Crystal and molecular structure of two geometrically restricted chemotactic tripeptides, analogues of formyl‐methionine‐leucine‐phenylalanine*

Abstract

The crystal structures of HCO-Met-Leu-Phe-OC(CH,), , (CH2,H3,NjO5S), fMLP-OtBu, and HCO-Met$ [CSNHI-Leu-Phe-OCH,, (C22H33 N,O,Sz), f M s LP-OMe, have been determined by single crystal X-ray diffraction, and their conformational properties investigated by molecular mechanics energy calculations. Crystals of fMLP-OtBu are monoclinic, space group P2,, a = 12.027(4), b = 9.492(3), c = 12.660(4)A, = 101.99(3)", Z = 2; those of fM'LP-OMe are orthorhombic, space group P21212,, a = 7.130(1), b = 12.097(2), c = 31.060(5) A, Z = 4 . The first compounds f M L P -0 t h is the t-butyl esterof the tripeptide f M L P that represents one of the most potent compounds in inducing the lysozyme release from human neutrophils that reflects the chemotactic activity. From the crystal structure, it is shown that the orientation of the phenylalanine side chain is largely affected by the presence of the bulky group. fMs LP-OMe was shown to be inactive after thionation of the methionine residue in the original tripeptide.Nevertheless, the crystal structure does not reveal any influence of the presence of the thionated peptidic bond on the backbone conformation. The X-ray results have been used to generate parameters for empirical energy calculations. Subsequently, a strategy based on random generation of conformations followed by energy-minimization was applied to investigate the conformational space of thiopeptides, in comparison with normal peptides. From molecular free energy calculations, it is shown that the main influence of the introduction of a thioamide bond on the molecular structure is to prevent the existence of C&onformations involving the thiomethionine residue. Consequently, a larger number of conformers are found to form intramolecular hydrogen bonds involving the formyl group, reducing its availability to interact with the receptor. For the first time, the theoretical prediction of the existence of C,& conformations for f M L P is made. The resulting conformers are compared to previously active structures of these chemotactic agents.

Endothioxopeptides: A conformational overview

“…The first NMR experimental demonstration was published on the ability of a 12‐mer endothioxopeptide to fold in a β‐hairpin conformation, with the thionated Gly at position i + 2 of the type‐II′ β‐turn . A more constrained β‐hairpin conformation is adopted by a mono‐thionated tetrapeptide which results in a more efficient behavior as asymmetric catalyst . An NMR study confirmed that the ‐ d ‐Proψ[CSNH]Aib‐ sequence is folded in a type‐II′ β‐turn conformation.…”

“…In the first section of this paper treating experimental data on the conformation of endothioxopeptides, we describe the X‐ray diffraction results of our survey on the crystal‐state geometry and 3D‐structure of peptides (not simple amides nor amino acid derivatives) characterized by one (or more) thionamide groups, where we identified 30 publications of interest . Two of them are review articles, but the first, although dealing exclusively with endothioxopeptides, was published almost 30 years ago (with only six cited 3D structures) and the second treated this topic marginally, as a single part of its numerous sections.…”

“…A limited number shows building blocks different from α‐amino acids in the sequence. Most important is the observation that as many as 10 of them are analogs or short sequences of relevant bioactive peptides (melanostatin, peptaibols, cyclosporin, the formyl‐Met‐Leu‐Phe‐OH chemotactic tripeptide, enkephalins, an oxytocin – receptor ligand, and vancomycin).…”