Reactive collisions in quadrupole cells. 3: H/D exchange reactions of protonated aromatic amines with ND3

The H/D exchange reactions of a variety of protonated aromatic amines with ND₃ m the collision cell of a hybrid BEqQ tandem mass spectrometer have been studied. The MH⁺ ions were prepared by CH₄, t-C₄H₁₀, and NH₃ chemical ionization (CI) and, for some amines, by fast-atom bombardment (FAB). Evidence is presented that the kinetic energy of the incident ion as well as its internal energy must be dissipated by nonexchanging collisions before exchange occurs, once deactivated the MH⁺ ions exchange efficiently, which leads, in most cases, to [MHJ⁺ d _x ions m which all active hydrogens have been exchanged. The MH⁺ ion of 1,3-phenylenediamine formed by gas-phase CI exchanges only very slightly with ND₃ whereas a significant fraction of the MH⁺ ions formed by FAB exchange efficiently. This difference is rationalized in terms of dominant formation of the ring-protonated species in gas-phase CI reactions and significant formation of the N-protonated species by FAB with only the N-protonated species exchanging efficiently. Similar, although less pronounced, differences are observed for the MH⁺ ion of m-anisidine. In a number of cases apparent exchange of aromatic hydrogens also is observed. Evidence is presented for the interchange of ring and amine hydrogens in protonated aromatic amines and it is suggested that only the N-protonated species undergoes significant exchange with ND₃.     

H/D exchange reactions of protonated diglycine; an electrospray ionization–flow tube reactor experiment

The H/D exchange reactions of protonated diglycine, GLY₂H⁺, with ND₃ were studied under thermal conditions with a combination of an electrospray ion source and a flow tube reactor. Consecutive exchange of the five labile hydrogens is observed with increasing flow rate of ND₃. Collision complexes corresponding to the consecutive H/D exchanges are monitored for the first time. The role of multiple exchanges in a single collision event with ND₃ is probed. Results will be discussed in the light of previously suggested mechanisms of H/D exchange of GLY₂H⁺ with deuterated ammonia.     

The bimolecular hydrogen-deuterium exchange behavior of protonated alkyl dipeptides in the gas phase

As part of an ongoing characterization of the intrinsic chemical properties of peptides, thermal hydrogen-deuterium exchange has been studied for a series of fast-atom-bombardment-generated protonated alkyldipeptides and related model compounds in the reaction with D₂O, CH₃OD, and ND₃ in a Fourier transform ion cyclotron resonance mass spectrometer. Despite the very large basicity difference between the dipeptides and the D₂O and CH₃OD exchange reagents, efficient exchange of all active hydrogen atoms occurs. From the kinetic data it appears that exchange of the amino, amide, and hydroxyl hydrogens proceeds with different efficiencies, which implies that the proton in thermal protonated dipeptides is immobile. The selectivity of the exchange at the different basic sites is governed by the nature of both the dipeptide and the exchange reagent. The results indicate that reversible proton transfer in the reaction complexes, which effectuates the deuterium incorporation, is assisted by formation of multiple hydrogen bonds between the reagents. Exchange is considered to proceed via the intermediacy of different competing intermediate complexes, each of which specifically leads to deuterium incorporation at different basic sites. The relative stabilization of the competing intermediate complexes can be related to the relative efficiencies of deuterium incorporation at different basic sites in the dipeptide. For all protonated dipeptides studied, the exchange in the reaction with ND₃ proceeds with unit efficiency, whereas all active hydrogen atoms are exchanged equally efficiently. Evidently specific multiple hydrogen bond formations are far less important in the reversible proton transfers with the relatively basic ammonia, which allows effective randomization of all active hydrogen atoms in the reaction complexes.     

Gas phase hydrogen / deuterium exchange in electrospray ionization mass spectrometry as a practical tool for structure elucidation

Two methods for gas phase hydrogen/deuterium exchange have been developed for the analysis of small molecules. Hydrogen/deuterium exchange has been implemented by making simple modifications to the plumbing for the nebulizer and curtain gases on a nebulization-assisted electrospray ion source. The nebulizer gas exchange method has demonstrated deuterium exchange levels of 84–97% for a variety of molecules representing a wide range of structural classes containing up to 51 potentially exchangeable hydrogens; this allowed determination of the number of exchangeable hydrogens for all of the molecules studied containing ≤ 25 labile hydrogens (M _r ≤ 3000). ND₃ gas consumption is minimized in the nebulizer method by toggling the nebulizer from air to ND₃ for only a few scans of the total sample elution period. The curtain gas exchange method is more variable, yielding exchange levels of 32–98% for the same set of molecules; this was still sufficient to allow determination of > 70% of the molecules studied containing ≤ 25 labile hydrogens. Gas consumption is minimized in the curtain method by replacing ≤ 10% of the curtain gas flow with ND₃. Neither the nebulizer nor curtain exchange method requires the use of deuterated or aprotic solvents at typical 2 μL/min flow rates.     

One‐Electron Oxidation and Sensitivity of Uric Acid in On‐Line Electrochemistry and in Electrospray Ionization Mass Spectrometry

The sensitivity of detection of uric acid (H₂U) in positive ion mode electrospray ionization mass spectrometry (ESI MS) was enhanced by uric acid oxidation during electrospray ionization. With a carrier solution of pH 6.3>pK_a1=5.4 of H₂U, protonated unoxidized uric acid [H₂U+H]⁺ (m/z 169) was detected together with the protonated uric acid dimer [2H₂U+H]⁺ (m/z 337). The dimer likely forms by 1e^? oxidation of urate (HU^?) followed by rapid radical dimerization. A covalent structure of the dimer was verified by H/D exchange experiments. Efficiency of 2e^?, 2H⁺ oxidation of uric acid is low during ESI in pH 6.3 carrier solution and improves when a low on‐line electrochemical cell voltage is floated on the high voltage of the ES in on‐line electrochemistry ESI MS (EC/ESI MS). The intensity of the uric acid dimer decreases with an increase in the low applied voltage. In a carrier solution with 0.1 M KOH, pH 12.7>pK_a2=9.8 of H₂U, allantoin (Allnt) (MW 158.04), the final 2e^?, 2H⁺ oxidation product of uric acid, was detected as a potassium complex [K(Allnt)+K]⁺ (m/z 235) and the [2H₂U+H]⁺ dimer was not detected. In direct ESI MS analysis of 1000‐fold diluted urine [NaHU+H]⁺ (pK_sp NaHU=4.6) was detected in 40/60 (vol%) water/methanol, 1 mM NH₄Ac, pH ca. 6.3 carrier solution. A new configuration of the ESI MS instrument with a cone‐shaped capillary inlet significantly enhanced sensitivity in ESI and EC/ESI MS measurements of uric acid.     

Study of H/D-exchange reaction kinetics of polypeptides

One of the possible methods for 3D protein structure investigation is the study of gas-phase hydrogen/deuterium (H/D) exchange reaction between protein ions and a D-containing reactant gas, e.g., D₂O or ND₃. A segmented radio frequency quadrupole (RFQ) was used as a molecule-ion reactor to study gasphase H/D-exchange of protonated ions of three different peptides. The ions were produced in an electrospray ion source. The RFQ is a part of ion transport interface of a high resolution orthogonal time-of-fiight mass spectrometer (O-TOF MS). The RFQ was modified for a linear ion trap (LIT) mode of operation to increase a dwell time of target ions inside the RFQ. Phase-sensitive operation of the LIT and the O-TOF MS was controlled by custom developed PC executive program. The reaction mixture of N₂ and ND₃ was injected into the reactor, while keeping its partial pressure in the range of 10⁻³–10⁻² mbar, and corresponding ND3 concentration in the range of 10¹³–10¹⁴ cm⁻³. It was possible to vary the ion dwell time in the reactor between 30 ms and 1 s. H/D-exchange was studied for leucine enkephalin, gramicidin S and apamin. The data analysis based on statistical approach has shown a principal possibility to distinguish different mobile H-atoms of peptides, taking part in H/D-exchange, according to reaction rates.     

Deamination of protonated amines to yield protonated imines

Primary and secondary amines, when examined in atmospheric pressure chemical ionization, electrospray ionization, or chemical ionization, display protonated imines in their mass spectra. These products arise formally by nucleophilic substitution at the α-carbon with loss of both ammonia and molecular hydrogen. Collision-induced dissociation (CID) is used to characterize the product ions by comparison with authentic protonated imines. Gas-phase ion/molecule reactions of protonated amines with neutral amines also yield products that correspond to protonated imines (deamination and dehydrogenation), as well as providing simple deamination products. The reaction mechanism was investigated further by reacting the deamination product, the alkyl cation, with a neutral amine. The observed dehydrogenation of the nascent protonated secondary amine indicates that the reaction sequence is loss of ammonia followed by dehydrogenation even though the isolated protonated secondary amines did not undergo dehydrogenation upon CID. Formation of the deamination products in the protonated amine/amine reaction is competitive with proton-bound dimer formation. The proton-bound dimers do not yield deamination products under CID conditions in the ion trap or in experiments performed using a pentaquadrupole instrument. This demonstrates that the geometry of the proton-bound dimer, in which the α-carbons of the alkylamines are well separated [C _a -N-H-N-C _a ], is an unsuitable entry point on the potential energy hypersurface for formation of the imine [C _a -N-C _a ]. Isolation of the proton-bound dimers in the quadrupole ion trap is achieved with low efficiency and this characteristic can be used to distinguish them from their covalently bound isomers.     

H/D exchange kinetics: Experimental evidence for formation of different b fragment ion conformers/isomers during the gas-phase peptide sequencing

Electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) combined with H/D exchange reactions was utilized to explore the existence of different b₅⁺ and b₄⁺ fragment ion conformers/isomers of hexapeptide WHWLQL in the gas phase. Distinct H/D exchange trends for protonated WHWLQL ([M + H]⁺) and its b₅⁺ and b₄⁺ fragment ions (with ND₃) were observed. Isolated ¹²C_all isotopomers of both b₅⁺ and b₄⁺ fragment ions yielded bimodal distributions of H/D exchanged product ions. The H/D exchange reaction kinetics also confirmed that b₅⁺ and b₄⁺ fragment ions exist as combination of slow-exchanging (“s”) and fast-exchanging (“f”) species. The calculated rate constant for the first labile hydrogen exchange of [M + H]⁺ (k_{[M + H]} ⁺ = 3.80 ± 0.7 × 10⁻¹⁰ cm³ mol⁻¹ s⁻¹) was ∼30 and ∼5 times greater than those for the “s” and “f” species of b₅⁺, respectively. Data from H/D exchange of isolated “s” species at longer ND₃ reaction times confirmed the existence of different conformers or isomers for b₅⁺ fragment ions. The sustained off-resonance irradiation collision-activated dissociation (SORI-CAD) of WHWLQL combined with the H/D exchange reactions indicate that “s” and “f” species of b₅⁺ and b₄⁺ fragment ions can be produced in the ICR cell as well as the ESI source. The significance of these observations for detailed understanding of protein sequencing and ion fragmentation pathways is discussed.     

Structural characterization of methionine and leucine enkephalins by hydrogen/deuterium exchange and electrospray ionization tandem mass spectrometry

Conformational changes in two endogenous opioid active pentapeptides methionine enkephalin (Met-enk) and leucine enkephalin (Leu-enk) induced by trifluoroethanol (TFE) were identified using hydrogen/deuterium exchange (HDX), coupled with electrospray ionization (ESI) mass spectrometry. The exchange features in individual amino acid residues were characterized by acquiring tandem mass spectra of the deuterated peptides. The exact identity of the labile hydrogens involved in HDX reveals that the monomer forms of both peptides adopt an unfolded conformation in aqueous solvent, but prefer the 5-->2 beta-turn secondary structure under the membrane-mimetic environment. The ESI mass spectra of Met-enk and Leu-enk also reveal that the dimer structure of these peptides coexists with the monomer conformation. The extent of the dimer structure is dependent on the peptide concentration and nature of the solvent. The non-polar solvents facilitate the dimer formation.     

Phase transitions and molecular motions in [Ni(ND3)6](ClO4)2

[Ni(ND₃)₆](ClO₄)₂ has three solid phases between 100 and 300 K. The phase transitions temperatures at heating (T_C1^h=164.1 K and T_C2^h=145.1 K) are shifted, as compared to the non-deuterated compound, towards the lower temperature of ca. 8 and 5 K, respectively. The ClO₄⁻ anions perform fast, picosecond, isotropic reorientation with the activation energy of 6.6 kJ mol⁻¹, which abruptly slow down at T_C1^c phase transition, during sample cooling. The ND₃ ligands perform fast uniaxial reorientation around the Ni-N bond in all three detected phases, with the effective activation energy of 2.9 kJ mol⁻¹. The reorientational motion of ND₃ is only slightly distorted at the T_C1 phase transition due to the dynamical orientational order-disorder process of anions. The low value of the activation energy for the ND₃ reorientation suggests that this reorientation undergoes the translation-rotation coupling, which makes the barrier to the rotation of the ammonia ligands not constant but fluctuating. The phase polymorphism and the dynamics of the molecular reorientations of the title compound are similar but not quite identical with these of the [Ni(NH₃)₆](ClO₄)₂.     

Structural characterization by infrared multiple photon dissociation spectroscopy of protonated gas-phase ions obtained by electrospray ionization of cysteine and dopamine

Structural characterization of protonated gas-phase ions of cysteine and dopamine by infrared multiple photon dissociation (IRMPD) spectroscopy using a free electron laser in combination with theory based on DFT calculations reveals the presence of two types of protonated dimer ions in the electrospray mass spectra of the metabolites. In addition to the proton-bound dimer of each species, the covalently bound dimer of cysteine (bound by a disulfide linkage) has been identified. The dimer ion of m/z 241 observed in the electrospray mass spectra of cysteine has been identified as protonated cystine by comparison of the experimental IRMPD spectrum to the IR absorption spectra predicted by theory and the IRMPD spectrum of a standard. Formation of the protonated covalently bound disulfide-linked dimer ions (i.e. protonated cystine) from electrospray of cysteine solution is consistent with the redox properties of cysteine. Both the IRMPD spectra and theory indicate that in protonated cystine the covalent disulfide bond is retained and the proton is involved in intramolecular hydrogen bonding between the amine groups of the two cysteine amino acid units. For cysteine, the protonated covalently bound dimer (m/z 241) dominated the mass spectrum relative to the proton-bound dimer (m/z 243), but this was not the case for dopamine, where the protonated monomer and the proton-bound dimer were both observed as major ions. An extended conformation of the ethylammonium side chain of gas-phase protonated dopamine monomer was verified from the correlation between the predicted IR absorption spectra and the experimental IRMPD spectrum. Dopamine has the same extended ethylamine side chain conformation in the proton-bound dopamine dimer identified in the mass spectra of electrosprayed dopamine. The structure of the proton-bound dimer of dopamine is confirmed by calculations and the presence of an IR band due to the shared proton. The presence of the shared proton in the protonated cystine ion can be inferred from the IRMPD spectrum.

Reaction of cyclohexanone with [NH4]+ under chemical ionization conditions. 1—formation of protonated unsubstituted imines

High resolution and metastable decomposition spectra of the ions [M + NH₄]⁺ (and [M + ND₄]⁺) formed by reaction of [NH₄]⁺ (and [ND₄]⁺) with cyclohexanone have been measured. The results provide evidence that the m/z 98 ion, which is abundant in the chemical ionization (NH₃) spectrum of cyclohexanone, is in fact composed of two isobaric ions: a protonated imine ion and the molecular ion of cyclohexanone. The former is formed by a mechanism analogous to that occurring in solution.     

Observation of the ND4 Schüler band in a neutralized ion beam experiment

The emission of the ND₄ Schüler band was observed after neutralization of a mass-selected ND ₄ ⁺ ion beam. Thus the assignment of this band to the ammonium radical was confirmed. The lifetime of the upper state (3p ² F ₂) was determined to be 4.2 ns, which is much shorter than ab initio values of the radiative lifetime, showing that this state decays predominantly by predissociation. The Schuster band was not observed, neither after neutralization of ND ₄ ⁺ , nor of ND ₃ ⁺ .     

Non‐covalent dimers of the lysine containing protonated peptide ions in gaseous state: electrospray ionization mass spectrometric study

Study of the non‐covalent molecular complexes in gas phase by electrospray ionization mass spectrometry (ESI‐MS) represents a promising strategy to probe the intrinsic nature of these complexes. ESI‐MS investigation of a series of synthetic octapeptides containing six alanine and two lysine residues differing only by their positions showed the formation of non‐covalent dimers, which were preserved in the gas phase. Unlike the monomers, the dimers were found to show only singly protonated state. The decrease in the solvent polarity from water to alcohol showed enhanced propensity of formation of the dimer indicating that the electrostatic interaction plays a crucial role to stabilize the dimer. Selective functionalization studies showed that ε‐NH₂ of lysine and C‐terminal amide (? CONH₂) facilitate the dimerization through intermolecular hydrogen bonding network. Copyright © 2010 John Wiley & Sons, Ltd.     

Fourier transform ion cyclotron resonance study of multiply charged aggregates of small singly charged peptides formed by electrospray ionization

Aggregates of singly protonated peptides formed with a nanoelectrospray ion source have been observed in the gas phase using Fourier transform ion cyclotron resonance (FT-ICR). Employment of “soft” ion sampling conditions in the source, which were developed previously to generate water clusters of biomolecules, provides significant yields of aggregates of singly protonated GGDPG ([2GGDPG + 2H]²⁺), GGEPG ([2GGEPG + 2H]²⁺), and VEPIPY (2VEPIPY + 2H]²⁺). With peptide mixtures, heteroaggregates, e.g., [GGDPG + GGEPG + 2H]²⁺ have also been observed along with the homoaggregates. These weakly bound noncovalent complexes undergo facile exothermic dissociation into the corresponding singly protonated monomer species with normal operation of the electrospray ion source. For example, the aggregates were not observed in FT-ICR experiments utilizing a conventional electrospray ionization (ESI) or fast atom bombardment source or with a quadrupolar ion trap mass spectrometer equipped with a conventional ESI source. The formation and metastability of these aggregates are dependent on highly specific intermolecular hydrogen bonding between the monomers. The amino acid sequence (DPG) of GGDPG mimics the well-known β reverse turn of proteins and semiempirical calculations show that it provides excellent hydrogen bonding sites for a protonated N-terminus amino group. Support for this conjecture is provided by the failure to observe aggregate formation of singly protonated peptides with several larger peptides, including hexaglycine and hexaalanine.     

Ultrafast fragmentation and ionisation dynamics of ammonia clusters

The formation of protonated and unprotonated ammonia cluster ions is studied by femtosecond two colour two photon pump-probe techniques applied to (NH₃) _n and (ND₃) _n clusters withn up to 8. The fourth harmonic (～ 200 nm, 6.2 eV, 160 fs) of a Ti: Sapphire laser pulse is used to excite the clusters in a state corresponding to theà state of NH₃ while the third harmonic (267 nm, 4.65 eV) is used for the subsequent ionisation step. Employing a combination of the optical Bloch equations for the excitation process and rate equations for the cluster dynamics we calibrate the zero time delay and carefully analyse the time dependence of the pump-probe signal. Several distinct intermediate steps in the time evolution can be distinguished, having characteristic time constants ranging from 40 fs to over 100 ps. They are discussed in a consistent scheme for the excitation, ionisation and protonation dynamics, accounting also for characteristic differences observed between deuterated and undeuterated species. A particularly remarkable time dependence of the homogeneous (NH₃) ₂ ⁺ cluster ion signal is interpreted as a fingerprint of internally protonated neutral precursors of the type NH₃NH₂NH₄.     

On a polarographic double wave of some Cu(II) alkanoates involving metal-metal interaction

Electrode behaviour of some Cu(II) alkanoates, known to exhibit metal-metal exchange interaction to varying degrees, has been studied in various organo-aqueous solutions under varying experimental conditions, at DME. All the Cu(II) alkanoates except Cu(II) monochloroacetate have been observed to undergo diffusion-controlled, two-electron transfer, irreversible electroreduction in sodium perchlorate and gelatin. In 25% alcohol and 25% acetone, all these alkanoates undergo irreversible electroreductions. In 50% alcohol or 50% acetone, all these Cu(II) alkanoates except Cu(II) trichloroacetate undergo a double wave electroreduction and similar are the observations for Cu(II) formate and acetate in 25% dioxane. The first part of this double wave may be attributed to the monomer electroreduction and the second part to the dimer, which is supposed to be in sluggish equilibrium with the monomer. E_1/2 data have been reported.     

Elucidation de structure et comportement de produits naturels sous ionisation chimique—II: Aminolyse et substitution nucleophile produites—en phase gazeuse avec l'ammoniac—sur la cinerubine a

The structure of cinerubine A has been studied by chemical ionisation mass spectrometry. In the NH₃/Cl mode, this type of compound with polyfunctional sites undergoes as aminolysis reaction, i.e. hydroxyl and carbonyl groups are substituted by NH₃. The protonated molecular ions formed in the ion source fragment to give intense ions which provide useful structural information. Unimolecular fragmentations in the first field-free region permit determination of the reactive sites. Use of ND₃ as reagent gas provides information on the number of mobile H atoms and permits the assignment of structure for these ionic species.     

Use of deuterium–hydrogen exchange to characterize the fragmentation pathways of arteether and its metabolites in a thermospray mass spectrometer

Whereas the thermospray mass spectra of most compounds consist of only the pseudo-molecular ion with little fragmentation, the thermospray mass spectra of arteether (a cyclic endoperoxide) and its metabolites are relatively complex. Assignments of structures to individual fragments from normal spectra was particularly ambiguous because of uncertainties as to which fragments arose from ammonium ion or methanol adducts. In this study, these assignments could be resolved through the comparison of the regular spectrum with the deuterium-exchange spectrum (in an ND₄O₂CCH₃–CD₃OD–D₂O mobile phase) achieved using ‘sandwiched slug’ injection technique. The mass spectra of arteether and four of its metabolites all showed [M + ND₄]⁺ pseudo-molecular ions with greater than 91% H/D exchange, indicating a high efficiency with a minimal use of deuterated mobile phase. Most fragments showed H/D exchange rates in the 70–90% range and the isotope shift of individual spectral lines (ΔM) was found to be extremely useful in determining the structure of the fragment.     

Fluorine-bridged chain compounds as models in magnetochemistry: structures and magnetism of (imidH)2[Fe2oxF6] and [Fe2oxF4(DMSO)4]

The crystal structure of a new Fe(III) dimer obtained by fluorination of FeCl₃ with Me₃SnF in the presence of oxalic acid and crystallization from dimethyl sulfoxide (DMSO) has been determined: [Fe₂oxF₄(DMSO)₄] 1, Space group P2₁/n, Z=2, a=7.259(2), b=11.409(3), c=13.374(2) Å, β=97.26(1), R=4.47%. It shows a tetradentate bridging oxalato, equatorial cis fluorine and axial trans DMSO ligands.From an aqueous solution of FeF₃·3H₂O, oxalic acid, and imidazolium fluoride, crystals of (imidH)₂[Fe₂oxF₆] 2 could be precipitated. Space group Pban, Z=2, a=9.143(2), b=20.837(6), c=3.890(1) Å, R=2.51%. The structure shows anionic chains formed by bridging trans fluorine ligands connected, like in the dimer above, by oxalate anions to form a double chain. The magnetic properties were determined on powders by SQUID measurements. Mössbauer experiments were performed on the S=5/2 Fe³⁺ double chain of (imidH)₂[Fe₂oxF₆], where the anisotropy is of dipolar origin and is therefore very weak. The striking result is the characteristic aspect of two magnetic split spectra below T_N and additionally, the coexistence of a temperature dependent paramagnetic component (doublet) and the magnetic split spectra in the temperature range T_N=14.5 and T_H=40 K. The origin of that unusual behaviour is attributed to nonlinear excitations (magnetic solitons) in systems with small local anisotropy. Consequently, the domain wall width should be large. The subspectrum of the doublet was identified with intraband solitons. The relative intensity of the fast relaxing component increases with increasing temperature as ∝exp(−E_s/kT). From the slope of fractional intensity of the broadened doublet as a function of inverse temperature, the activation energy E_s/k=40(1) K was derived. Considering the results of magnetic susceptibility measurements of the intra-chain exchange constant J/k=−17.3(2) K, we found an anisotropy constant of D/k=0.15(2) K in agreement with our magnetic study. Additionally, parameters as the inter-chain constants J′/k=0.08(2) K and J″/k=0.60(5) K were calculated. Speculatively, from the experimental data of the magnetically split spectra the ratio (domain length/domain wall width) ≈ 2 was estimated at T_N, in agreement with the theoretical expectations. Finally, the results obtained for the double chain of (imidH)₂[Fe₂oxF₆] with weak anisotropy are compared with previously reported Mössbauer experiments on -doped powder of (ND₄)₂MnF₅, inelastic neutron scattering, and magnetic susceptibility measurements on single crystals of (ND₄)₂MnF₅ with strong anisotropy.