A new method for studying the folding kinetics of proteins is described. The method combines a continuous flow mixing technique with an electrospray mass spectrometer. Different protein conformations in solution are detected by the different charge states they produce during electrospray ionization. Unfolded proteins generally have more accessible protonation sites and give higher charge states than native proteins. The method is applied to study the refolding of acid-denatured cytochrome c. Global data analysis is used to obtain the exponential lifetimes which are associated with the refolding process. The kinetics can be described by two lifetimes of 0.17 +/- 0.02 and 8.1 +/- 0.9 s which are in accordance with the results of stopped flow experiments previously described in the literature. These lifetimes are associated with roughly 90 and 10% of the total intensity changes in the mass spectrum, respectively, and most likely reflect fast and slow refolding subpopulations of cytochrome c in solution.

No abstract

No abstract

The optical absorption spectra of a series of small gold clusters and their cations have been measured, between 1.9 and 5.6 eV, using a method based upon the photodepletion of a molecular beam of their van der Waals complexes containing one and two xenon atoms. This method provides size-specific information even though the molecular beam contains a wide range of cluster sizes. There is little difference between the spectra of complexes containing one or two xenon atoms. However there is a pronounced odd–even alternation in the spectra of gold clusters with differing numbers of valence s electrons. This alternation is described in terms of a simple electron pairing scheme. The spectrum for Au13 is in reasonable agreement with Dirac scattered-wave molecular orbital considerations for icosahedral Au13 [A. F. Ramos, R. Arratia-Perez, and G. L. Malli, Phys. Rev. B 35, 3790 (1987)]. This description of the molecular and electronic structure of small gold clusters in terms of localized molecular orbitals is contrasted with other models based upon jellium potentials and delocalized excitations that have been used to describe small clusters of alkali metals and silver. The bonding in gold clusters is influenced by relativistic effects that increase the degree of sd hybridization in the molecular orbitals. Even though gold clusters can be described in this way, some evidence for electron shells is also presented. Thus, it is concluded that structural motifs other than jellium potentials can lead to shell structure in cluster properties.

A detailed description of a linear ion trap time-of-flight (TOF) mass spectrometer system, capable of sequential mass spectrometry (MS(n)), is given. Many improvements have been incorporated since the initial description of this system (Rapid Commun. Mass Spectrom. 1998; 12: 1463-1474). The pressure in the trap has been lowered from 7.0 to 1.8 mTorr, resulting in an increase in the mass resolution of ion excitation from 75 to 240. Use of the system for MS(3) is demonstrated. Dipole excitation of the n = 1 harmonic, instead of the n = 0 fundamental frequency of ion motion, is shown to have a higher frequency resolution, f/Deltaf, but lower mass resolution, m/Deltam. Both experiments and modeling demonstrate that at the lower pressure there is less collisional cooling of ions in the axial and radial directions of the trap. The efficiency of trapping is shown to be nearly 100% for periods up to 5 s. The demonstrated mass range for mass analysis has been extended to greater than m/z 16 250. To avoid the formation of adduct ions when trapping protein ions for extended times requires ultra-high vacuum cleanliness conditions, even though the trap operates in the mTorr-pressure range. Upgrading the TOF to a reflectron with higher quality ion optics results in an increase in the mass resolution of the TOF mass spectrometer to about 5000 at m/z 750.