Studies On The Synthesis, Characterization And Bioactivity Of Heterocycle-Tryptophan Schiff Base Complexes

It has been proved that amino acid Schiff bases and amino acid Schiff basecomplexes have good biological activities (such as anticancer, antibacterial andinteractions with DNA), oxygen-carrying properties and catalytic performances,which has been a hot topic in the field of biological inorganic chemistry and materialsscience. As an important way for the protein degradation of eukaryotic cells,Ubiquitin-proteasome pathway (UPP) plays a critical role in the degradation of theproteins involve in the cell cycle control and tumor growth. Specified block thispathway will affect the regulation of many key proteins in cells, and then affect manyimportant cellular processes, eventually leading to cell apoptosis. Hence, theproteasome becomes an ideal target for the design and development of new anticancerdrugs. In recent years, the anticancer activities of Schiff base metal complexesreceived much attention. However, the study of tryptophan Schiff base metalcomplexes as proteasome inhibitors induce apoptosis in cancer cells is rarely reported.Therefore, it has an important guiding significance for the development of antitumordrugs to design and synthesize some novel amino acid Schiff base metal complexes,and study chemical structures, biological activity and mechanism, especially theinhibitory effect and mechanism on tumor cells by targeting cellular proteasome.In this paper, four carbonyl heterocyclic compounds with multiple structureshave been used to react with tryptophan and then coordinate with metal ions,preparing25different heterocycle-tryptophan Schiff base metal complexes, including11single crystals. The structures of these metal complexes were characterized byelemental analysis, IR spectrum, UV spectrum,1H NMR, and TG-DTG analysis, etc.Furthermore, the crystal structures were detected by X-ray crystal diffraction. Theinteraction mode between two complexes and calf thymus DNA has been tested, too. Moreover, the inhibition of proteasomal chymotrypsin-like activity and induction ofapoptosis in various types of tumor cells by heterocycle-tryptophan Schiff base metalcomplexes have also been studied by targeting the cellular proteasome. The mainresearches of this paper are as follows:(1) Five metal complex crystals with Schiff base2-acetylpyridine-L-tryptophanwere obtained, which were composed of M(C18H16N3O2)2·2CH3OH (M=Mg(II),Ni(II), Cu(II), Cd(II)), Zn(C18H16N3O2)2·2CH3CH2OH. The crystallographic structuralanalysis reveals that all five crystals crystallize in the tetragonal crystal system, spacegroup P43212. Each metal atom coordinates with two Schiff base ligands. It iscoordinated by six atoms, namely, two nitrogen atoms from C=N, two nitrogen atomsfrom pyridine rings and two carboxylic oxygen atoms in different ligands, forming atype of the4N+2O neutrality complex. In addition, there are two solvent moleculesin the crystalline lattice which are not bound to the metal ions. Take the complexMg(C18H16N3O2)2·2CH3OH for example. The cell parameters are as follows: a=b=11.6532(14), c=29.025(3),===90, V=3941.5(8)3, F(000)=1480,calcd=1.181g/cm3, the final R1=0.0670and wR2=0.1763(I>2(I)). As a result ofthe alternate arrangement of chains through inter-chained N-H…O hydrogen bondinginteractions, a2-D fabricated layer.(2) Five metal complexes with Schiff base ligand2-acetylpyrazine-L-tryptophanwere synthesized. The single crystal of Ni(C17H15N4O2)2·2CH3OH was also obtained.The crystallographic structural analysis reveals that the crystal crystallizes in thetetragonal crystal system, space group P43212with cell parameters a=b=11.8069(10), c=28.961(2),===90, V=4037.2(6)3, F(000)=1544,calcd=1.213g/cm3, the final R1=0.0548and wR2=0.1466(I>2(I)). Each nickel atomcoordinates with two Schiff base ligands. It is coordinated by six atoms, namely, twonitrogen atoms from C=N, two nitrogen atoms from pyrazine rings and twocarboxylic oxygen atoms in different ligands, forming a neutral4N+2O complex. Inaddition, there are two free solvent methanol molecules in the crystalline lattice. Eachligand bridges two Ni(II) centers through N–H…O intermolecular hydrogen bonds,leading to a one-dimensional coordination polymer. Two adjacent chains are bridged by one type of hydrogen bond, involving N–H…O interactions. As a result of thealternate arrangement of chains through inter-chained hydrogen bonding interactions,a2-D layer is formed.The chemical compositions of the remaining four metal complexes are as follows:M(C17H15N4O2)2·2CH3OH (M=Mn(II), Cu(II), Cd(II)), Zn(C17H15N4O2)2·4CH3OH.(3) Five metal complexes with Schiff base ligand5-methylfurfural-L-tryptophanwere synthesized. The chemical compositions of these metal complexes are as follows:M(C17H15N2O3)2·2CH3OH (M=Mn(II), Ni(II), Cu(II), Cd(II)),Zn(C17H15N2O3)2·4CH3OH.(4) Five metal complexes with Schiff base5-bromo-2-thiophenecarbaldehyde-L-tryptophan were synthesized. The chemical compositions of these metal complexesare as follows: M(C16H12N2O2SBr)2·2CH3OH (M=Mn(II), Ni(II), Cu(II), Zn(II),Cd(II)).(5) The crystal of2-acetylpyridine-D-tryptophan nickel complexNi(C17H15N4O2)2·2CH3OH was obtained. The crystallographic structural analysisreveals that the crystal crystallizes in the monoclinic crystal system, space group C2/cwith cell parameters a=17.4878(17), b=11.3696(12), c=18.6213(18),==90,=106.0470(10), V=3558.2(6)3, F(000)=1544,calcd=1.377g/cm3, thefinal R1=0.0356，wR2=0.0866(I>2(I)).(6) Four metal complex crystals with Schiff base2-acetylpyridine-D-tryptophanwere obtained, which were composed of M(C18H16N3O2)2·2CH3OH (M=Mg(II),Ni(II), Cd(II)), Zn(C18H16N3O2)2·4CH3OH. The crystallographic structural analysisreveals that all four crystals crystallize in the tetragonal crystal system, space groupP41212. Each metal atom coordinates with two Schiff base ligands. It is coordinatedby six atoms, namely, two nitrogen atoms from C=N, two nitrogen atoms frompyridine rings and two carboxylic oxygen atoms in different2-acetylpyridine-D-tryptophan ligands, forming a type of the4N+2O neutralitycomplex.(7) The energies and components of molecular orbital, natural chargesdistribution and electrostatic potential of Mg(C18H16N3O2)2·2CH3OH were calculated by the density functional theory with the gradient corrected B3LYP method usingGaussian03program package based on the crystal structure. All the data obtainedfrom quantum chemistry calculation are consistent with those from determination,indicating that the calculation model is stabilized.(8) The interactions between complexes Mg(C18H16N3O2)2·2CH3OH,Ni(C17H15N4O2)2·2CH3OH and calf thymus DNA were studied by UV absorptionspectra, fluorescence emission spectra and viscometric method. The nature of thebinding seems to be mainly an electrostatic interaction between DNA and the complex.However, other binding modes, such as hydrogen bonding, may also be present in thissystem. It has been demonstrated that when ligand is less coplanar, it is morefavorable to interact with DNA in the way of electrostatic binding.(9) Anticancer activities and mechanism of heterocycle-tryptophan Schiff basecadimum complexes as the proteasome inhibitor were studied. The antiproliferaionactivities of25tryptophan Schiff base complexes were studied by MTT method inhuman breast cancer MDA-MB-231cells. It shows that2-acetylpyridine-L-tryptophan cadimium complex Cd1,2-acetylpyrazine-L-tryptophan cadimiumcomplex Cd2and5-methylfurfural-L-tryptophan cadimium complex Cd3are moreeffective than the other metal complexes with same series in the inhibition of cellproliferation of human breast cancer MDA-MB-231cells. According to theproteasomal chymotrypsin-like activity assay, western blotting analysis and cellularapoptotic morphology assay, we proved that inhibition of proteasomal activity(especially, CT-like activity) by these three complexes, can strongly induce apoptosisin cultured breast cancer MDA-MB-231cells.5-Bromo-2-thiophenecarbaldehyde-L-tryptophan cadimium comples Cd4, whichhas a-Br atom in the thiophene ring, cannot actually enter cancer cells. Although itcould inhibit CT-like activity of the cell-free proteasome in vitro, it cannot inhibitcellular proteasome activity, as a result, Cd4cannot induce apoptosis in culturedMDA-MB-231cells.Cadimium complexes Cd1, Cd2, Cd3and Cd4could inhibit cancer cellproliferation of LN-CaP human prostate cancer cells and ANBL6-V10R human multiple myeloma cancer cells in a concentration-dependent manner. Take Cd4as anexample, it shows that Cd4is a potent inhibitor of proteasomal chymotrypsin-likeactivity, further capable of inducing apoptosis of LN-CaP human prostate cancer cellsand ANBL6-V10R human multiple myeloma cancer cells in a cancer cell-specificmanner.The combination of Cd4and Bortezomib could highly inhibit the activity ofproteasome and then induce apotosis of ANBL6-V10R human multiple myelomacancer cells compared with Cd4or Bortezomib alone.