyrosine was necessary, but not sufficient for binding. [Met]-enkephalin, a pentapeptide, showed two different modes of binding in overlapping subsites. In one subsite, preferred over the second in a ratio of 1.3:1.0, the side chain of amino-terminal tyrosine penetrated through the floor of the main cavity to lodge in the deep binding pocket about 20 A from the entrance. The remainder of the peptide spanned the length of the main cavity in an extended conformation. In the second subsite the amino end was restricted to the main cavity and the peptide backbone turned abruptly upward at residue 3 to interact with external surfaces. An (Arg-6, Phe-7) heptapeptide extension of [Met]-enkephalin entered the deep pocket and assumed an extended conformation in the main cavity like the pentapeptide. Its last two residues flattened against the external surfaces. [Leu]-enkephalin and its analogues displayed a combination of internal and external binding like [Met]-enkephalin in its secondary subsite. Enkephalin analogues with D-amino acids in position 2 generally adopted conformations which were more convoluted than those in the L-isomers. Moreover, external interactions tended to be more prominent in the D-derivatives. The beta-casomorphin-7 heptapeptide penetrated into the deep pocket and traversed the main cavity in as extended a conformation as the presence of two proline residues would permit. On removal of the ligand there was an unexpected hysteresis effect involving permanent structural alterations in the walls of the binding region. beta-casomorphins-4 and -5 were bound in the main cavity with the carboxyl ends protruding from the entrance.(ABSTRACT TRUNCATED AT 400 WORDS)

The crystal structures of [Met5]enkephalin and a third form of [Leu5]enkephalin: observations of a novel pleated beta-sheet

The structures of [Met5]enkephalin (Tyr-Gly-Gly-Phe-Met) and [Leu5]enkephalin (Tyr-Gly-Gly-Phe-Leu) have been determined from single crystal x-ray diffraction data and refined to residuals of 0.100 and 0.092, respectively. The [Met5]enkephalin structure consists of dimers forming antiparallel beta-sheets extending in the monoclinic ac plane with 10.6 water molecules per dimer. The two molecules, related by pseudo two-fold axes, have similar backbone conformations and similar tyrosine and phenylalanine side-chain conformations. Both methionine residues are disordered and the disorder is different in the two independent molecules. Additional hydrogen bonds connect adjacent dimers to form infinite sheets normal to the b axis. The water molecules are found mainly in the interstices between the sheets. [Leu5]Enkephalin crystallizes as a monohydrate that is isomorphous with the [Met5]enkephalin structure with respect to the beta-sheet but different with respect to the tyrosine and phenylalanine side-chain conformations and water content. The peptide chains in both structures are fully extended and more nearly planar than pleated. The planes of the peptide chains in the dimers form an angle of 143.3 degrees with one another in [Met5]enkephalin and 156.0 degrees in [Leu5]enkephalin. This produces a zigzag pattern or pleat in the beta-sheets perpendicular to the direction of the peptide chains and, therefore, perpendicular to the normal beta-sheet pleat. The average repeat distance between Ni and Ni+2 in the peptide chains of both structures is 7.10 A, versus an ideal value of 6.68 A.

Leucine enkephalin analogues containing a conformationally restrained N-terminal amino acid residue

Three analogues of leucine enkephalin, in which the terminal tyrosine-1 residue has been replaced by conformationally restrained aromatic amino acids, have been synthesized by classical solution methods. Their opiate agonist potencies on electrically stimulated guinea pig ileum and mouse vas deferens preparations were determined and compared with morphine, Met enkephalin, and Leu enkephalin. None of these analogues had analgesic properties when evaluated on the above tissue preparations or when evaluated by the hot-plate test in mice after subcutaneous and intracerebroventricular administration.

Crystal structure of leucine-enkephalin

The crystal structure of leucine-enkephalin has been determined in a crystal form that has four independent enkephalin molecules and much water and dimethylformamide solvent in the asymmetric unit. All four enkephalins have extended peptide backbones with the Tyr, Phe and Leu side chains above and below the plane of the backbone. There is evidence that this extended conformation may provide an acceptable model for enkephalin binding to opiate mu-receptors.