Field-flow fractionation in bioanalysis: A review of recent trends

Field-flow fractionation (FFF) is a mature technique in bioanalysis, and the number of applications to proteins and protein complexes, viruses, derivatized nano- and micronsized beads, sub-cellular units, and whole cell separation is constantly increasing. This can be ascribed to the non-invasivity of FFF when directly applied to biosamples. FFF is carried out in an open-channel structure by a flow stream of a mobile phase of any composition, and it is solely based on the interaction of the analytes with a perpendicularly applied field. For these reasons, fractionation is developed without surface interaction of the analyte with packing or gel media and without using degrading mobile phases. The fractionation device can be also easily sterilized, and analytes can be maintained under a bio-friendly environment. This allows to maintain native conditions of the sample in solution.In this review, FFF principles are briefly described, and some pioneering developments and applications in the bioanalytical field are tabled before detailed report of most recent FFF applications obtained also with the hyphenation of FFF with highly specific, sensitive characterization methods. Special focus is finally given to the emerging use of FFF as a pre-analytical step for mass-based identification and characterization of proteins and protein complexes in proteomics.     

Polymorphism and Phonon Dynamics of α‐Quaterthiophene

Two 4T: Low‐frequency micro‐Raman spectroscopy coupled with lattice dynamics calculations is an invaluable tool for investigating polymorphism in organic semiconductors. The Raman spectra of the low‐temperature (LT) and high‐temperature (HT) polymorphs of α‐quaterthiophene (4T) are presented and interpreted (see picture). Raman mapping is applied to investigate the phase purity.

     

Regular interval Cantor sets of <Emphasis Type="Italic">S</Emphasis><Superscript>1</Superscript> and minimality

It is known that not every Cantor set of S ¹ is C ¹-minimal. In this work we prove that every member of a subfamily of what we here call regular interval Cantor set is not C ¹-minimal. We also prove that no member of a class of Cantor sets that includes this subfamily is C ^1+∈-minimal, for any ∈ > 0. Partially supported by CNPq-Brasil and PEDECIBA-Uruguay.     

Understanding the inelastic electron-tunneling spectra of alkanedithiols on gold

We present results for a simulated inelastic electron-tunneling spectra (IETS) from calculations using the "gDFTB" code. The geometric and electronic structure is obtained from calculations using a local-basis density-functional scheme, and a nonequilibrium Green's function formalism is employed to deal with the transport aspects of the problem. The calculated spectrum of octanedithiol on gold(111) shows good agreement with experimental results and suggests further details in the assignment of such spectra. We show that some low-energy peaks, unassigned in the experimental spectrum, occur in a region where a number of molecular modes are predicted to be active, suggesting that these modes are the cause of the peaks rather than a matrix signal, as previously postulated. The simulations also reveal the qualitative nature of the processes dominating IETS. It is highly sensitive only to the vibrational motions that occur in the regions of the molecule where there is electron density in the low-voltage conduction channel. This result is illustrated with an examination of the predicted variation of IETS with binding site and alkane chain length.     

Hollow-fiber flow field-flow fractionation: a gentle separation method for mass spectrometry of native proteins

Low-impact ionization sources like electrospray ionization (ESI) and matrix-assisted, laser desorption/ionization (MALDI) equipped with time-of-flight (TOF) mass analyzers provide intact protein analysis over a very wide molar mass range. ESI/TOFMS provides also indications on the higher-order structure of intact proteins and non-covalent protein complexes. However, direct analysis of intact proteins mixtures in real samples shows limited success, mainly because spectra become very complex to interpret. This is also due to sample contaminants, and to the mechanism of competitive ionization in ESI or MALDI. Rapid and efficient sample clean-up and separation methods can significantly enhance the power of TOFMS for intact protein analysis. However, if protein native conditions want to be maintained, the methods should affect neither the three-dimensional structure nor the non-covalent chemistry of the proteins. Reversed-phase (RP) HPLC, size-exclusion chromatography (SEC), and capillary zone electrophoresis (CZE) are on-line or off-line coupled to ESI/TOFMS or MALDI/TOFMS. In fact, these separation methods often show limitations when applied to the analysis of native proteins. Organic modifiers and saline buffers are required in the case of RP HPLC or CZE. They can induce protein degradation or affect ionization when MS is performed after separation. High voltages used in CZE can contribute to alter proteins from their native form. In the case of high molar mass proteins, SEC is scarcely selective, and barely able to detect protein aggregates. Sample entanglement/adsorption on the stationary phase can also occur.     

Aflatoxin M1 determination in cheese by liquid chromatography-tandem mass spectrometry

A new method for determining aflatoxin M1 (AFM1) in cheese by liquid chromatography-tandem mass spectrometry has been developed. Two methodologies were compared for sample extraction. The first one involves sample extraction with dichloromethane for hard, aged cheese or acetone for fresh cheese and includes a preliminary matrix solid-phase dispersion-extraction step before solid-phase extraction (SPE) clean-up by a Carbograph-4 cartridge. The second method uses a water/methanol solution (90:10, v/v) extraction at 150 degrees C before clean-up. The average recoveries of AFM1 from samples spiked at levels of 0.25-0.45 microg/kg, were 81-92% and the precision (RSD) ranged from 3 to 7% with the first method, whilst the average recoveries were 79-84%, and RSD ranged from 7 to 15% for the second method. Due to different matrix effect, the quantification limits were 0.019-0.025 microg/kg in the first case and 0.048-0.143 microg/kg in the second one, depending on cheese typology.     

Proteomics of bovine myelin sheath: Characterization of a truncated form of P0 by MALDI-TOF/TOF mass spectrometry

The glycoprotein P0, the major structural protein of the peripheral nerve myelin, plays a critical role in holding myelin lamellae together via interaction of both extracellular and cytoplasmic domains. Mutations in the human P0 gene give rise to severe and progressive forms of dominantly inherited peripheral neuropathies like CMT1B. Here we report on the characterization of a bovine P0-derived protein of nearly 26 kD that corresponds to the P0 protein truncated in its cytoplasmic domain. Matrix assisted laser desorption ionization (MALDI)-time-of-flight/time-of-flight (TOF/TOF) mass spectrometry (MS) analysis on its tryptic digest has provided a peptide mapping, the main difference of which from the normal P0 analog was represented by the absence of the cluster of peaks at m/z 1513.7501, 1530.7701, and 1546.7651. The latter corresponds to the P0 fragment QTPVLYAMLDHSR and to its pyroglutamic and methionine-oxidized derivatives. The species at 1530.7701 covering the sequence 186-198 of P0 is not an artifact and might have a functional role in the myelin architecture.