private function performMethionineExcision(array $peptides)
    {
        $nmePeptides = array();
        foreach ($peptides as $peptide) {
            $entry = $peptide->getProteins()[0];

            if ($entry->getStart() > 0) {
                continue;
            }

            $sequence = $peptide->getSequence();

            if (strlen($sequence) <= 1) {
                continue;
            }

            if ($sequence[0] != 'M') {
                continue;
            }

            $nmePeptide = new Peptide();
            $nmePeptide->setSequence(substr($sequence, 1));
            $nmePeptide->addProtein($entry->getProtein(), 1, $entry->getEnd());
            $nmePeptide->setMissedCleavageCount($peptide->getMissedCleavageCount());

            $nmePeptides[] = $nmePeptide;
        }

        return array_merge($peptides, $nmePeptides);
    }