Selection of Collision Energies in Proteomics Mass Spectrometry Experiments for Best Peptide Identification: Study of Mascot Score Energy Dependence Reveals Double Optimum

International audienceCollision energy is a key parameter determining the information content of beam-type collision induced dissociation tandem mass spectrometry (MS/MS) spectra, and its optimal choice largely affects successful peptide and protein identification in MS-based proteomics. For an MS/MS spectrum, quality of peptide match based on sequence database search, often characterized in terms of a single score, is a complex function of spectrum characteristics, and its collision energy dependence has remained largely unexplored. We carried out electrospray ionization-quadrupole-time of flight (ESI-Q-TOF)-MS/MS measurements on 2807 peptides from tryptic digests of HeLa and E. coli at 21 different collision energies. Agglomerative clustering of the resulting Mascot score versus energy curves revealed that only few of them display a single, well-defined maximum; rather, they feature either a broad plateau or two clear peaks. Nonlinear least-squares fitting of one or two Gaussian functions allowed the characteristic energies to be determined. We found that the double peaks and the plateaus in Mascot score can be associated with the different energy dependence of b- and y-type fragment ion intensities. We determined that the energies for optimum Mascot scores follow separate linear trends for the unimodal and bimodal cases with rather large residual variance even after differences in proton mobility are taken into account. This leaves room for experiment optimization and points to the possible influence of further factors beyond m/z

Scientific Workflow Optimization for Improved Peptide and Protein Identification

Background: Peptide-spectrum matching is a common step in most data processing workflows for massspectrometry-based proteomics. Many algorithms and software packages, both free and commercial, have beendeveloped to address this task. However, these algorithms typically require the user to select instrument- andsample-dependent parameters, such as mass measurement error tolerances and number of missed enzymaticcleavages. In order to select the best algorithm and parameter set for a particular dataset, in-depth knowledgeabout the data as well as the algorithms themselves is needed. Most researchers therefore tend to use defaultparameters, which are not necessarily optimal.Results: We have applied a new optimization framework for the Taverna scientific workflow management system(http://ms-utils.org/Taverna_Optimization.pdf) to find the best combination of parameters for a given scientificworkflow to perform peptide-spectrum matching. The optimizations themselves are non-trivial, as demonstrated byseveral phenomena that can be observed when allowing for larger mass measurement errors in sequence databasesearches. On-the-fly parameter optimization embedded in scientific workflow management systems enables expertsand non-experts alike to extract the maximum amount of information from the data. The same workflows could beused for exploring the parameter space and compare algorithms, not only for peptide-spectrum matching, but alsofor other tasks, such as retention time prediction.Conclusion: Using the optimization framework, we were able to learn about how the data was acquired as well asthe explored algorithms. We observed a phenomenon identifying many ammonia-loss b-ion spectra as peptideswith N-terminal pyroglutamate and a large precursor mass measurement error. These insights could only be gainedwith the extension of the common range for the mass measurement error tolerance parameters explored by theoptimization framework