Proteomics for Microalgae Extracts by High-Resolution Mass Spectrometry

Two protocols of protein extraction from Arthrospira platensis (spirulina) microalgae to study their proteome by mass spectrometry (MS) are here presented. The first is based on an aqueous buffer solution of Tris-HCl and the second on cold acetone. The identification of proteins was carried out by a bottom-up approach, which involves enzymatic digestion of extracted proteins followed by either matrix-assisted laser desorption ionization with time-of-flight (MALDI-TOF) MS or liquid chromatography (LC) coupled with electrospray ionization (ESI) and Fourier-transform tandem MS. While MALDI-TOF MS allowed for a fast peptide mass fingerprinting (PMF) check yet identifying less than 20 proteins in the extracted samples, the data-dependent acquisitions (DDA) mode of reversed-phase (RP) LC-ESI tandem FTMS/MS separations allowed us to recognize more than one hundred proteins by searching into dedicated spectral libraries. The application of MALDI-TOF MS analysis was found, however, of great support for preliminary investigations of cyanobacteria samples before proceeding with the RPLC-ESI-MS/MS DDA investigation, which definitively allows for a qualitative proteome analysis also of minor spirulina proteins in processed foodstuffs. Although the protein content in spirulina can be influenced by cultivation and environmental conditions, e.g., light intensity, climate, and water/air quality, here the qualitative chemical profiles of the examined samples were characterized by similar composition in high-quality proteins as phycocyanins and phycoerythrins.

Synthesis and Investigation of Novel CHCA-Derived Matrices for Matrix-Assisted Laser Desorption/Ionization Mass Spectrometric Analysis of Lipids

A significant area of study and upgrading for increasing sensitivity and general performances of matrix-assisted laser-desorption ionization (MALDI) mass spectrometry (MS) is related to matrix design. Several efforts have been made to address the challenge of low-mass-region interference-free for metabolomics analysis and specifically for lipidomics. To this aim, rationally designed matrices as 4-chloro-α-cyanocinnamic acid (ClCCA) were introduced and reported to provide enhanced analytical performances. We have taken this rational design one step further by developing and optimizing new MALDI matrices with a range of modifications on the CHCA core, involving different functionalities and substituents. Of particular interest was the understanding of the electron-withdrawing (e.g., nitro-) or donating (e.g., methoxy-) effects along with the extent of conjugation on the ionization efficiency. In the present work, ten matrices were designed on a reasonable basis, synthesized, and characterized by NMR and UV spectroscopies and laser desorption ionization. With the assistance of these putative MALDI matrices, samples containing phospholipids (PL), and neutral di-/tri-acylglycerols (DAG, TAG) were investigated using milk, fish, blood, and human plasma extracts. In comparison with CHCA and ClCCA, four of them, viz. [(2E,4E)-2-cyano-5-(4-methoxyphenyl)penta-2,4-dienoic acid] (1), [(2E,4E)-2-cyano-5-(4-nitrophenyl)penta-2,4-dienoic acid] (2), [(E)-2-cyano-3-(6-methoxynaphthalen-2-yl)acrylic acid] (6) and [(E)-2-cyano-3-(naphthalen-2-yl)acrylic acid] (7) displayed good to even excellent performances as MALDI matrices in terms of ionization capability, interference-free spectra, S/N ratio, and reproducibility. Especially compound 7 (cyano naphthyl acrylic acid, CNAA) was the election matrix for PL analysis and matrix 2 (cyano nitrophenyl dienoic acid, CNDA) for neutral lipids such as DAG and TAG in positive ion mode.

Ligand-Tuning of the Stability of Pd(II) Conjugates with Cyanocobalamin

Besides the well-known functions performed by vitamin B₁₂ (CblCN) in biochemical processes of the human body, an increasing interest has been raised by the possibility of its use as a transmembrane drug carrier, capable, among others, of enhancing the accumulation of inorganic cytostatics in cancer cells. The present study was aimed at determining the possibility of the formation of CblCN conjugates with Pd(II) complexes. A key aspect was their stability, which we attempted to tune by appropriate choice of ligands. Syntheses, spectroscopic analysis of postreaction systems and kinetic investigations of conjugate formation reactions, have been complemented by DFT modelling. The obtained results showed that ligand charge, geometry and electron affinity may have a significant impact on carrier binding and release leading to the activation of the Pd(II) complex. This provides a rationale to expect that with appropriate composition of the coordination sphere, it will be possible to extend the spectrum of less toxic inorganic chemotherapeutics.

In vitro reactions of a cyanocobalamin-cisplatin conjugate with nucleoside monophosphates

RATIONALE

Cisplatin (CP) is a widely used anticancer drug characterized by toxic side effects that could be alleviated using novel delivery systems including CP prodrugs. The in vitro incubation of a putative prodrug, obtained from cyanocobalamin (CNCbl) and cis-diamminemonochloroplatinum(II) (mCP), with nucleoside monophosphates (NMPs) was investigated.

METHODS

The in vitro reactions between the putative prodrug CNCbl-mCP and the NMPs of adenosine (AMP), guanosine (GMP), cytidine (CMP) and uridine (UMP) were carried out in slightly acidic water-methanol solutions at 37°C for 24 h. Each sample was examined using reversed-phase liquid chromatography coupled with electrospray ionization in positive ion mode and tandem mass spectrometry (RPLC/ESI-MS/MS) by collision-induced dissociation in a linear ion-trap mass spectrometer.

RESULTS

Seven adducts were recognized as formed by substitution reactions of the chloride ligand in planar CP. Comparison between observed and theoretical isotopic patterns together with MS/MS fragmentation pathways revealed the presence of single or multiple binding sites depending on the NMP involved. The CNCbl-mCP conjugate was found to interact with N⁷ or O⁴ atoms of GMP and UMP, respectively, generating single adducts, while two isomeric adducts were observed for CMP. Finally, AMP gave rise to three isomeric adducts.

CONCLUSIONS

In agreement with literature data relevant to the interaction between CP and NMPs, the most reactive nucleotides were AMP and GMP. The present RPLC/ESI-MS/MS approach is very promising for investigation of the reactions of CP conjugates with ribonucleotides not only in vitro but also in vivo.

Characterization of 6-bromoferulic acid as a novel common-use matrix for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

RATIONALE

Ferulic acid (FA) is a standard matrix used for analyzing proteins. In this study, the ability of a halogenated FA to serve as an effective MALDI matrix was investigated. Various halogenated FAs were synthesized, and the characteristics and performance of each were compared with those of the standard matrices α-cyano-4-hydroxycinnamic acid (CHCA) and 2,5-dihydrobenzoic acid (DHBA).

METHODS

The abilities of 6-bromoferulic acid (6-BFA), ferulic acid (FA), and eight other halogenated FA derivatives to ionize eight synthetic peptides were examined. Absorption measurements, MM2 structure optimizations, and proton affinity (PA) calculations were also performed for 6-BFA and FA. The suitabilities of these compounds as matrices for matrix-assisted laser desorption/ionization (MALDI) for lipids, sugar chains, polymers, cyanocobalamin, synthetic peptides, and tryptic peptides originating from two types of serum proteins were also tested.

RESULTS

The 6-position of FA was found to be the best site for introducing a bromine because the generated compound allowed facile detection of cyanocobalamin and several peptides. 6-BFA exhibited good sensitivity for large peptides (3-5 kDa) and peptides containing acidic amino acids or proline. 6-BFA was also shown to be a suitable matrix for tandem mass spectrometry (MS/MS) analysis when using MALDI time-of-flight (TOF) mass spectrometry (MS) with a quadrupole ion trap (QIT) system.

CONCLUSIONS

The properties of 6-BFA as a MALDI matrix differed from those of DHBA and CHCA. 6-BFA appears to be a useful matrix for de novo sequencing using MALDI-QIT-TOF-MS.

Tandem mass spectrometry characterization of a conjugate between oleuropein and hydrated cis-diammineplatinum(II)

RATIONALE

Oleuropein (Ole) has been claimed to mitigate cisplatin (CP)-induced acute injury in kidney and liver of mice. In vitro reactivity of hydrated CP species with Ole, and an Ole metabolite, hydroxytyrosol (HT), is of great interest as the preliminary step for gathering in vivo information on the possible physiological role of the Ole/HT-cis-diammineplatinum(II) (Ole/HT-cis-DAP) conjugate.

METHODS

Reversed-phase liquid chromatography coupled to electrospray ionization mass spectrometry using a linear ion trap instrument (RPLC/ESI-MS) and tandem mass (MS/MS) measurements, both in positive and negative ion mode, revealed the molecular identity of platinum-based conjugates.

RESULTS

The Ole-cis-DAP conjugate (i.e., C₂₅ H₃₆ N₂ O₁₃ Pt^II ) features two cis-ammine non-leaving ligands and a bidentate catechol ligand moiety belonging to Ole; the coordination of the central Pt(II) is square-planar with non-equivalent bond angles compared with the ideal arrangement of 90°. HT, the free Ole metabolite excreted in human urine, acts as bidentate O,O-donor ligand of cis-DAP as well.

CONCLUSIONS

The first evidence, together with structural information, is provided about the in vitro formation of a conjugate between cis-DAP and Ole or its urinary metabolite HT. Presuming that such conjugates are also generated in vivo, the mechanisms by which they might contribute to reduce CP toxicity remain to be elucidated.

Structural Elucidation of Cisplatin and Hydrated cis-Diammineplatinum(II) Complex Conjugated with Cyanocobalamin by Liquid Chromatography with Electrospray Ionization-Mass Spectrometry and Multistage Mass Spectrometry

Pt(II)-based derivatives bearing a cyanocobalamin (CNCbl) unit were synthesized in aqueous solutions, and the reaction mixtures were examined by reversed-phase liquid chromatography with electrospray ionization and linear ion trap mass spectrometry (MS). Isotopic pattern analysis, multistage mass-spectra (MS/MS and MS³) interpretation, and differential isotopic labeling were used to establish the chemical composition and to suggest the chemical structures of reaction products. When cisplatin (cis-[PtCl₂(NH₃)₂]) was used as a Pt(II) drug derivative, a coordination bond between diamminemonochloroplatinum(II) and the cyano group of CNCbl, in turn linked covalently to the vitamin Co(III) ion, occurred. The resulting conjugate with a Co^III-CN-Pt^II bridge was MS detected as a doubly positive charged ion with the prevailing isotopologue at m/z 810.26 (empirical formula [C₆₃H₉₅ClCo^IIIN₁₆O₁₄PPt]²⁺). Likewise, a peak signal centered at m/z 811.26 was observed when ¹⁵N-labeled cisplatin cis-[PtCl₂(¹⁵NH₃)₂] was used as Pt(II) complex, thus confirming the presence of both the cisplatin amino groups in the conjugate. A bifunctional conjugate was obtained between CNCbl and the cis-diamminediaquaplatinum(II), that is, cis-[Pt(NH₃)₂(H₂O)₂]²⁺; in this case, the planar coordination complex of Pt(II) was also involved in a covalent bond with the oxygen atom of one of the CNCbl amide moieties. The peak signal detected at m/z 792.26 (empirical formula [C₆₃H₉₄Co^IIIN₁₆O₁₄PPt]²⁺) changed to m/z 793.26 when the labeled cis-[Pt(¹⁵NH₃)₂(H₂O)₂]²⁺ complex was adopted for conjugation. Comparison between MS/MS spectra allowed an extended structural characterization of both conjugates, as such or ¹⁵N-labeled. Two-dimensional heteronuclear (¹H-¹⁵N) single quantum correlation NMR spectroscopy, applied to ¹⁵N-labeled conjugates, supported the hypotheses made on the Pt(II) coordination in both cases.