Modern Approaches in de novo Sequencing of Nontryptic Peptides of Ranid and Hylid Frogs by Means of Mass Spectrometry: A Review

Modern mass spectrometry is the most demanded method of analysis of biopolymers, first of all, proteins and peptides. The novel methods of ionization of biomolecules developed at the end of the 20th century allowed increasing the efficiency of mass spectrometry in life sciences, first of all, proteomics. The methods of qualitative and quantitative analysis of these compounds are permanently improving. They become faster, more reliable, informative, sensitive, and selective. Significant role plays automation based on the original algorithms of the machine data processing and usage of spectral databases. One of the challenging questions of mass spectrometry involves analysis of polypeptides of various living organisms, whose genomes remain unknown so far. Amphibians represent a typical example of that species. In these cases, de novo sequencing of novel peptides is required. Development of an automated method of the de novo sequencing exclusively by mass spectrometry is of high scientific priority. It deals with top-down analysis, when an array of tandem mass spectrometry tools allows establishing amino acid sequence of each peptide using their polyprotonated molecules as precursor ions and eliminating assembling based on the hydrolysates of these compounds. A number of nuances still require time-consuming manual spectral elucidation. The present review throws light upon certain problems of mass spectrometry for the de novo sequencing of natural peptides of ranid and hylid frogs as typical representatives of amphibians. It tackles the questions related to post translational modifications, differentiation of isobaric and isomeric amino acid residues, cyclization of short peptides, and application of complementary methods of tandem mass spectrometry to trigger fragmentation. The benefits of mass spectrometry in taxonomy studies, e.g., for the reliable differentiation of closely related ranid and hylid frog species and even populations, are also discussed.