Abstract
Crown ethers with different ring sizes and substituents (18-crown-6, dibenzo-18-crown-6, dicyclohexano-18-crown-6, a chiral tetracarboxylic acid-18-crown-6 ether, dibenzo-21-crown-7, and dibenzo-30-crown-10) were evaluated as shift reagents to differentiate epimeric model peptides (tri-and tetrapeptides) using ion mobility mass spectrometry (IM-MS). The stable associates of peptide epimers with crown ethers were detected and examined using traveling-wave ion mobility time-of-flight mass spectrometer (Synapt G2-S HDMS) equipped with an electrospray ion source. The overall decrease of the epimer separation upon crown ether complexation was observed. The increase of the effectiveness of the microsolvation of a basic moiety - guanidine or ammonium group in the peptide had no or little effect on the epimer discrimination. Any increase of the epimer separation, which referred to the specific association mode between crown substituents and a given peptide sequence, was drastically reduced for the longer peptide sequence (tetrapeptide). The obtained results suggest that the application of the crown ethers as shift reagents in ion mobility mass spectrometry is limited to the formation of complexes differing in stoichiometry rather than it refers to a specific coordination mode between a crown ether and a peptide molecule.
Highlights
Peptides belong to the most biologically relevant group of molecules
Addition of any crown ether presented in Scheme 1 to the solution containing a model tri- or tetrapeptide with a general sequence Ac-(H)FRW-NH2 led to the formation of noncovalently bound complex, which was detected as a protonated ion in the mass spectrum
The specificity was observed in the association of DB30C10 and 18C6 to the guanidinium group and the ammonium group, respectively, and it was suggested that it may be used to determine the presence of arginine or lysine moiety in a peptide sequence, [30] in the non-competitive binding conditions, both ethers along with these shown in Scheme 1 form stable associates with the model peptides containing the arginine residue
Summary
Peptides belong to the most biologically relevant group of molecules. it has long been assumed that in higher animals proteins and peptides are built exclusively from Lamino acids, the analysis of D-amino acid-containing peptides as the source of naturally and unnaturally occurring modifications in living organisms is becoming an increasingly important issue. [1, 2] to other types of isomers, differing in chemical, physical, and biological properties, they remain undistinguishable using direct mass spectrometry measurements, due to the identical ratio of the mass of ion to its charge (m/z). The ion mobility shifts of the dipeptides isobars: RA, KV, LN upon complexation with crown ethers were mainly due to the different complexation stoichiometry, which in turn was closely related to the selectivity and specificity of the formation of the complexes between basic amino acid side chain or NH2- terminus with crown ethers depending on the crown size. In this manuscript the effect of microsolvation of the amino acid basic side-chain on the ion mobility shift of epimeric peptides is described. The IM peaks were fitted with Gaussian distributions using SigmaPlot 12.0
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