Are the anions MeO(CO)n- (n = 1 and 2) methoxide anion donors in the gas phase? A theoretical investigation.

1. The anions CH(3)O-(-)CO and CH(3)OCO-(-)CO are both methoxide anion donors. The processes CH(3)O-(-)CO --> CH(3)O(-) + CO and CH(3)OCO-CO --> CH(3)O(-) + 2CO have DeltaG values of +8 and -68 kJ mol(-1), respectively, at the CCSD(T)/6-311++G(2d, 2p)//B3LYP/6-311++G(2d,2p) level of theory. 2. The reactions CH(3)OCOCO(2) (-) --> CH(3)OCO(2) (-) + CO (DeltaG = -22 kJ mol(-1)) and CH(3)COCH(O(-))CO(2)CH(3) --> CH(3)COCH(O(-))OCH(3) + CO (DeltaG = +19 kJ mol(-1)) proceed directly from the precursor anions via the transition states (CH(3)OCO...CO(2))(-) and (CH(3)COCHO...CH(3)OCO)(-), respectively. 3. Anion CH(3)COCH(O(-))CO(2)CH(3) undergoes methoxide anion transfer and loss of two molecules of CO in the reaction sequence CH(3)COCH(O(-))CO(2)CH(3) --> CH(3)CH(O(-))COCO(2)CH(3) --> [CH(3)CHO (CH(3)OCO-(-)CO)] --> CH(3)CH(O(-))OCH(3) + 2CO (DeltaG = +9 kJ mol(-1)). The hydride ion transfer in the first step is a key feature of the reaction sequence.

Diagnostic fragmentations of adducts formed between carbanions and carbon disulfide in the gas phase. A joint experimental and theoretical study.

Selected carbanions react with carbon disulfide in a modified LCQ ion trap mass spectrometer to form adducts, which when collisionally activated, decompose by processes which in some cases identify the structures of the original carbanions. For example (i) C(6)H(5)(-) + CS(2)--> C(6)H(5)CS(2)(-)--> C(6)H(5)S(-) + CS, occurs through a 3-membered ring ipso transition state, and (ii) the reaction between C(6)H(5)CH(2)(-) and CS(2) gives an adduct which loses H(2)S, whereas the adduct(s) formed between o-CH(3)C(6)H(5)(-) and CS(2) loses H(2)S and CS. Finally, it is shown that decarboxylation of C(6)H(5)CH(2)CH(2)CO(2)(-) produces the beta-phenylethyl anion (PhCH(2)CH(2)(-)), and that this thermalized anion reacts with CS(2) to form C(6)H(5)CH(2)CH(2)CS(2)(-) which when energized fragments specifically by the process C(6)H(5)CH(2)CH(2)CS(2)(-)--> C(6)H(5)CH(2)(-)CHC(S)SH --> [(C(6)H(5)CH(2)CH[double bond, length as m-dash]C[double bond, length as m-dash]S) (-)SH] --> C(6)H(5)CH(2)CCS(-) + H(2)S. Experimental findings of processes (ii) and (iii) were aided by deuterium labelling studies, and all reaction profiles were studied by theoretical calculations at the UCCSD(T)/6-31+G(d,p)//B3LYP/6-31+G(d,p) level of theory unless indicated to the contrary.

Anions [N(CH2)3]- and [ON(CH2)2]- are stable in the gas phase, but can they be charge stripped to form the radicals N(CH2)3 and ON(CH2)2? A joint experimental and theoretical study.

Collision-induced activation of deprotonated trimethylamine N-oxide yields the two anions [N(CH(2))(3)](-) and [ON(CH(2))(2)](-) following losses of H(2)O and CH(4), respectively. These two anions decompose by minor losses of H(*) and H(2) when collisionally activated: no other fragmentations are noted. Calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31+G(d) level of theory indicate that these trigonal anions are stable, and should not rearrange following collisional activation. Collisional-induced charge stripping of the anions [N(CH(2))(3)](-) and [ON(CH(2))(2)](-), respectively, form N(CH(2))(3) and ON(CH(2))(2). Some of these neutrals are energised and undergo rearrangement and dissociation. From a consideration of experiment and theory, it is proposed (i) that energised N(CH(2))(3) may cyclise to form the 1-aziridinylcarbinyl radical. This species may ring open to CH(2)=NCH(2)CH(2) which then decomposes to CH(2)N and C(2)H(4) and (ii) energised ON(CH(2))(2) may undergo OC cyclisation followed by ring opening to energised CH(2)=NCH(2)O which may fragment to yield CH(2)N and CH(2)O.

A theoretical study of the cyclization processes of energized CCCSi and CCCP.

Calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31+G(d) level of theory have shown that cyclization of both the ground state triplet and the corresponding singlet state of CCCSi may rearrange to give cyclic isomers which upon ring opening may reform linear C(3)Si isomers in which the carbon atoms are scrambled. The cyclization processes are energetically favorable with barriers to the transition states from 13 to 16 kcal mol(-1). This should be contrasted with the analogous process of triplet CCCC to triplet rhombic C(4), which requires an excess energy of 25.8 kcal mol(-1). A similar cyclization of doublet CCCP requires 50.4 kcal mol(-1) of excess energy; this should be contrasted with the same process for CCCN, which requires 54.7 kcal mol(-1) to effect cyclization.

The fallaxidin peptides from the skin secretion of the Eastern Dwarf Tree Frog Litoria fallax. Sequence determination by positive and negative ion electrospray mass spectrometry: antimicrobial activity and cDNA cloning of the fallaxidins.

The glandular skin secretion of the Eastern Dwarf Tree Frog Litoria fallax contains nine peptides named fallaxidins. The sequences of these peptides were elucidated using a combination of positive and negative electrospray mass spectrometry together with Edman sequencing. Among these peptides are: (i) fallaxidins 1.1 and 2.1 which have the sequences YFPIPI-NH2 and FWPFM-NH2. The activities of these peptides are unknown, but it has been shown that they are not smooth muscle active, opioids or antimicrobially active, nor do they effect proliferation of lymphocytes; (ii) two weakly active antibiotics, fallaxidins 3.1 and 3.2 (e.g. fallaxidin 3.1, GLLDLAKHVIGIASKL-NH2), and a moderately active antibiotic fallaxidin 4.1 (GLLSFLPKVIGVIGHLIHPPS-OH). Fallaxidin 4.1 has an unusual sequence for an antibiotic, containing three Pro residues together with a C-terminal CO2H group. cDNA cloning has confirmed the identity of the nine isolated peptides from L. fallax, together with five additional peptides not detected in the peptide profile. The pre-regions of the nine preprofallaxidins are conserved and similar to those of the caerin peptides from L. caerulea and L. splendida, suggesting that the fallaxidin and caerin peptides, although significantly different in sequence, originated from a common ancestor gene.

Characteristic negative ion fragmentations of deprotonated peptides containing post-translational modifications: mono-phosphorylated Ser, Thr and Tyr. A joint experimental and theoretical study.

Peptides and proteins may contain post-translationally modified phosphorylated amino acid residues, in particular phosphorylated serine (pSer), threonine (pThr) and tyrosine (pTyr). Following earlier work by Lehmann et al., the [M-H]- anions of peptides containing pSer and pThr functionality show loss of the elements of H3PO4. This process, illustrated for Ser (and using a model system), is CH3CONH-C(CH2OPO3H2)CONHCH(3) --> [CH3CONHC(==CH2)CONHCH3 (-OPO3H2)] (a) --> [CH3CONHC(==CH2)CONHCH3-H]- + H3PO4, a process endothermic by 83 kJ mol(-1) at the MP2/6-31++G(d,p)//HF/6-31++G(d,p) level of theory. In addition, intermediate (a) may decompose to yield CH3CONHC(==CH2)CONHCH3 + H2PO4 - in a process exothermic by 3 kJ mol(-1). The barrier to the transition state for these two processes is 49 kJ mol(-1). Characteristic cleavages of pSer and pThr are more energetically favourable than the negative ion backbone cleavages of peptides described previously. In contrast, loss of HPO3 from [M-H]- is characteristic of pTyr. The cleavage [NH2CH(CH2-C6H4-OPO3H-)CO2H] --> [NH2C(CH2-C6H4-O-)CO2H (HPO3)] (b) --> NH2CH(CH2-C6H4-O-)CO2H + HPO3 is endothermic by 318 kJ mol(-1) at the HF/6-31+G(d)//AM1 level of theory. In addition, intermediate (b) also yields NH2CH(CH2-C6H4-OH)CO2H + PO3 - (reaction endothermic by 137 kJ mol(-1)). The two negative ion cleavages of pTyr have a barrier to the transition state of 198 kJ mol(-1) (at the HF/6-31+G(d)//AM1 level of theory) comparable with those already reported for negative ion backbone cleavages.