Synthetic enkephalins. Addicting properties and conformational studies in solution

Circular dichroic properties of the tyrosine residues in tetrazole analogues of opioid peptides

CD studies on tetrazole analogues of opioid peptides show that peptides sharing the same N-terminal sequence, H-TyrPsi[CN(4)]Gly-, give very large Cotton effects of the Tyr side chain in the near-UV region. CD spectra of five such peptides: H-TyrPsi[CN(4)]Gly-Gly-Phe-Leu-OH (I), H-TyrPsi[CN(4)]Gly-Phe-Pro-Gly-Pro-Ile-NH(2) (II), H-TyrPsi[CN(4)]Gly-Phe-Pro-NH(2) (III), H-TyrPsi[CN(4)]Gly-Phe-Gly-Tyr-Pro-Ser-NH(2) (IV), and H-TyrPsi[CN(4)]Gly-Phe-Asp-Val-Val-Gly-NH(2) (V), and two others for comparison: H-Tyr-GlyPsi[CN(4)]Gly-Phe-Leu-OH (VI) and H-TyrPsi[CN(4)]Ala-Phe-Gly-Tyr-Pro-Ser-NH(2) (VII), were measured in methanol, 2,2,2-trifluoroethanol, and water at different pH values. The spectra show that the conformations of the Tyr(1) residue in peptides I-V are very similar in all solvents used but differ distinctly from those observed for VI and VII. Strong Tyr bands in the aromatic region result probably from the rigid structure of the common N-terminal part of peptides I-V. These bands are weaker for IV, which maybe due to the presence of a second Tyr residue in that peptide, giving an opposite contribution to the CD spectrum as that arising from Tyr1. It seems that the rigid structure of the N-terminal part of I-V results from the interaction of the Tyr(1) side chain and the tetrazole ring. The CD bands of the Tyr residues of VI and VII are much smaller than those of I-V in all solvents, except VII in trifluoroethanol (TFE) where Tyr bands comparable in intensity to those of I-V are observed. This spectral property may derive from the same sign contribution of both Tyr residues of VII to the CD spectrum.

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.

Synthesis of four diastereomeric enkephalins incorporating cyclopropyl phenylalanine

Four diastereomeric D-Ala2, Leu5-enkephalins have been synthesized and their CD spectra and rat brain binding affinities determined. Only the peptides containing Z-cyclopropyl phenylalanine residues showed strong binding affinity. No correlation between CD spectra and bioactivity could be made.

In vitro interactions of opioid peptides with phospholipids. II. Additional spectral evidence

The interaction of leu-enkephalin with phosphatidylserine has been studied with ultraviolet and circular dichroism spectroscopy methods as well as with fluorescence anisotropy techniques. The data reported hereunder confirm the existence of binding between the two species, and also support the hypothesis that not only the tyrosine, but also the phenylalanine residue in the leu-enkephalin molecule is involved in peptide-lipid interaction. In addition, ultraviolet and CD evidence, taken together, tend to suggest that both aromatic residues are bound, with a different degree of involvement, to the same region of the lipid molecule. The data reported are discussed in terms of the interaction model previously proposed by us.

Conformational energy calculations on enkephalins and enkephalin analogs. Classification of conformations to different configurational types

Conformational energy calculations were carried out on the peptide enkaphalins (ENK) and selected analogs to find those conformers of low energy. The analogs studied include [D-Ala2]Enk-NH2, [D-Ala2]Enk, [D-Met2, Pro5]Enk-NH2, [D-Ala2, D-Phe5]Enk, [D-Ala2, D-Leu5]Enk, [D-Ala2, (N-Me)Phe4, Met5] Enk-NH2 and [D-Ala2, (N-Me)Met5]Enk-NH2. When the low-energy conformers for all the analogs are compared, different allowed backbone conformations are found which orient the functional side-chains such that three classifications of structures appear. Each classification shows a unique configuration of side-chain positions in space even though different backbone conformations are found within each classification.

Hexahydro derivative of (D-Met2, Pro-NH2(5)]-enkephalin gives rise to physical dependence

Filippi et al. (1979) have claimed that substitution of cyclohexylalanine for phenylalanine in leucine-enkephalin and its L-Ser3 derivative resulted in synthetic analogs which were non-addicting. We have examined the hexahydro derivative of [D-Met2, Pro-NH2(5)]-enkephalin, a potent analgesic agent, to determine if dependence liability can be separated from short-term opiate actions after hydrogenation of the phenylalanine residue. In contrast to Filippi et al.'s observations, we found that the hexahydro derivative of [D-Met2, Pro-NH2(5)]-enkephalin gives rise to physical dependence and that its short-term potencies generally paralleled its long-term ability to produce physical dependence.