Mass Spectrometry Measures the Mass of Proteins and Peptides

A powerful technique for measuring the mass of molecules such as proteins and peptides is mass spectrometry. This

▲ EXPERIMENTAL FIGURE 3-37 The molecular weight of proteins and peptides can be determined by time-of-flight mass spectrometry. In a laser-desorption mass spectrometer, pulses of light from a laser ionize a protein or peptide mixture that is absorbed on a metal target (1). An electric field accelerates the molecules in the sample toward the detector ( 2 and 3). The time to the detector is inversely proportional to the mass of a molecule. For molecules having the same charge, the time to the detector is inversely proportional to the mass. The molecular weight is calculated using the time of flight of a standard.

technique requires a method for ionizing the sample, usually a mixture of peptides or proteins, accelerating the molecular ions, and then detecting the ions. In a laser desorption mass spectrometer, the protein sample is mixed with an organic acid and then dried on a metal target. Energy from a laser ionizes the proteins, and an electric field accelerates the ions down a tube to a detector (Figure 3-37). Alternatively, in an electrospray mass spectrometer, a fine mist containing the sample is ionized and then introduced into a separation chamber where the positively charged molecules are accelerated by an electric field. In both instruments, the time of flight is inversely proportional to a protein's mass and directly proportional to its charge. As little as 1 X 10—15 mol (1 femtomole) of a protein as large as 200,000 MW can be measured with an error of 0.1 percent.

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