Gas-phase formation of large neutral alkaline-earth metal tryptophan complexes

Abstract

We report on the first observation of isolated large neutral metal amino acid complexes such as TrpnMek, with Me == Ca, Ba, Sr, cluster combinations covering n = 1. ..33, k = 0..2 and masses beyond 6500 u. The cluster beam is generated using UV laser desorption from a mixed powder of alkaline-earth metal salts and tryptophan inside a cluster mixing chalU1el. The particles are detected using VUV photoionization followed by time-of-flight mass spectroscopy. The enhanced stability of metal amino acid clusters over pure amino acid clusters is substantiated in molecular dynamics simulations by determining the gain in binding energy related to the inclusion of the metal atoms. . The majority of studies so far focused on the generation and use of ionic species. Our work, presented here, focuses on the gas-phase formation of neutral biomolecular clusters as did already some very early studies in the field [1]. Such experiments are often motivated by the desire to better identify the role of solvation and desolvation for the relative importance of various binding forces in organic clusters and molecules [5].The particle's neutrality is also an important criterion in efforts to establish new beam methods for quantum interferometry with molecular matter waves [6-9] as well as for precision metrology on large molecules [10][11][12]. Neutral molecules are much better isolated from any electromagnetic environment and they may therefore propagate coherently, without noticeable perturbation, even in a rather noisy field environment.Biomolecular clusters are exciting study objects for future matter wave experiments as they permit one to scale the mass in well-defined steps, while at the same time changing the internal properties in an intriguing way: the number of possible conformations as well as the number of allowed configurational and vibrational transitions scales exponentially with the size of the cluster [13], and it is still an open question how this will affect de Broglie interferometry [14], which formally considers only the center of mass motion alone. (19]. But it has still remained a challenge to find reproducible methods for photoionizing biomolecules and complexes with masses exceeding 3000 u [17,20,21].The formation and photo-detection of small amino acid [22] and nucleotide clusters [23], both hydrated and solvent-free, were already reported for objects up to 1400 u. Ultracold small bioclusters can be formed by pick-up in helium nano-droplets [24]. Complexes formed between metal atoms and amino acids were also studied to understand their structure [25], their fragmentation pathways [26,27], and metal ion affinities [28].Here we extend these investigations to much larger organometallic complexes composed of up to more than 30 tryptophan molecules with masses exceeding 6500 u, that can still be detected using VUV photoionization and time-of-flight mass spectrometry. ExperimentalWe generate the molecular macroclusters using pulsed laser evaporation and collisional cooling: the pulsed beam of aNd:YAG laser (Quantel...

Fragmentation of the tryptophan cluster [Trp9–2H]2− induced by different activation methods

“…Non‐covalent amino acid clusters have been the subject of intense scrutiny over the past two decades. Diverse areas of interest have included: (i) the use of fragmentation of proton‐bound dimers to determine the gas‐phase proton affinities1 and gas‐phase acidities2–4 of amino acids via Cooks's kinetic method:5–7 (ii) establishing whether amino acids can exist in their zwitterionic form within clusters;8–10 (iii) the search for magic numbers, which has most notably revealed the homochiral serine octamer;11–13 (iv) the use of silver amino acid cluster precursors to assembly silver cluster and silver‐hybrid cluster cations in the gas phase for subsequent reactivity14, 15 and spectroscopic studies;16, 17 (v) peptide bond formation within amino acid clusters;18 (vi) analytical applications aimed at establishing the chirality of substrates using metal clusters of amino acids;19, 20 and using large non‐specific clusters of known concentrations of an amino acid with an analyte to provide ‘standard‐free quantitation’ of the analyte;21, 22 (vii) formation and ionisation of neutral, non‐covalent amino acid complexes23 including electron attachment to clusters embedded in helium nanodroplets;24, 25 and (viii) studies aimed at examining the formation26–31 and fragmentation reactions of multiply charged clusters using combinations of electrospray ionization (ESI) or its variants together with collision‐induced dissociation (CID)30 and electron‐induced dissociation (EID) 31…”

Alkaline earth metal salts of CaCO3, BaCO3, and SrCO3 as matrix for tryptophan SIMS analysis

“…The samples were prepared in three ways using one component matrix of CaCO 3 in one solid and one liquid forms, and three component matrices of CaCO 3 BaCO 3 and SrCO 3 as one solid form. To prepare solid samples consisting of Trp, alkaline earth metal salts, and cellulose, Marksteiner's procedure was used 22. The liquid sample consits of 0.01g CaCO 3 diluted in 0.5 ml of 2 M citric acid, directly mixed with aqueous solution of Trp (0.05g of Trp and 0.5 ml of water).…”

“…Due to the fact that indol functional group absorbs strongly in the near ultraviolet part of the spectrum, Trp photoionization has been studied under various conditions 18–21. MALDI mass spectrometry of Trp ions was demonstrated by using alkaline earth metal salts to enhance stability of metal amino acid clusters over pure amino acid clusters 22. As a result, isolated large neutral metal amino acid complexes were observed within < 6500 u 22.…”

“…MALDI mass spectrometry of Trp ions was demonstrated by using alkaline earth metal salts to enhance stability of metal amino acid clusters over pure amino acid clusters 22. As a result, isolated large neutral metal amino acid complexes were observed within < 6500 u 22. Our contribution has ambition to compare the MALDI and SIMS results, although we are very much aware of different ionization mechanisms.…”

New Prospects for de Broglie Interferometry

“…the observation of Poisson's spot. Recent developments of new nanoparticle sources [14,15,16] and novel detection methods [17] may soon allow one to experimentally access them in a mass regime between 10 4 and 10 6 atomic mass units (amu). Grating diffraction has already been thoroughly studied with electrons [18,19,20], neutrons [21], atoms [22,23,24] and molecules [25,26,27,28].…”