Ultraflex 2 spectrometer

The 25-residue SRP2 were prepared by stepwise solid-phase synthesis using 9-fluorenylmethoxycarbonyl (Fmoc) chemistry on an ABI model 431A automatic peptide synthesizer (Applied Biosystems; Foster City, CA). For peptide synthesis CLEAR amide resin (0.3 mmol g^–1; Peptides International, Louisville, KY) and N^α-F-moc amino acids (Anas-pec Inc., San Jose, CA) were used. Following de-protection and cleavage, the linear peptide was cyclized by formation of a disulfide bond between C8 and C19. The peptide was purified to >95% purity by HPLC (System Gold HPLC; Beck-man Instruments, Inc., Fullerton, CA) with a Phenomenex reversed-phase C-18 column (Torrance, CA), and eluted using a linear gradient of 10–90% acetonitrile containing 0.1% trifluoroacetic acid at 1 mL min^–1. HPLC-purified peptide was characterized by matrix-assisted-laser desorption time-of-flight (MALDI-TOF) mass spectrometric analysis using a Bruker Ultraflex II spectrometer (Bruker Daltronics, Billerica, MA). After characterization, peptide was lyophilized and stored as dry powder until use.