Background: In bacteria, such as Salmonella typhimurium, tryptophan is synthesized from indole-3-glycerole phosphate (IGP) by a tryptophan synthase αββα heterotetramer. Plants have evolved multiple α (TSA) and β (TSB) homologs, which have probably diverged in biological function and their ability of subunit interaction. There is some evidence for a tryptophan synthase (TS) complex in Arabidopsis. On the other hand maize (Zea mays) expresses the TSA-homologs BX1 and IGL that efficiently cleave IGP, independent of interaction with TSB.

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The development and validation of analytical methods is a key to succeed in investigating noncovalent interactions between biomolecules or between small molecules and biomolecules. Electrospray ionization mass spectrometry (ESI-MS) was applied with a Fourier transform ion cyclotron resonance mass spectrometer (FTICR-MS) as well as a quadrupole/time-of-flight tandem mass spectrometer (QqToF-MS) for a systematic investigation of noncovalent complexes based on nucleobase pairing in an artificial and noncharged backbone topology. Synthetical beta-peptide helices covalently modified with nucleobases were organized by recognition of a sequence of four nucleobases. Specific duplexes of beta-peptide helices were obtained on the basis of hydrogen bonding base pair complementarity. Oligomer interactions were detected with defined stoichiometry and sensitivity for the respective duplex stability. FTICR-MS and QqToF-MS were used equally well to indicate double strand stabilities in agreement with the dissociation data determined by UV spectroscopy. Furthermore, the dissociation energies of gas phase ions of the noncovalent complexes were analyzed with collision induced dissociation (CID)-MS/MS and infrared multiphoton dissociation (IRMPD)-MS/MS. The CID conditions turned out to be too harsh for a differentiation of the duplex stabilities, whereas IRMPD might be developed as a technique to detect even small interaction energy differences.

Specific helix oligomerization between transmembrane segments (TMSs) is often promoted by motifs like GxxxG. Disruption of this motif in the transmembrane segments of vesicular stomatitis virus G-protein and of glycophorin A results in a reduced dimerization level studied by in vivo systems like ToxR. This paper reports the influence of sequence motifs like GxxxG in solution and the gas phase.The transmembrane segments may behave differently in the gas and liquid phase, because of the absence of surrounding solvent molecules in the gas phase. Comparison of experiments depending on peptide properties performed in the gas and liquid phase discloses that the peptides retain 'some memory' of their liquid-phase structure in the gas phase. A direct correlation has been found between helicity in solution as determined by circular dichroism and dimerization in the gas phase monitored by electrospray mass spectrometry. These results show that a proper folding in solution is required for oligomerization.On the other hand, sequence-specific oligomerization depending on the GxxxG motif was not observed with the mass spectrometric detection. Further on, neither concentration-dependent complex studies nor studies regarding complex stability in the gas phase - via collision-induced dissociation (CID) - led to sequence-specific differences.Finally, the findings show that in mass spectrometric measurements noncovalent interactions of studied TMSs is rather more dependent on the secondary structure and proper folding than on their primary structure.