Negative SPR Signals during Low Molecular Weight Analyte Recognition.

Surface plasmon resonance (SPR) is a powerful technique for studying biomolecular interactions mainly due to its sensitivity and real-time and label free advantages. While SPR signals are usually positive, only a few studies have reported sensorgrams with negative signals. The aim of the present work is to investigate and to explain the observation of negative SPR signals with the hypothesis that it reflects major changes in ligand conformation resulting from target binding. We demonstrated that these negative unconventional signals were due to the negative complex (ligand/analyte) refractive index increment (RII) deviation from the sum of the RII of the individual entities which counterbalanced the theoretical increase of the signal triggered by the target recognition and the ligand folding. We also found that the conformation change of biomolecules can induce a negative or a positive complex RII deviation depending on its sequence and immobilization mode.

Multiphoton ionization of large water clusters.

Water clusters are multimers of water molecules held together by hydrogen bonds. In the present work, multiphoton ionization in the UV range coupled with time of flight mass spectrometry has been applied to water clusters with up to 160 molecules in order to obtain information on the electronic states of clusters of different sizes up to dimensions that can approximate the bulk phase. The dependence of ion intensities of water clusters and their metastable fragments produced by laser ionization at 355 nm on laser power density indicates a (3+1)-photon resonance-enhanced multiphoton ionization process. It also explains the large increase of ionization efficiency at 355 nm compared to that at 266 nm. Indeed, it was found, by applying both nanosecond and picosecond laser ionization with the two different UV wavelengths, that no water cluster sequences after n = 9 could be observed at 266 nm, whereas water clusters up to m/z 2000 Th in reflectron mode and m/z 3000 Th in linear mode were detected at 355 nm. The agreement between our findings on clusters of water, especially true in the range with n > 10, and reported data for liquid water supports the hypothesis that clusters above a critical dimension can approximate the liquid phase. It should thus be possible to study clusters just above 10 water molecules, for getting information on the bulk phase structure.

Aldehydic oligonucleotide: a key intermediate for the preparation of oligonucleotide conjugates through oxime bond formation.

Oligonucleotides functionalized with an aldehyde group are the key intermediates used for the preparation of peptide-oligonucleotide conjugates through the formation of an oxime linkage. Herein, we describe a brief overview of various synthetic protocols developed in our laboratory for the preparation of aldehyde containing oligonucleotides and their subsequent conjugation with peptides.

Pro-oligonucleotide synthesis using allyl and allyloxycarbonyl protections: direct MALDI-TOF MS analysis on solid support.

The solid-support synthesis of pro-oligonucleotide heteropolymer chimeras had been performed with allyloxycarbonyl group (AOC) for the protection of nucleobases and of allyl and S-acetyl-2-thioethyl (MeSATE) for phosphate protections to respectively generate phosphodiester and MeSATE phosphotriester linkages.

Direct MALDI-TOF MS analysis of oligonucleotides on solid support through a photolabile linker.

MALDI-TOF mass spectrometry was used to analyze oligonucleotides still anchored to long-chain alkylamine controlled-pore glass (LCAA-CPG) through a photolabile linker. This technique is useful to follow supported chemical reactions in real time and monitor by-products formation.

Analysis of solid-supported oligonucleotides by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

This study presents matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) as a powerful tool to analyze and characterize oligonucleotides covalently linked to a solid support during their synthesis. The analysis of the fragment ions generated either in negative or positive mode allows direct and easy access to the nucleotide sequence and identification of the internucleosidic linkage. The mechanisms of the fragmentation of the solid-supported oligonucleotides induced by MALDI-TOFMS are discussed. Copyright 2000 John Wiley & Sons, Ltd.

Sensitivity analysis of differential absorption lidar measurements in the mid-infrared region.

The availability of new laser sources that are tunable in the IR spectral region opens new perspectives for differential absorption lidar (DIAL) measurements. A region of particular interest is located in the near IR, where some of the atmospheric pollutants have absorption lines that permit monitoring of emissions from industrial plants and in urban areas. In DIAL measurements, the absorption lines for the species to be measured must be carefully chosen to prevent interference from other molecules, to minimize the dependence of the absorption cross section on temperature, and to optimize the measurements with respect to the optical depth. We analyze the influence of these factors and discuss a set of criteria for selecting the best pairs of wavelengths (lambda(on) and lambda(off)) to be used in DIAL measurements of several molecular species (HCl, CO, CO(2), NO(2), CH(4), H(2)O, and O(2)). Moreover, a sensitivity study has been carried out for selected lines in three different regimes: clean air, urban polluted air, and emission from an incinerator stack.

Aerosol observations by lidar in the nocturnal boundary layer.

Aerosol observations by lidar in the nocturnal boundary layer (NBL) were performed in Potenza, Southern Italy, from 20 January to 20 February 1997. Measurements during nine winter nights were considered, covering a variety of boundary-layer conditions. The vertical profiles of the aerosol backscattering coefficient at 355 and 723.37 nm were determined through a Klett-modified iterative procedure, assuming the extinction-to-backscattering ratio within the NBL has a constant value. Aerosol average size characteristics were retrieved from almost simultaneous profiles of the aerosol backscattering coefficient at 355 and 723.37 nm, the measurements being consistent with an accumulation mode radius not exceeding 0.4 microm. Similar results in terms of aerosol sizes were obtained from measurements of the extinction-to-backscattering ratio profile at 355 nm performed on six nights during the measurement campaign. Backscattering profiles at 723.37 nm were also converted into profiles of aerosol liquid water content.

Self-aligning lidar for the continuous monitoring of the atmosphere.

In a lidar system an accurate transmitter-receiver alignment is essential for correct results. Usually this optical adjustment is time consuming and requires the intervention of highly qualified personnel. As a solution to this problem, a fast and precise automatic alignment procedure is presented, based on a simple model of the transmitter-receiver overlap. The lidar mounted at the Naples University is used to test this method. A centering precision of few microradians is obtained through dedicated software controlling a gimbal-mounted mirror. The automatic alignment procedure is then assessed. In particular, the correctness of the center and of its error is determined. Finally, the system is applied to the monitoring of tropospheric aerosols, leading to the continuous retrieval of profiles with fine spatiotemporal resolution.

Analysis of the receiver response in lidar measurements.

We report on the calculation of the effective telescope area in lidar applications by a ray-tracing approach. This method allows one to consider the true experimental working conditions and hence to obtain accurate values of the effective telescope area as a function of the height. This in turn allows the retrieval of the signal from the ranges where the overlap function is not constant (e.g., lower ranges), thus increasing the useful range interval. Moreover, we show that the spherical mirrors are more appropriate than the parabolic ones for most of the lidar measurements, although a particular alignment procedure, such as the one we describe, must be used.

Mode size and time duration fluctuations in a picosecond Nd:YAG laser.

A new technique is successfully used to analyze in real time the pulse-to-pulse fluctuations of mode size and time duration in a picosecond Nd:YAG laser. In particular we show that the pulse length (30 psec) of our active-passive mode-locked Nd:YAG laser is stable to within 10% when the cavityis perfectly tuned and the saturable absorber is fresh. This technique is experimentally shown to be effective and reliable for real-time analysis of the stability of ultrashort laser pulses undera broad range of experimental conditions.