Acid digestion, accelerated

While all proteases convert proteins to amino acids, each enzyme acts on a particnlar section of the protein. Endopeptidases hydrolyze peptide bonds that are adjacent to specific amino acid side chains, thns converting long protein chains into many shorter peptides. The exopeptidases hydrolyze peptide bonds at either the carboxyl- or amino-terminal ends of these peptides. The endopeptidases, therefore, prodnce more snbstrates for the exopeptidases so that the rate of protein digestion accelerates almost like a chain reaction and then declines as the hydrolytic processes approach completion. 

The higher efficiency of microwave ovens to permit the warming of the samples allows drying, favors the digestion with acid, and accelerates chemical reactions that are controlled by a slow thermal stage. 

MALDI-MS was performed using a Kratos Kompact MALDI III mass spectrometer fitted with a standard 337 nm nitrogen laser, and operated in the linear mode at an accelerating voltage of 20 kV. Two sample preparation methods were used (1) for collected peptides, 0.3 pL aliquots of sample and matrix (a-cyano-4-hydroxy cinnamic acid, Biomolecular Separations, Inc.) were mixed on the probe slide and allowed to air dry or (2) for unfiactionated digests, a thin polycrystalline film was prepared according to (11) (with modifications for use on a probe slide (12)), matrix and sample aliquots were mixed (usually 0.3 pL each) on this surface and prior to drying, rinsed twice with 2 pL of deionized water. 

---