Kinetics and mechanism of protection of thymine from sulphate radical anion under anoxic conditions

The rates of photooxidation of thymine in presence of peroxydisulphate (PDS) have been determined by measuring the absorbance of thymine at 264 nm spectrophotometrically. The rates and the quantum yields (φ) of oxidation of thymine by sulphate radical anion have been determined in the presence of different concentrations of caffeic acid. Increase in [caffeic acid] is found to decrease the rate of oxidation of thymine suggesting that caffeic acid acts as an efficient scavenger of SO ₄ ^•- and protects thymine from it. Sulphate radical anion competes for thymine as well as for caffeic acid. The rate constant of sulphate radical anion with caffeic acid has been calculated to be 1.24 x 10¹⁰ dm³ mol^-1s^-1. The quantum yields of photooxidation of thymine have been calculated from the rates of oxidation of thymine and the light intensity absorbed by PDS at 254 nm, the wavelength at which PDS is activated to sulphate radical anion. From the results of experimentally determined quantum yields (φ_expt1) and the quantum yields calculated (φ_cl) assuming caffeic acid acting only as a scavenger of SO ₄ ^•- radicals show that φ_expt1 values are lower than φ_cl values. The φ ’ values, which are experimentally found quantum yield values at each caffeic acid concentration and corrected for SO ₄ ^•- scavenging by caffeic acid, are also found to be greater than φ_expt1 values. These observations suggest that the thymine radicals are repaired by caffeic acid in addition to scavenging of sulphate radical anions.     

Oxidation of caffeine by phosphate radical anion in aqueous solution under anoxic conditions

The photooxidation of caffeine in presence of peroxydiphosphate (PDP) in aqueous solution at natural pH (∼7.5) has been carried out in a quantum yield reactor using a high-pressure mercury lamp. The reactions were followed spectrophotometrically by measuring the absorbance of caffeine at λ_max (272 nm). The rates of reaction were calculated under different experimental conditions. The quantum yields were calculated from the rates of oxidation of caffeine and the intensity of light at 254 nm which was measured by using peroxydisulphate solution as a standard chemical actinometer. The reaction rates of oxidation of caffeine by PDP increase with increase in [PDP] as well as with increase in light intensity, while they are independent of [caffeine]. The quantum yields of oxidation of caffeine by PDP are independent of [PDP] as well as light intensity. However, quantum yields of oxidation of caffeine by PDP increase with increase in caffeine concentration. On the basis of these experimental results and product analysis, a probable mechanism has been suggested in which PDP is activated to phosphate radical anions (PO₄ ^·2−) by direct photolysis of PDP and also by the sensitizing effect of caffeine. The phosphate radical anions thus produced react with caffeine by electron transfer reaction, resulting in the formation of caffeine radical cation, which deprotonates in a fast step to produce C8-OH adduct radicals. These radicals might react with PDP to give final product 1,3,7-trimethyluric acid and PO₄ ^·2− radicals, the latter propagates the chain reaction.     

THE EFFECT OF MEDIUM POLARITY ON THE HEMATOPORPHYRIN-SENSITIZED PHOTOOXIDATION OF l-TRYPTOPHAN

Abstract The 83 μM hematoporphyrin (HP)-sensitized photooxidation of 0.1 mM tryptophan in aqueous solution buffered at pH 7.4 or in binary mixtures of phosphate buffer and organic solvents of higher (formamide) or lower (N,N-dimethylformamide, methanol, ethanol, tetrahydrofuran) polarity proceeds by a pure singlet oxygen (¹O₂) mechanism as suggested by azide quenching experiments, the rate-enhancing action of deuterated solvents, and the lack of any significant reaction between triplet HP and tryptophan. Both the first-order rate constant of the photoprocess and the photooxidation quantum yield (φ= 0.011 in phosphate buffer at pH 7.4) increase when the medium polarity is increased (e.g. φ= 0.024 in 90% formamide); this results mainly from the greater quantum yield of ¹O₂ generation and the longer lifetime of ¹O₂. The intrinsic reactivity of ¹O₂ with tryptophan is independent of formamide concentration. A moderate decrease in the medium polarity (e.g. in the range 0-30% methanol) enhances the efficiency of tryptophan photooxidation (φ= 0.014 in 30% methanol) as a result of the enhanced quantum yields of triplet HP and ¹O₂ formation. In contrast, the overall photooxidation rate is depressed at high concentrations of low-polarity organic solvents (e.g. φ= 0.0039 in 90% methanol) due to a 5.5-fold drop of the rate constant for the ¹O₂-tryptophan reaction which counteracts the enhancement of the lifetime and quantum yield of triplet HP and ¹O₂. The solvent composition also affects the equilibria between monomeric and multimeric forms of HP. However, under our experimental conditions, the aggregation state of HP appears to exert only a minor influence on the efficiency of tryptophan photooxidation.     

Mechanisms of Calcium and Phosphate Ion Association in Aqueous Solution

The initial steps of nucleation of calciumphosphates in aqueous solution are elucidated by means of quantum / classical molecular mechanics simulations. A special focus is dedicated to the role of the protonation state of the phosphate ion. The crystallization of calciumphosphates including entirely deprotonated phosphate ions is found at much lower pH values than required for finding the (PO₄)^3? species in water. In such cases the depronation of the hydrogenphosphate ion has to occur during crystal growth. According to our findings, the [Ca²⁺··(PO₄)^3?··Ca²⁺] ion triple is the smallest stable aggregate, which may be expected to contain an entirely deprotonated phosphate ion.     

DAMAGE TO URACIL- AND ADENINE-CONTAINING BASES, NUCLEOSIDES, NUCLEOTIDES AND POLYNUCLEOTIDES: QUANTUM YIELDS ON IRRADIATION AT 193 AND 254 NM

Abstract Photoreactions, such as base release and decomposition of the base moiety, induced by either 20 ns laser pulses at 193 nm or continuous 254 nm irradiation, were studied for a series of uracil and adenine derivatives in neutral aqueous solution. The quantum yield of chromophore loss (φ) depends significantly on the nature of the nucleic acid constituent and the saturating gas (Ar, N₂O or O₂). In the case of polynucleotides the destruction of nucleotides was measured by high-performance liquid chromatography after hydrolysis; the quantum yields (φ) are comparable to those of chromophore loss or larger. The φ_cl and aφ_dn of 0.04–0.1 for poly(U) and poly(dU), obtained for both wavelengths of irradiation, are due to processes originating from the lowest excited singlet state, i.e. formation of photohydrates and photodimers, and a second part from photoionization using λ_irr= 193 nm. Irradiation at 193 nm effectively splits pyrimidine dimers and thus reverts them into monomers. The quantum yield for release of undamaged bases (φ_br) from nucleosides, nucleotides and polynucleotides upon irradiation at 254 nm is typically φ_br= (0.1–1) × 10^?4 Breakage of the N-glycosidic bond is significantly more efficient for λ_irr=193 nm, e.g. φ_br= 1.1 × 10^?3, 0.8 × 10^?3, 4.3 × 10^?3 and 0.5 × 10^?3 for poly(A), poly(dA), poly(U) and poly(dU) in Ar-saturated solution, respectively. Enhanced φ values for λ_irr= 193 nm, essentially for adenine and its derivatives, are caused by photo-processes that are initiated by photoionization.     

Mechanism of protection of adenosine from sulphate radical anion and repair of adenosine radicals by caffeic acid in aqueous solution

The photooxidation of adenosine in presence of peroxydisulphate (PDS) has been studied by spectrophotometrically measuring the absorbance of adenosine at 260 nm. The rates of oxidation of adenosine by sulphate radical anion have been determined in the presence of different concentrations of caffeic acid. Increase in [caffeic acid] is found to decrease the rate of oxidation of adenosine suggesting that caffeic acid acts as an efficient scavenger of SO ₄ ^• and protects adenosine from it. Sulphate radical anion competes for adenosine as well as for caffeic acid. The quantum yields of photooxidation of adenosine have been calculated from the rates of oxidation of adenosine and the light intensity absorbed by PDS at 254 nm, the wavelength at which PDS is activated to sulphate radical anion. From the results of experimentally determined quantum yields (Φ_expt1) and the quantum yields calculated (Φ_cal) assuming caffeic acid acting only as a scavenger of SO ₄ ^• show that Φ_expt1 values are lower than Φ_expt1 values. The ǵf values, which are experimentally found quantum yield values at each caffeic acid concentration and corrected for ₄ ^• scavenging by caffeic acid, are also found to be greater than Φ_expt1 values. These observations suggest that the transient adenosine radicals are repaired by caffeic acid in addition to scavenging of sulphate radical anions.     

BIOLOGICAL DEACTIVATION AND SINGLE-STRAND BREAKAGE OF PLASMID DNA BY PHOTOSENSITIZATION USING TRIS(2,2''- BIPYRIDYL)RUTHENIUM(II) AND PEROXYDISULFATE

Abstract— Single-strand break formation and biological deactivation of plasmid pBR322 DNA in the presence of tris(2,2′-bipyridyl)-ruthenium(II), Ru(bpy)2/3;⁺, and K₂S₂O₈, upon irradiation with visible light(400–500 nm), were studied in aqueous solution at room temperature. Conditions of complete binding of Ru(bpy)2/3;⁺ to the strand were employed. The damage is initiated mainly by the SO^2/3;; radical anion. Under anoxic conditions at a ratio of nucleotide to sensitizer concentrations (N/S) of 18 and S₂O2/8^- concentrations of 0.5 mM the quantum yield of single-strand break (ssb) formation is φ_ssb= 8.4 times 10^-3 while that of biological deactivation (bd) is Ø_bd= 7.6 times 10^-3 (φ_ssb= 5.2 times 10^-36.4 times 10^-3, 6.0 times 10^-3 and φ_bd= 4.2 times 10^-3, 5.2 times 10^-3, 4.8 times 10^-3 at N/S=3, 6, and 9, respectively). The quantum yields are approximately 2.5 times smaller in air-saturated solutions. At N/S = 18 about 33 SO₄^-radical anions are required per one lethal event. φ_bd increases linearly with the S₂Oφ^- concentration (up to 0.5 mM). The damage to DNA is drastically reduced on addition of mono- or divalent salts (e.g. NaC10₄, MgCl₂). These additives cause the release of Ru(bpy)2⁺ from the strand. The observed damage to DNA is thus the result of a site specific reaction. When the phenanthroline analogue, Ru(phen)φ⁺, is used as sensitizer, φ_ssb and φ_bd are three times smaller.     

Kinetics of oxidation of adenosine by t-butoxyl radical: Protection and repair by caffeic acid

The kinetics of oxidation of adenosine and caffeic acid by t-BuO^• has been studied by the photolysis of t-BuOOH in the presence of t-BuOH. The rates and the quantum yields (φ) of oxidation of caffeic acid by t-BuO^• radicals have been determined in the absence and presence of varying concentrations of adenosine. An increase in the concentration of adenosine has been found to decrease the rate of oxidation of caffeic acid suggesting that adenosine and caffeic acid compete for t-BuO^• radicals. From competition kinetics, the rate constant of t-BuO^•–caffeic acid reaction has been calculated to be 8.15 × 10⁸ dm³ mol⁻¹ s⁻¹. The results of experimentally determined quantum yield (φ_exptl) values of oxidation of caffeic acid and the quantum yield values calculated (φ_cal) by assuming that caffeic acid reacts only with t-BuO^• radicals suggest that caffeic acid not only protects adenosine from t-BuO^• radicals but also repairs adenosine radicals formed by the reaction of t-BuO^• radicals. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 515–521, 2005     

Properties of A Phosphate Sensitive Solid State Electrode Based On Cerium-Iv-Hydrogenphosphate Mixed With Pvc

Abstract

The development of a phosphate sensitive electrode based on a heterogeneous (Ce-O-Ce) (HPO₄)₃·H₂O / PVC membrane is described. At a certain salt / PVC - mixture ratio an electrode for H₂PO₄ ^? with a slope of -50 mV/activity decade (AD) and a detection limit of approximately 20 ppm H₂PO₄ ^? is obtained. the selectivity coefficients for the interfering ions So₄ ^2?, cl^?, NO₃ ^?     

Primary processes in the thiacyanine dimer-photosensitized reduction of p-nitroacetophenone in water by

Primary processes in the reduction of p-nitroacetophenone (p-NAP) by ascorbic acid (AA) in water photosensitized by thiacyanine dimers M ₂ ^2? have been considered. For M ₂ ^2? , the quantum yields of fluorescence and intersystem crossing to the triplet state (M ₂ ^2? )_T increases in comparison to the monomers M^?. The dimers (M ₂ ^2? )_T enter into the reactions of both one-electron photoreduction by ascorbic acid to give AA^+· and M ₂ ^3? and one-electron photooxidation by p-nitroacetophenone to give p-NAP^?· and the dimeric radical anion M ₂ ^? which dissociates to M^? and M^· within 25–30 μs. The primary oxidative or reductive photosensitization in the ternary systems containing (M ₂ ^2? )_T, p-NAP, and AA affords p-NAP^?· and AA^+·.     

Viscosity dependence of the fluorescence lifetimes of cryptocyanine,pinacyanol and DDI

Viscosity dependence of the fluorescence lifetimes of cryptocyanine, pinacyanol and I, I'-diethyl-2,2'-dicarbocyanine iodide (DDI) has been studied by making relative quantum yield measurements in five solvents. It has also been established. by means of flash kinetic spectroscopy, that, in rigid solvents, φ^o_F+φ^o_IC ≈ 1 for all three dyes, where φ^o_F and φ^o_IC stand for the intrinsic quantum yields of fluorescence emission and internal conversion, respectively. Values of the intrisic rate constant of internal conversion are also presented.     

Synthesis of Ammonium‐ and 4‐Dimethylaminopyridinium Amidofluorophosphates and Crystal Structure of [DMAPH][PO2F(NH2)]

NH₄[PO₂F(NH₂)] has been prepared by the reaction of a betaine py·PO₂F with excess ammonia in acetonitrile solution, while the ammonolysis of DMAP·PO₂F with a stoichiometric amount of NH₃ yields [DMAPH][PO₂F(NH₂)]. The crystal structure of the latter was determined by single‐crystal X‐ray diffraction, which revealed that the anions [PO₂F(NH₂)]^— are linked to infinite chains by double N—H···O bridges. Additional strong N—H···O bridging bonds connect each anion with its [DMAPH]⁺ counterion. The formation of a new betaine NH₃·PO₂F in the solution of py·PO₂F in liquid ammonia was proved by ³¹P NMR spectroscopy and by identification of its hydrolysis products.     

Specificity of the redox photoreaction of dimers of thiadicarbocyanine dyes

The primary steps of the redox reaction of dimers of the thiadicarbocyanine dye and its 5,5′-dichloro derivative in aqueous solutions were studied in the presence of 4-nitroacetophenone, ascorbic acid, or hydroquinone. In water the dye molecules (anion, M^?) mainly exist as dimers M₂ ^2?. The laser pulse irradiation (10 ns, 532 nm) results in the population of the lowest triplet level M₂ ^2?, whose depletion occurs due to both intersystem crossing to the ground state and photoinduced transition to the highest triplet state of the dimer followed by photoionization. Photoionization at low intensities of a laser pulse proceeds via the one-quantum mechanism going to the two-quantum mechanism with an increase in the laser pulse intensity. The photooxidation of the dimer in the lowest triplet state with 4-nitroacetophenone results in the formation of unstable radical anion M₂ ^?· that spontaneously dissociates to monomer M^? and radical M^· of the dye. In the presence of electron donors (ascorbic acid, hydroquinone), the dimers in the triplet state are not photoreduced, but the electron donors reduce M₂ ^?· and M^· to the dye dimer and monomer, respectively.     

Fluoroquinolone Antimicrobials: Singlet Oxygen, Superoxide and Phototoxicity

The fluoroquinolone antibacterial agents possess photo-sensitizing properties that lead to phototoxic responses in both human and animal subjects. The phototoxicity order reported in humans is: fleroxacin > lomefloxacin, pefloxacin > ciprofloxacin ? enoxacin, norfloxacin and ofloxacin. Studies both in vivo and in vitro have related this phototoxicity to the generation of reactive oxygen species including hydrogen peroxide and the hydroxyl radical. We determined the quantum yields of singlet oxygen generation (φ_Δ,) by detection of the singlet oxygen (¹O₂) luminescence at 1270 tun for several fluoroquinolones, naphthyridines and other structurally related compounds. All the fluoroquinolones examined have low φ_Δ values ranging from 0.06 to 0.09 in phosphate buffer at pD 7.5. We also determined the ¹O₂ quenching constants for these compounds and their values were on the order of 10⁶M^?1 s¹, except for lomefloxacin whose rate constant was 1.8 × 10⁷M^?1 s^?1. The φ_Δ values were significantly decreased in a solvent of lower polarity such as methanol (0.007 ≤φ_Δ≤ 0.02). The production of ¹O₂ by these antibiotics did not correlate with the order reported for their phototoxicity. We also measured the photogeneration (λ > 300 nm) of superoxide by these antibacterials in dimethylsulfoxide using electron paramagnetic resonance and the spin trap 5,5-dimethyl-l-pyrroiine N-oxide. Although there is not a one-to-one correspondence between the relative rates of superoxide generation and the phototoxicity ranking of the fluoroquinolones, the more phototoxic compounds tended to produce superoxide at a faster rate. Nevertheless, the magnitudes of the observed differences do not appear sufficient to explain the range of fluoroquinolone phototoxicity potencies in human and animal subjects in general and the high activity of fleroxacin and lomefloxacin in particular. For these latter drugs the photoinduced loss of the F₈ atom as fluoride and the concomitant generation of a highly reactive carbene at C-8 provide a more plausible mechanism for their potent phototoxic and photocarcinogenic properties.     

Thermal-lensing and phosphorescence studies of the quantum yield and lifetime of singlet molecular oxygen (1 delta g) sensitized by hematoporphyrin and related porphyrins in deuterated and non-deuterated ethanols

Time-resolved thermal-lensing was used to measure the absolute quantum yield (φ_Δ) of singlet molecular oxygen, O₂(¹Δ_g), produced by hematoporphyrin photosensitization in ethanol. Deuteration of the solvent did not affect the value of φ_Δ. The value of φΔ= 0.53 was then used as reference to evaluate φΔ in O₂ (¹Δ_g) phosphorescence experiments with the related porphyrins, monohydroxyethylvinyl deuteroporphyrin and dihematoporphyrin ether. The φ_Δ values, in conjunction with the respective quantum yields of intersystem crossing (measured using a nanosecond laser flash photolysis technique) served to evaluate efficiencies, S_Δ, of O₂ (¹Δ_g) production from the porphyrin triplet states. The lifetime T_Δ in monodeuterated ethanol was measured as 29 ± 3 μs and 30 ± 1 (xs by time-resolved thermal lensing and phosphorescence detection, respectively. T_Δ in ethanol and fully deuterated ethanol were in good agreement with values reported in the literature.     

Na2Co(H2PO4)4·4H2O

In the title compound, disodium cobalt tetrakis­(dihydrogen­phosphate) tetrahydrate, the Co^II ion lies on an inversion centre and is octahedrally surrounded by two water molecules and four H₂PO₄ groups to give a cobalt complex anion of the form [Co(H₂PO₄)₄(OH₂)]^2?. The three‐dimensional framework results from hydrogen bonding between the anions. The relationship with the structures of Co(H₂PO₄)₂·2H₂O and K₂CoP₄O₁₂·5H₂O is discussed.     

ESR.-Studies of Nonalternant Radicals Structurally Related to Phenalenyl

The radical anion and the radical cation of azuleno[1,2,3-cd]phenalene (III) have been investigated by ESR. spectroscopy, along with the radical anion of 2-phenylazulene (IV). Also studied has been the neutral radical obtained by one-electron reduction of cyclohepta[cd]phenalenium-cation (VI^⊕). Assignment of the proton coupling constants for the radical ions III. ·?, III ·⊕ and IV·⊕, and the radical VI · is supported by comparison with the ESR. spectra of specifically deuteriated derivatives III-d₅ ·?, III-d₅ ·⊕, IV-d₂ ·? and VI-d₁′. The experimental results are in full accord with qualitative topological arguments and predictions of HMO models. Whereas the radical anion III ·? exhibits α-spin distribution similar to that of IV ·?the corresponding radical cation III ·⊕ and the neutral radical VI · are related in this respect to phenalenyl (V·). It is noteworthy that oxidation of III by conc. H₂SO₄ yields a paramagnetic species (IIIa ·⊕) which has a similar – but not an identical – structure as the radical cation III ·⊕ produced from III with AlCl₃ in CH₃NO₂.     

PHOTOLESIONS AND BIOLOGICAL INACTIVATION OF PLASMID DNA ON 254 nm IRRADIATION AND COMPARISON WITH 193 nm LASER IRRADIATION

Plasmid pTZ18R and calf thymus DNA in aerated neutral aqueous solution were irradiated by continuous 254 nm light. The quantum yields are φ_ssb= 4.0 × 10^-5 and φ_dsb= 1.4 × 10^-6 for single- and double-strand break formation, respectively, φ_br= 2.3 × 10^-5 for base release, φ_dn= 2.1 × 10^-3 for destruction of nucleotides, and φ_icl×φ_lds× 1 × 10^-6 for interstrand cross-links and locally denatured sites, respectively. The presence of Tris-HCI/ ethylenediaminetetraacetic acid (10:1, pH 7.5) buffer strongly reduces φ_ssb, The corresponding φ values, obtained on employing pulsed 193 nm laser irradiation, are much larger than those using λ_irr, = 254 nm. This is ascribed to a contribution of chemical reactions induced by photoionization, which is absent for 254 nm irradiation. The quantum yields of inactivation of plasmid DNA (λ_irr= 254 nm) were measured by transformation of the Escherichia coli strains AB1157 (wild type), φ_ina(1157) = 1.6 × 10^-4, AB1886 (uvr^-), φ_ina(1886) = 4.2 × 10^-4, AB2463 (rec^-), φ_ina(2463) = 4.1 × 10^-4 and AB2480 (uvr^- rec^-), φ_ina(2480) = 3.1 × 10^-3. The quantum yields of inactivation of plasmid DNA are compared with those of the four E. coli strains (denoted as chromosomal DNA inactivation) obtained from the literature. The results for E. coli strain AB2480 show that the chromosomal DNA and the plasmid DNA are both inactivated by a single pyrimidine photodimer per genome. With the E. coli strain AB2463 inactivation of plasmid and chromosomal DNA is the same for the same total damage per genome and is ～ 10 times smaller than for AB2480. This is explained by photodimer repair in chromosomal and plasmid DNA and by the absence of dsb repair in both cases. In the repair wild-type strain AB1157, inactivation of the plasmid DNA is roughly 100 times higher than that of the chromosomal DNA. We postulate that a portion of this difference is due to repair of dsb by the recA system in chromosomal DNA and that such repair does not take place in the plasmid DNA. The biological results from 254 nm irradiation are compared with those from 193 nm laser irradiation.     

Single crystal NQR and NMR study of (NH4)2InBr6·H2O

The ⁸¹Br NQR tensors of (NH₄)₂InBr₅·H₂O have been studied on single crystals by combining a 4π Zeeman goniometer with Fourier transform NQR. AT T=293K the coupling constants are 136.27 e²φ_ZZQh^?(su81Br)/Mhz ≦ 146.14 and the asymmetry parameter 0.013 ≦ η (⁸¹Br) ≦ 0.133. The EFG axes φ_zz(⁸¹Br) are almost parallel to the bond directions InBr. The strong changes of φ_yy(¹¹⁵In) and φ_zz(¹¹⁵In) with temperature can be explained by assuming small changes of the In(OH₂) bond distance in the complex anion (InBr₅·H₂Cl^2?. The D₂O molecule within the deuterated complexes flips freely in the range studied (100 ≦ T/K ≦ 298). Small EFG's at the ¹⁴N site in the cation ND^?₄ are due to the EFG of the surrounding lattice charges.     

Formation and Dissociation of Phosphorylated Peptide Radical Cations

In this study, we generated phosphoserine- and phosphothreonine-containing peptide radical cations through low-energy collision-induced dissociation (CID) of the ternary metal?Cligand phosphorylated peptide complexes [Cu^II(terpy) _p M]^·2+ and [Co^III(salen) _p M]^·+ [ _p M: phosphorylated angiotensin III derivative; terpy: 2,2':6',2''-terpyridine; salen: N,N '-ethylenebis(salicylideneiminato)]. Subsequent CID of the phosphorylated peptide radical cations ( _p M^·+) revealed fascinating gas-phase radical chemistry, yielding (1) charge-directed b- and y-type product ions, (2) radical-driven product ions through cleavages of peptide backbones and side chains, and (3) different degrees of formation of [M ?C H₃PO₄]^·+ species through phosphate ester bond cleavage. The CID spectra of the _p M^·+ species and their non-phosphorylated analogues featured fragment ions of similar sequence, suggesting that the phosphoryl group did not play a significant role in the fragmentation of the peptide backbone or side chain. The extent of neutral H₃PO₄ loss was influenced by the peptide sequence and the initial sites of the charge and radical. A preliminary density functional theory study, at the B3LYP 6-311++G(d,p) level of theory, of the neutral loss of H₃PO₄ from a prototypical model??N-acetylphosphorylserine methylamide??revealed several factors governing the elimination of neutral phosphoryl groups through charge- and radical-induced mechanisms.