Enkephalins conformational restriction

The multiple conformational restriction of dermorphin-related tet-rapeptide analogues that was performed represents a rational design of opioid peptidomimetics characterized by a high degree of structural rigid-ification. This is indicated by the fact that the p-selective agonist H-Hat-D-Orn-Aic-Glu-NH2 contains only two freely rotatable bonds, whereas there are 14 freely rotatable bonds in [Leu5]enkephalin. 

The work of Veber et al. established that valuable information about the bioactive conformation of a flexible peptide could be obtained by applying the principles of conformational restriction, and several additional examples soon were reported that followed this strategy. Conformationally restricted enkephalin analogs, e.g., 02-13), were formed by cyclizing between positions 2 and 5 of enkephalins (4a-b) (39). Cyclization of a-mela-notropin (14) gave the unusually active analog... 

However, while the Indane analog shows considerably less activity in the guinea pig ileum and mouse vas deferens assays chan [Leu ]enkephalln, the tetralin analog is more active in the guinea pig ileum (p-receptor) assay and much less active in the mouse vas deferens (6-receptor) assay than [leu ]enkephalin. Implying an Increase of receptor specificity by this conformational restriction. The use of dehydro amino acids to Induce rigidity and increase hydrophoblclty of the enkephalins has resulted in the u-selective [AAla , Leu ]enkephalln and the 6-selective [D -Ala ,... 

In contract to these eompoynds, the myre conformatlonally constrained [d Pen, D-Cys ]-, [D-Pen. Cys ]-, [0 -Pen, D -Pen ]-, and C)-Pen. Pen ]-enkephalins exhibit exceptional 6 receptor selectivity. Conformational differences associated with the Pen residues appear to be responsible for the high receptor selectivity. Finally, a series of cyclic retro-Inverso enkephalins have been designed to incorporate favorable conformational restriction and Increased resistance to enzymatic degradation. 

The conformationally restricted cyclic, disulfide containing, enkephalin analogue (D-Pen, D-Pen) enkephalin (DPDPE) was synthesized by solid-phase methods. Its purification was accomplished previously by partition on Sephadex G-25 block polymerizate using the solvent system (1-butanol acetic acid water 4 1 5), followed by gel filtration on Sephadex G-15 with 30% acetic acid as the eluent. DuCCC demonstrated a highly efficient and one step method for the purification of DPDPE. The crude DPDPE (500 mg), which contained impurities and salts, was purified by DuCCC with a two-phase solvent system... 

Conformational Restrictions for Receptor Specific Analogs. Development of d Receptor Selective Enkephalins... 

The enkephalins have been the most extensively modified of the opioid peptides, and thousands of analogs of these pentapeptides have been prepared (see Refs. 653-658 for reviews). The naturally occurring enkephalins exhibit some selectivity for 8 receptors (see Table 7.9), but these peptides are rapidly degraded by a variety of peptidases (see Section 6.8 below). Therefore one major goal of structural modification of these small peptides has been to increase metabolic stability. Depending on the nature of the modifications made, both jLt- and S-selective enkephalin derivatives have been prepared (enkephalin derivatives generally have very low affinity for k opioid receptors). These derivatives have included both linear peptides and conformationally constrained derivatives. Conformational constraints have included cyclizations between residues in the peptide chain and local constraints by incorporation of an amino acid whose side-chain conformation is restricted. 

Conformationally Constrained Analogs. Local restriction of conformational freedom can be accomplished by incorporating conformationally constrained amino acids (see Refs. 679,680 for reviews). In the case of the enkephalins, incorporation of 2-amino-6-hy-droxy-2-tetralincarboxylic acid (Hat, Fig. 7.43) in place of Tyr in [Leu ]enkephalin methyl ester results in a /u,-selective compound [IC50 ratio (MVD/GPI) = 5.11, whereas the analog containing 2-hydroxy-5-hydroxy-2-indane-carboj rlic acid (Hai) is virtually inactive (681). Incorporation of 2, 6 -dimethyltyrosine (Dmt,... 

Conformationally Constrained Analogs. Local restriction of conformation in the linear enkephalins has included incorporation of dehydroamino acids [e.g., dehydrophenylalanine (APhe, Fig. 7.43)] and cyclopropyl-methylphenylalanine (VPhe) into the peptides (see Ref 704 for a review). In the case of the dehydrophenylalanine (APhe ) derivatives of [D-Ala, Leu ]enkephalin, the Z isomer exhib-... 

Molecular dynamics simulation of leucine-enkephalin has shown that the conformational space occupied by the phenyl group of Phe is restricted to the region of the indolic benzene moiety [15] (Fig. 1). However, it appeared imlikely that the Phe phenyl group can assume a conformation identical with that of the indolic benzene moiety because superposition of both rings was not observed dining the simulations. In this regard, many of the Phe phenyl conformations were nearly perpendicular to the plane of the indole system. 

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