Direct analysis of samples using atmospheric pressure ionization (API) provides a more rapid method for analysis of volatile and semivolatile compounds than vacuum solids probe methods and can be accomplished on commercial API mass spectrometers. With only a simple modification to either an electrospray (ESI) or atmospheric pressure chemical ionization (APCI) source, solid as well as liquid samples can be analyzed in seconds. The method acts as a fast solids/liquid probe introduction as well as an alternative to the new direct analysis in real time (DART) and desorption electrospray ionization (DESI) methods for many compound types. Vaporization of materials occurs in the hot nitrogen gas stream flowing from an ESI or APCI probe. Ionization of the thermally induced vapors occurs by corona discharge under standard APCI conditions. Accurate mass and mass-selected fragmentation are demonstrated as is the ability to obtain ions from biological tissue, currency, and other objects placed in the path of the hot nitrogen stream.

Multiply charged ions, similar to those obtained with electrospray ionization, are produced at atmospheric pressure (AP) using standard MALDI conditions of laser fluence and reflective geometry. Further, the charge state can be switched to singly charged ions nearly instantaneously by changing the voltage applied to the MALDI target plate. Under normal AP-MALDI operating conditions in which a voltage is applied to the target plate, primarily singly charged ions are observed, but at or near zero volts, highly charged ions are observed for peptides and proteins. Thus, switching between singly and multiply charged ions requires only manipulation of a single voltage. As in ESI, multiple charging, produced using the AP-MALDI source, allows compounds with molecular weights beyond the mass-to-charge limit of the mass spectrometer to be observed and improves the fragmentation relative to singly charged ions.

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Matrix-assisted laser desorption/ionization mass spectrometry (MS) has been shown to provide most probable peak (M p ) values for poly(methyl methacrylate) polymers that are low relative to manufacturers, M p values measured by size exclusion chromatography (SEC). Comparison of theoretical M p values determined by MS or SEC is shown to be a function of how the data are displayed. For narrow polymer distributions, the theoretical M p value determined by MS will be 2 monomer units smaller than the M p determined by SEC. For wide polydispersity, the M p value determined by MS will be considerably lower than those obtained from SEC. The M p value reported should be reserved for weight fraction vs log mass plots as in SEC. The modal molecular mass, M m , is recommended for data represented as number fraction vs linear mass as with MS data.

The molecular oxygen in our atmosphere is a product of a water-splitting reaction that occurs in the oxygenevolving complex of photosystem II of oxygenic photosynthesis. The catalytic core of the oxygen-evolving complex Is an ensemble of four manganese atoms arranged in a cluster of undetermined structure. The pulsed electron paramagnetic resonance (EPR) technique of electron spin-echo envelope modulation (ESEEM) can be used to measure nuclear spin transitions of nuclei magnetically coupled to paramagnetic metal centers of enzymes. We report the results of ESEEM experiments on the cyanobacterium Synechocystis PCC 6803 selectively labeled with 15N at the two nitrogen sites of the imidazole side chain of histidine residues. The experiments demonstrate that histidine is bound to manganese in the oxygen-evolving complex.Photosynthetic oxygen evolution is a cyclic process involving five redox states, So through S4, where the subscript refers to the number of oxidizing equivalents that have accumulated on the oxygen-evolving complex (OEC) (1). These accumulate sequentially with each light-driven charge separation of the photosystem II (PSII) reaction center. Upon attaining the S4 state an oxygen molecule is released, and the OEC returns to the So state. The catalytic core of the OEC consists of a tetranuclear manganese cluster of undetermined structure. Electron paramagnetic resonance (EPR) signals arising from the manganese cluster of the OEC poised in the S2 state have been observed in the g = 2 and g = 4 regions of the EPR spectrum (2-5). The first electron spin-echo envelope modulation (ESEEM) study of the g = 2 "multiline" signal, using PSII membranes isolated from spinach, revealed a broad structured peak centered near 5 MHz (6). This peak was shown to arise from 14N by ESEEM experiments on PSII particles isolated from a thermophilic cyanobacterium (Synechococcus sp.) grown alternatively with MATERIALS AND METHODSIsolation of a Histidine-Tolerant Strain. Labarre et al. (15) described the isolation of a histidine-tolerant mutant of Synechocystis PCC 6803 in which the transport of this normally toxic amino acid was impaired. As our application required incorporation of labeled histidine into cellular protein, we needed to both maintain histidine transport in a histidine-tolerant organism and suppress endogenous biosynthesis. The latter could be accomplished either through inactivation of genes involved in the biosynthetic pathway or through feedback inhibition of the pathway by the exogenous amino acid. A spontaneous histidine-tolerant strain showing feedback inhibition of the histidine biosynthetic pathway was obtained by growing wild-type cells photoautotrophically in Preparation of PSII Oxygen-Evolving Core Complexes.The histidine-tolerant strain was grown photoautotrophically in two 10-liter batches of BG-11 medium bubbled with 5% CO2 in air and containing 240 uM DL-histidine. In one batch, the histidine contained only natural-abundance 14N. In the other batch, both of the imidazole nitrogens were...

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Electrospray ionization (ESI) is capable of ionizing many soluble polymers. The ESI spectra are complex because of overlap of the multiply charged ions of the oligomer distribution, causing current computer transform programs to fail. However, it is possible to determine the origin of the multiply charged ions, making it feasible to write a program designed to transform ESI polymer spectra. To assess the value of such a program for polymer analysis, isolated monodisperse methyl methacrylate (MMA) oligomers (25 and 50 repeat units) were used to determine molar signal response and propensity for fragmentation.The sum of the peak areas for the multiply charged MMA 50-mer was found to be only about 66% of the summed peak areas for the 25-mer for the same molar concentration. However, conversion of the multiply charged peak areas to the singly charged representations, with peak area compression taken into account, gave equal signal responses for the 25-and 50-mers. Signal response variations due to the tacticity of the MMA oligomers were not observed. Fragmentation of the MMA oligomers also was shown not to occur under normal ESI conditions. Therefore, transformation of the polymer spectra to the singly charged molecular ion distribution should allow accurate calculation of average molecular weights, polydispersity, end group mass, and repeat unit mass.