Polypeptide mass standard

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was used to analyze the purified peptide. AXIMA CFR mass spectrometer (Kratos Analytical) was operated in linear and positive ion mode. Polypeptide mass standard (Kratos Analytical) is used as an external calibration.

Mass spectrometry analysis was performed on a matrix-assisted laser desorption ionization time-of-flight mass spectrometer (MALDI-TOF–MS) AXIMA CFR (Kratos Analytical). α-Cyano-4-hydroxycinnamic acid (CHCA) was used as matrix. The specific parameters were as follows: the ion acceleration voltage was 20 kV, the accumulating time of single scanning was 50 s, polypeptide mass standard (Kratos Analytical) serving as external standard.

The purified peptide was subjected to amino acid sequencing by Edman degradation on an Applied Biosystems pulsed liquid-phase sequencer, model 491. The Cys sulfhydryl group was alkylated by reaction with 4-vinyl pyridine according to the method described by Friedman et al. [7] . The actual molecular mass of the peptide was measured by a Matrix-Assisted Laser Desorption Ionization Time-Of-Flight mass spectrometer (MALDI-TOF-MS, AXIMA CFR (Kratos Analytical)) in positive ion and linear mode. α-Cyano-4-hydroxycinnamic acid (CHCA) was used as matrix. The specific operating parameters were as follows: the ion acceleration voltage was 20 kV; the accumulating time of single scanning was 50 s. The polypeptide mass standard (Kratos Analytical) was used as the external standard.