Chlorophyll and pheophytin protonated and deprotonated ions: Observation and theory

Pheophytin a and chlorophyll a have been investigated by electrospray mass spectrometry in the positive and negative modes, in view of the importance of the knowledge of their properties in photosynthesis. Pheophytin and chlorophyll are both observed intensely in the protonated mode, and their main fragmentation route is the loss of their phytyl chain. Pheophytin is observed intact in the negative mode, while under collisions, it is primarily cleaved beyond the phytyl chain and loses the attaching propionate group. Chlorophyll is not detected in normal conditions in the negative mode, but addition of methanol solvent molecule is detected. Fragmentation of this adduct primarily forms a product (-30 amu) that dissociates into dephytyllated deprotonated chlorophyll. Semi-empirical molecular dynamics calculations show that the phytyl chain is unfolded from the chlorin cycle in pheophytin a and folded in chlorophyll a. Density functional theory calculations have been conducted to locate the charges on protonated and deprotonated pheophytin a and chlorophyll a and have found the major location sites that are notably more stable in energy by more than 0.5 eV than the others. The deprotonation site is found identical for pheophytin a and the chlorophyll a-methanol adduct. This is in line with experiment and calculation locating the addition of methanol on a double bond of deprotonated chlorophyll a.

UV photodissociation dynamics of deprotonated 2'-deoxyadenosine 5'-monophosphate [5'-dAMP-H]-

The UV photodissociation dynamics of deprotonated 2'-deoxyadenosine 5'-monophosphate ([5'-dAMP-H](-)) has been studied using a unique technique based on the coincident detection of the ion and the neutral fragments. The observed fragment ions are m/z 79 (PO(3)(-)), 97 (H(2)PO(4)(-)), 134 ([A-H](-)), 177 ([dAMP-H-A-H(2)O](-)), and 195 ([dAMP-H-A](-)), where "A" refers to a neutral adenine molecule. The relative abundances are comparable to that found in previous studies on [5'-dAMP-H](-) employing different excitation processes, i.e., collisions and UV photons. The fragmentation times of the major channels have been measured, and are all found to be on the microsecond time scale. The fragmentation mechanisms for all channels have been characterized using velocity correlation plots of the ion and neutral fragment(s). The findings show that none of the dissociation channels of [5'-dAMP-H](-) is UV specific and all proceed via statistical fragmentation on the ground state after internal conversion, a result similar to fragmentations induced by collisions.

Reactions of laser-ablated zirconium atoms within a supersonic expansion: insertion versus radical mechanism

In a laser ablation type microreactor followed by supersonic expansion, zirconium atoms have been reacted with methyl fluoride, CH(3)F (MeF), and mixtures of MeF and dimethylether, CH(3)-O-CH(3) (DME) seeded in He. With both mixtures, only a number of simple fluorinated products are formed, and they have been identified by one-photon ionization. All products can be linked to radical reactions either with F atoms, CH(3), or ZrF(1, 2, 3) radicals. No insertion products of the Grignard reagent type, F -Zr-CH(3) could be identified with or in the absence of DME. On the other hand, evidence has been found for the presence of organometallic compounds of the type ZrC(2)H(n=2, 4, 6), which could result from radical attack. Thus, even in conditions where intense solvation is at work, induced by clustering with polar DME molecules, which can act as stabilizing agents, a direct insertion mechanism into the C-F bond involving barrier suppression is not at work in our conditions. The reactivity due to radicals is very effective in this type of reactor, and the products that are efficiently formed can be quickly stabilized in the expansion. The radical attack supersedes, in the case of zirconium solvated by DME, the metastable mechanism with Zr(4d)(3)(5s)(1), that is certainly energetically impossible in the absence of strong reaction barrier suppression by a solvent. High level ab initio calculations performed at the CASPT2 level of theory are used for characterizing the electronic and geometric structure of the inserted products. They also reveal striking features of the reaction mechanism that support the absence of observation of inserted products within solvated clusters of zirconium.

Bidentate ligation of magnesium by 1,2-dimethoxyethane in the gas phase

The 1:1 Mg...1,2-dimethoxyethane (Mg-DXE) complexes are studied experimentally and theoretically. They are generated by a laser ablation source in a supersonic expansion. They are studied spectroscopically by resonance two-photon ionization. Density functional theory/Becke three-dimensional Lee, Yang, and Parr and ab initio calculations using the MOLPRO quantum chemistry package are performed to document their ground and excited states in a series of geometry ranging from monodentate to bidentate ligation of Mg by the O atoms of DXE. An absorption band is observed in the 27 800-30 500 cm(-1) range, which, thanks to the calculations, is attributed to the bidentate complex. The structure of the band is discussed in terms of the excitation of electronic states of the complex correlating adiabatically to the 3s3p (1)P and 3s4s (1)S states of Mg at large separation between Mg and DXE.