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

The crystal structures of HCO-Met-Leu-Phe-OC(CH3)3, (CH25H39N3O5S), fMLP-OtBu, and HCO-Met psi CSNH]-Leu-Phe-OCH3, (C22H33N3O4S2), fMS 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(1), a = 12.027(4), b = 9.492(3), c = 12.660(4) A, beta = 101.99(3) degrees, Z = 2; those of fMS LP-OMe are orthorhombic, space group P2(1)2(1)2(1), a = 7.130(1), b = 12.097(2), c = 31.060(5) A, Z = 4. The first compounds fMLP-OtBu is the t-butyl ester of the tripeptide fMLP 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. fMSLP-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 C7(eq) conformations 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 C7eq conformations for fMLP is made. The resulting conformers are compared to previously active structures of these chemotactic agents.