Characterization of silsesquioxanes by size-exclusion chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Tandem mass spectrometry of electrosprayed polyhedral oligomeric silsesquioxane compounds with different substituents

RATIONALE

Polyhedral oligomeric silsesquioxanes (POSSs), defined as [RSiO(3/2) ](n) with R designating an organic substituent, were considered here as models of highly cross-linked polysiloxanes, to be further used as references in tandem mass spectrometric characterization of plasma polymers of hexamethyldisiloxane, expected to be composed of organic polydimethylsiloxane (PDMS) and inorganic (SiO(x) ) silica-based parts. The collision-induced dissociation (CID) behavior of [RSiO(3/2) ](8) compounds was then studied as a function of the R substituent.

METHODS

POSS compounds were produced in the gas phase as ammonium adducts and the product ions generated upon CID, amongst which was the protonated precursor, were accurately mass measured in an orthogonal acceleration time-of-flight mass analyzer.

RESULTS

The presence of eight propylamine substituents was shown to induce sequential dehydration of the protonated precursor, ultimately leading to a complete unfolding of the POSS cage. Similar opening of the octahedron structure in the protonated molecule substituted with OSi(CH(3) )(3) , OSiH(CH(3) )(2) or OH (formed upon methanolysis of dimethylsiloxy substituents) was proposed to account for the product ions generated during their CID. Sequential charge-remote transfers of a methyl group (and of H in the case of dimethylsiloxy substituents) from one substituent group to a neighboring one was shown to lead to a linear co-oligomeric chain composed of randomly distributed siloxy-based monomers.

CONCLUSIONS

All the peaks observed in CID could be accounted for by applying dissociation reactions typically occurring in protonated polysiloxane-like oligomers. The large number of product ions observed in MS/MS was found to result from the variety of possible structural rearrangements, producing numerous linear isomeric forms of the dissociating species.

Study of three-dimensional configurations of (γ-methacryloxypropyl)-silsesquioxanes by ultraviolet laser matrix-assisted desorption/ionization time-of-flight mass spectrometry and quantum chemical calculation

[(3-Methacryloxy)propyl]silsesquioxanes (MSSO) were prepared from the hydrolytic condensation of [(3-methacryloxy)-propyl]trimethoxysilane (MPMS) in the presence of an acid catalyst (HCOOH). The proposed MSSO structures were characterized with Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) ((1)H, (13)C and (29)Si), and were assigned by ultraviolet laser matrix-assisted desorption/ionization time-of-flight mass spectrometry (UV-MALDI-TOF MS). The large organic group connected to silicon was simplified for the quantum chemical calculation (QCC), and the correlation of the calculated total energies (E(T)) before and after simplification was analyzed by multiple linear regression, verifying no significant influence on the final conclusions of the research of structural formulas by a correlation coefficient (r). The geometric parameters (Si-O bond length and Si-O-Si, O-Si-O bond angles) and E(T) of the simplified MSSO were calculated by QCC to determine the relative stability of various MSSO structures. The structural geometry (silicon ring), the fraction of intramolecular cycles (f) and the number of the silicon rings (F) were also employed to qualitatively determine the relative stability. The results of the calculation showed that almost all of the cage structures had a lower E(T) than the isomeric ladder structures; therefore, most MSSO structures are of the cage type.

Hydrogen exchange mass spectrometry for the analysis of protein dynamics

Hydrogen exchange coupled to mass spectrometry (MS) has become a valuable analytical tool for the study of protein dynamics. By combining information about protein dynamics with more classical functional data, a more thorough understanding of protein function can be obtained. In many cases, protein dynamics are directly related to specific protein functions such as conformational changes during enzyme activation or protein movements during binding. The method is made possible because labile backbone hydrogens in a protein will exchange with deuterium atoms when the protein is placed in a D2O solution. The subsequent increase in protein mass over time is measured with high-resolution MS. The location of the deuterium incorporation is determined by monitoring deuterium incorporation in peptic fragments that are produced after the labeling reaction. In this review, we will summarize the general principles of the method, discuss the latest variations on the experimental protocol that probe different types of protein movements, and review other recent work and improvements in the field.