Peptidcluster in der Gasphase
In this work the formation of clusters caused by non-covalent interactions between matrix molecules and short-chain peptides (i.e. di-, tri- and tetra-peptides) has been investigated by using a laser mass spectrometer. After the desorption achieved by using IR laser radiation the molecules are transported with the support of a supersonic jet into the ionisation chamber. Thus their internal degrees of freedom are cooled. As a result the formation of weakly bonded clusters are observed. The formation of clusters in the gas phase consisting of peptides could be shown by using a combination of laser desorption and multi photon ionisation. Under the ionization conditions a direct ionization of these peptides is not possible because they lack an adequate chromophoric system. The cluster have been detected by mixing with several derivatives of benzoic acid and cinnamic acid, which are standard matrix substances to be used in the matrix assisted laser desorption / ionisation technique. The measurements demonstrate that the formation of peptide clusters in the gas phase are possible by applying nano-second laser pulses. Additionally the formation of protonated homogenous peptide clusters respectively heterogeneous clusters of the matrix can be observed. Besides the formation of clusters in the gas phase, a further result of this work shows the possibility of gas phase reaction in the cluster. The simple proton transfer reaction as well as reactions between then proton donator and the peptide are observed. The following results regarding the gas phase reaction with tryptophan as a proton donator show: · The separation of water from the ionised peptide-matrix-cluster shows that it is possible to observe the prolongation of the peptide chain and thus the formation of peptide bonding in the gas phase for the first time. · The relative intensity of the signals of the separation of water increases when changing from 260 nm to 290 nm. This means, that the reaction of peptide bond- formation has a lower activation energy. · During the dissociative proton transfer the proton is transferred from the nitrogen of the indole ring of tryptophan to the analyte molecule. This could be demonstrated in experiments with protected derivatives of tryptophan. · Characteristic patterns of fragmentation pathway could be shown with help of mass spectra for tri- and tetra-peptides. Based on the results of the fragmentation reactions in many experiments between di-, tri- and tetra-peptides and vanillic acid as matrix, and on additional experiments with complete deuteration of the phenolic proton and the carboxylic proton of the vanillic acid it could be demonstrated that the intramolecular proton transfer is performed from the n-terminal side of the peptide chain to the c-terminal side.