Supramolecular recognition of thymine (or its analogs) with various central cations can form magic number clusters. Dual nano-ESI via theta tip emitters was used to on-line synthesize clusters. Even thermodynamically unstable clusters can be detected by MS thanks to the very short life-time (∼ms) of the generated microdroplets. By recording characteristic cluster distributions, isomers can be clearly differentiated in a novel “bottom-up” way. Theoretical calculations were performed to explain the MS results.

The surface of a micro-droplet can be treated as a simplified model to study reactions on a cell membrane. A novel group of heterogeneous cations induced clusters (HeteroCICs) that formed at the surface of micro-droplets was discovered by mass spectrometry (MS). This novel category of clusters are produced through two types of cations co-induced self assemblies of thymine molecules. Applying this phenomenon, an on-line derivatization and quantitation method of “ambient MS” style has been established, which can be used to real time monitor fish freshness changes during shelf-life.

A multichannel rotating electrospray ionization (MRESI) mass spectrometry method is described. It is carried out by rotating three spray emitters infused with the same or different kinds of solvents or solutions. Rotation effect is shown as a critical factor in equalizing the sampling processes from different channels. The interactions of plumes generated from different spray emitters in MRESI has been systematically studied. Due to electrostatic repulsion, liquid fusion of charged droplets from different channels does not occur, while volatile reagents can induce reactions between sprays by vaporizing into neutral gas phase molecules which then extracted by other droplets. When the method is coupled to a laser desorption technique, the desorbed analytes can be post-ionized by multiple electrosprays concurrently, providing a more informative MS detection of a complex sample.

Besides analytical applications, ambient mass spectrometry (MS) is also a special platform to execute various chemical reactions. Chemistry in laser desorption-based, ambient MS ion sources has unique features. Previously, laser-ablated copper(I) ions were used to catalyze in-source click reactions. The advances of such in-source reaction are presented here. First, the encounter and structure elucidation of a copper(I) triazole dimer are demonstrated. Then, based on the quick appearance of such dimer in the HALDI-MS spectrum, the rate acceleration effect is studied. Last, the crystalized dimer is used to catalyze the in-flask click reaction, and confirms the proposed “di-nuclear mechanism” of the click reaction. This work is another example of how ambient in-source chemistry having unique features compares with traditional in-flask chemistry, and also shows that new insightful information can be obtained from results of ambient in-source reactions.

A label-free double amplification system has been developed by using a ternary DNA probe containing the poly(adenine-thymine) sequence assisted by exonuclease III degradation. The method achieved more than 600-fold signal amplification and allowed sensitive detection of single-stranded DNA and thrombin at the pM level by using liquid chromatography/mass spectrometry.

A click reaction (copper-catalyzed azide–alkyne cycloaddition) catalyzed by “naked” copper ions (without ligands) generated in situ from a copper substrate by laser ablation in a high-voltage assisted laser desorption/ionization (HALDI) ion source is demonstrated.

A novel method for the detection of histamine (HIM) via the formation of a self-assembled magic number cluster with thymine (T) by electrospray ionization tandem mass spectrometry (ESI-MS/MS) is described. The formation of the magic number cluster [T17 + HIM + 2H]2+ shifts the MS signal of histamine to the interference-free higher mass range and the signal intensity is increased by four orders of magnitude. In addition, the formation of [T17 + HIM + 2H]2+ is highly specific to histamine compared with its metabolite and other similar biogenic amines, which may be attributed to both of its amino and imidazole groups. The linear dynamic range of the method is in the range of 1 nM–20 μM, and the limit of detection can be as low as 0.1 nM. The feasibility of this method is further demonstrated by the quantitative analysis of histamine in a red wine sample. Since little sample preparation or separation is required before the analysis, this method provides a rapid new way for the sensitive and specific detection of histamine by MS.

An atmospheric pressure laser desorption ionization mass spectrometry (APLDI-MS) method was developed for the fast and direct analysis of active ingredients in drug tablets. The tablets containing several active ingredients were directly analyzed by APLDI-MS without any sample preparation, and the identities of the active ingredients were further confirmed by tandem MS. The ionization of the active ingredients in the tablets by APLDI did not need an applied high voltage or any auxiliary solvent or gas. In addition, the good penetration depth of the near-infrared pulse laser of APLDI enabled it to directly ionize the active ingredient (s) in a drug tablet even with a coated film. This simple, rapid and direct APLDI-MS method is very promising for the fast screening of drugs and for on-line quality control in pharmaceutical industry.No matter the tablets containing one active ingredient or more than one active ingredient, no matter the tablets coated with a film or not, they can be directly analyzed by APLDI-MS without any sample preparation.

We describe complexation reactions of insulin and other proteins with metal ions generated from the substrate surface by laser irradiation in laser desorption spray post-ionization mass spectrometry (LDSPI-MS). This particular type of complexation reaction in LDSPI-MS was investigated for the first time, which indicated that the mechanistic process of LDSPI-MS might be much more complicated than that proposed before for similar methods.

G-quadruplex motifs are known to be present in telomeres of human and other organisms. Recent bioinformatic studies also revealed the widespread existence of these motifs in promoter regions of human genes. Treatment of cultured cells with 5-bromo-2′-deoxyuridine (BrdU) is known to result in the substitution of DNA thymidine with BrdU; such replacement has been shown to sensitize cells to killing induced by UV light. Our previous studies revealed that the exposure of BrdU-carrying duplex DNA or BrdU-treated MCF-7 cells to UVB light could lead to the facile formation of intrastrand cross-link products initiated from BrdU. Here we found that the exposure of BrdU-bearing G-quadruplex DNA to UVA light could also give rise to the efficient formation of the G[8−5]U intrastrand cross-link, where the C8 of guanine in the external G-tetrad is covalently linked with the C5 of its adjacent 3′ uracil in the loop region. In addition, the yield for the cross-link product is dependent on the conformation of the G-quadruplex. Together, the formation of intrastrand cross-link in G-quadruplex motifs may account for the photocytotoxic effect induced by BrdU incorporation, and the BrdU-mediated photo-cross-linking may constitute a useful method for monitoring the different conformations of G-quadruplex folding.

ESI-MS and molecular dynamic calculations reveal that in the presence of K+, Rb+ and Cs+, uracil, thymine and their homologues form self-assembled quintet structures that are stabilized by hydrogen bonding and ion dipole interactions.

Upon exposure to UVA light, psoralens can induce DNA interstrand cross-links (ICLs), which can block DNA replication and transcription. Among the psoralen derivatives, 8-methoxypsoralen (8-MOP) is conventionally applied for psoriasis therapy, and amotosalen S59 is used to inactivate bacterial and viral pathogens in blood components. In addition to the ICL formation, psoralens also readily form various monoadducts (MAs) with thymidine residues in DNA when exposed to UVA light, and the biological implications for these monoadducts remain unclear. Here, we reported a method that encompassed digestion with a single enzyme (nuclease P1) and LC−MS/MS, for the simultaneous quantification of ICL and MAs induced in human cells exposed with 8-MOP or S59 and UVA light. Our results showed that the yield of ICL induced by S59, which increased from 3.9 to 12.8 lesions/103 nucleotides as the dose of UVA light increased from 0.5 to 10.0 J/cm2, was ∼100 fold more than that induced by 8-MOP. In addition, three and five products were identified as 8-MOP- and S59-MAs, respectively, and the yields of MAs were significantly lower than that for ICL. The yields of the three 8-MOP-MAs were 7.6−2.2, 1.9−9.9, and 7.2−51 per 106 nucleotides and those of the five S59-MAs were 215−19, 106−39, 25−21, 32−146, and 22−26 per 106 nucleotides as the dose of UVA light increased from 0.5 to 10.0 J/cm2. Although the yields of MAs induced by 8-MOP and S59 were lower than those of the respective ICLs under the same exposure conditions, the formation of appreciable amounts of MAs might account for some of the mutations induced by psoralens.

A label-free double amplification system has been developed by using a ternary DNA probe containing the poly(adenine-thymine) sequence assisted by exonuclease III degradation. The method achieved more than 600-fold signal amplification and allowed sensitive detection of single-stranded DNA and thrombin at the pM level by using liquid chromatography/mass spectrometry.

ESI-MS and molecular dynamic calculations reveal that in the presence of K+, Rb+ and Cs+, uracil, thymine and their homologues form self-assembled quintet structures that are stabilized by hydrogen bonding and ion dipole interactions.