Bioconjugation of Cyclometalated Gold(III) Lipoic Acid Fragments to Linear and Cyclic Breast Cancer Targeting Peptides.

Cell-targeting peptides (CTPs) are increasingly used in the field of cancer research due to their high affinity and specificity to cell or tissue targets. In the search for novel metal-based drug candidates, our research group is particularly focused on bioconjugates by utilizing peptides to increase the selectivity of cytotoxic organometallic compounds. Motivated by the relatively high cytotoxic activity of gold complexes, such as Auranofin (approved to treat rheumatoid arthritis), for the treatment of various diseases, we anticipated that gold peptide bioconjugates would present interesting candidates for novel breast cancer therapies. For this, we investigate the use of the natural compound lipoic acid (Lpa) as a bioconjugation handle to link Au complexes in the oxidation state +III to peptides using the dithiol moiety. Using this strategy, we have synthesized Au(III) complex bioconjugates linked to the linear LTVSPWY peptide and two cyclic DfKRG and KTTHWGFTLG tumor-targeting peptides. Solid-phase peptide synthesis (SPPS) was used to prepare the peptides, with lipoic acid introduced N-terminally as a conjugation handle. After peptide cleavage, the metal complex was introduced in solution by first reducing the internal disulfide bond, followed by reaction with Au(ppy)Cl2 (1, ppy: 2-phenyl-pyridine), to yield the Au(III)-Lpa-peptide bioconjugates. The new bioconjugates were successfully synthesized, purified by semi-preparative HPLC, and characterized by ESI-MS. Au(III)-peptide bioconjugates were tested as cytotoxic agents against two different human breast cancer cell lines (MCF-7 and MDA-MB-231) and normal human fibroblasts cells (GM5657T) and compared to cisplatin, the parent Au(III) dichloride complex, and metal-free peptides. These in vitro data show that the Au(III)-peptide bioconjugate 5, possessing the cyclic integrin-targeting RGD-derived peptide sequence in the structure, exhibits improved activity compared to the parent gold(III) compound Au(ppy)Cl2 (1) as well as to cisplatin or the metal-free peptide. Moreover, the excellent targeting properties of 5 are supported by the fact that a Au(III)-peptide conjugate with the exact same peptide sequence, but a linear rather than the cyclic form of 5 exhibits 10 times lower cytotoxic activity.

Synthesis of nisin AB dicarba analogs using ring-closing metathesis: influence of sp(3) versus sp(2) hybridization of the α-carbon atom of residues dehydrobutyrine-2 and dehydroalanine-5 on the lipid II binding affinity.

Herein the synthesis of two nisin AB dicarba analogs is described, focusing on amino acid modifications at positions 2 and 5. The nisin mimics were synthesized by a combination of solid phase synthesis of the linear peptides, followed by macrocyclization via ring-closing metathesis and fragment assembly by means of solution phase chemistry. The two N-terminal nisin AB-fragment mimics contain either the native dehydrobutyrine (Dhb)/dehydroalanine (Dha) amino acid residues or alanine at position 2 and 5, respectively. The native dehydrobutyrine at position 2 and dehydroalanine at position 5 were introduced as their precursors, namely threonine and serine, respectively, and subsequent dehydration was carried out by EDCI/CuCl as the condensing agent. Both AB-fragment mimics were analyzed in a lipid II binding assay and it was found that the Ala2/Ala5 AB-mimic (2) showed a reduced activity, while the Dhb2/Dha5 AB-mimic (3) was as active as the native AB-fragment (1).

The Fanconi anemia associated protein FAAP24 uses two substrate specific binding surfaces for DNA recognition.

To maintain the integrity of the genome, multiple DNA repair systems exist to repair damaged DNA. Recognition of altered DNA, including bulky adducts, pyrimidine dimers and interstrand crosslinks (ICL), partially depends on proteins containing helix-hairpin-helix (HhH) domains. To understand how ICL is specifically recognized by the Fanconi anemia proteins FANCM and FAAP24, we determined the structure of the HhH domain of FAAP24. Although it resembles other HhH domains, the FAAP24 domain contains a canonical hairpin motif followed by distorted motif. The HhH domain can bind various DNA substrates; using nuclear magnetic resonance titration experiments, we demonstrate that the canonical HhH motif is required for double-stranded DNA (dsDNA) binding, whereas the unstructured N-terminus can interact with single-stranded DNA. Both DNA binding surfaces are used for binding to ICL-like single/double-strand junction-containing DNA substrates. A structural model for FAAP24 bound to dsDNA has been made based on homology with the translesion polymerase iota. Site-directed mutagenesis, sequence conservation and charge distribution support the dsDNA-binding model. Analogous to other HhH domain-containing proteins, we suggest that multiple FAAP24 regions together contribute to binding to single/double-strand junction, which could contribute to specificity in ICL DNA recognition.

Semi-synthesis of biologically active nisin hybrids composed of the native lanthionine ABC-fragment and a cross-stapled synthetic DE-fragment.

The antimicrobial peptide nisin is a promising template for designing novel peptide-based antibiotics to improve its drug-like properties. First steps in that direction represent the synthesis of hybrid nisin derivatives that contain a native nisin ABC-part and synthesized cross-stapled DE-ring fragments and are described here. The biological activity of the newly synthesized nisin derivatives was evaluated in order to compare the bioactivity of the synthetic DE-ring containing mimic and native lanthionine-bridged DE-ring containing nisin. The native nisin ABC-ring system was obtained via chymotrypsin digestion of full-length nisin, and was subsequently functionalized at the C-terminal carboxylate with two different amino alkyne moieties. Next, nisin hybrids were successfully prepared using Cu(I)-catalyzed azide alkyne cycloaddition 'click' chemistry by chemo-selective ligation of the ABC-alkyne with the N-terminal azido functionalized dicarba-DE ring mimic. The newly synthesized compounds were active as potent lipid II binders and retained antimicrobial activity in a growth inhibition assay. However, pore formation was not observed, possibly either due to the different character of the 'staples' as compared to the parent sulfides, or due to the triazole moiety as a sub-optimal amide bond isostere.

Development of Potent Mcl-1 Inhibitors: Structural Investigations on Macrocycles Originating from a DNA-Encoded Chemical Library Screen.

Evasion of apoptosis is critical for the development and growth of tumors. The pro-survival protein myeloid cell leukemia 1 (Mcl-1) is an antiapoptotic member of the Bcl-2 family, associated with tumor aggressiveness, poor survival, and drug resistance. Development of Mcl-1 inhibitors implies blocking of protein-protein interactions, generally requiring a lengthy optimization process of large, complex molecules. Herein, we describe the use of DNA-encoded chemical library synthesis and screening to directly generate complex, yet conformationally privileged macrocyclic hits that serve as Mcl-1 inhibitors. By applying a conceptual combination of conformational analysis and structure-based design in combination with a robust synthetic platform allowing rapid analoging, we optimized in vitro potency of a lead series into the low nanomolar regime. Additionally, we demonstrate fine-tuning of the physicochemical properties of the macrocyclic compounds, resulting in the identification of lead candidates 57/59 with a balanced profile, which are suitable for future development toward therapeutic use.

Synthesis, antimicrobial activity, and membrane permeabilizing properties of C-terminally modified nisin conjugates accessed by CuAAC.

Functionalization of the lantibiotic nisin with fluorescent reporter molecules is highly important for the understanding of its mode of action as a potent antimicrobial peptide. In addition to this, multimerization of nisin to obtain multivalent peptide constructs and conjugation of nisin to bioactive molecules or grafting it on surfaces can be attractive methods for interference with bacterial growth. Here, we report a convenient method for the synthesis of such nisin conjugates and show that these nisin derivatives retain both their antimicrobial activity and their membrane permeabilizing properties. The synthesis is based on the Cu(I)-catalyzed alkyne-azide cycloaddition reaction (CuAAC) as a bioorthogonal ligation method for large and unprotected peptides in which nisin was C-terminally modified with propargylamine and subsequently efficiently conjugated to a series of functionalized azides. Two fluorescently labeled nisin conjugates together with a dimeric nisin construct were prepared while membrane insertion as well as antimicrobial activity were unaffected by these modifications. This study shows that C-terminal modification of nisin does not deteriorate biological activity in sharp contrast to N-terminal modification and therefore C-terminally modified nisin analogues are valuable tools to study the antibacterial mode of action of nisin. Furthermore, the ability to use stoichiometric amounts of the azide containing molecule opens up possibilities for surface tethering and more complex multivalent structures.

Improving the biological activity of the antimicrobial peptide anoplin by membrane anchoring through a lipophilic amino acid derivative.

The lipophilic amino acid, (S)-2-aminoundecanoic acid, was synthesized and incorporated at a number of specific positions within the peptide sequence of anoplin. These lipophilic anoplin analogs showed to be more active against Escherichia coli and Staphylococcus aureus compared to native anoplin, while the EC50-value of hemolysis was at least one order of magnitude lower than the MIC values. This was in sharp contrast to the N-acylated anoplin derivative, where a gain in activity also led to a complete loss of selectivity. Thus, the incorporation of a lipophilic amino acid residue into anoplin enhanced the antimicrobial activity, while selectivity towards microbial membranes was retained.

Synthesis and structural characterization of the individual diastereoisomers of a cross-stapled alkene-bridged nisin DE-ring mimic.

Herein, we describe the synthesis, structural characterization, and synthetic use as an advanced intermediate of a cross-stapled alkene-bridged hexapeptide to mimic the DE-ring of the lantibiotic nisin. The linear precursor was cyclized by ring-closing metathesis to give the correctly folded bicyclic hexapeptide in a single step, and the four individual diastereoisomers were isolated, structurally assigned and characterized by HPLC, NMR and MS, respectively. The bicyclic hexapeptide was used as a versatile advanced synthon and was modified at its C- and N-terminus, among others, with an azide moiety to access a building block suitable for Cu(I)-catalyzed alkyne-azide cycloaddition-based ligation reactions.

Scalable purification of the lantibiotic nisin and isolation of chemical/enzymatic cleavage fragments suitable for semi-synthesis.

Herein, we describe a scalable purification of the lantibiotic nisin via an extraction/precipitation approach using a biphasic system, which can be carried out up to 40-80 gram scale. This approach results in an at least tenfold enrichment of commercially available preparations of nisin, which usually contain only 2.5% of the desired peptide, to allow further purification by preparative HPLC. As a follow-up study, the enriched nisin sample was digested either by trypsin or chymotrypsin, or treated by CNBr, and these reactions were monitored by LC-MS to identify and characterize the obtained fragments. Two previously unknown cleavage sites have been identified: Asn20-Met21 and Met21-Lys22 for trypsin and chymotrypsin, respectively. Furthermore, a novel and convenient enzymatic approach to isolate the native nisin C-ring [nisin fragment (13-20)] was uncovered. Finally, by means of preparative HPLC, nisin fragments (1-12), (1-20), (22-34), and (22-31) could be isolated and will be used in a semi-synthesis approach to elucidate the role of each fragment in the mode of action of nisin as an antimicrobial peptide.

Main phenolic compounds of the melanin biosynthesis pathway in bruising-tolerant and bruising-sensitive button mushroom (Agaricus bisporus) strains.

Browning is one of the most common postharvest changes in button mushrooms, which often results in economic losses. Phenolic compounds, which are associated with browning, were extracted from the nonbruised and bruised skin tissue of various button mushrooms with a sulfite-containing solution and analyzed with UHPLC-PDA-MS. In total, 34 phenolic compounds were detected. Only small differences in the total phenolic content between bruising-tolerant and -sensitive strains were observed. The contents of γ-L-glutaminyl-4-hydroxybenzene (GHB) and γ-L-glutaminyl-3,4-dihydroxybenzene (GDHB) correlated with bruising sensitivity; for example, R(2) values of 0.85 and 0.98 were found for nonbruised brown strains, respectively. In nonbruised skin tissue of the strains with brown caps, the GHB and GDHB contents in sensitive strains were on average 20 and 15 times higher, respectively, than in tolerant strains. GHB and GDHB likely participate in the formation of brown GHB-melanin, which seemed to be the predominant pathway in bruising-related discoloration of button mushrooms.