A strategy for improving the sequencing of peptides by infrared multiphoton dissociation (IRMPD) in a linear ion trap mass spectrometer is described. We have developed an N-terminal derivatization reagent, 4-methylphosphonophenylisothiocyanate (PPITC), which allows the attachment of an IR-chromogenic phosphonite group to the N-terminus of peptides, thus enhancing their IRMPD efficiencies. After the facile derivatization process, the PPITCmodified peptides require shorter irradiation times for efficient IRMPD and yield extensive series of y ions, including those of low m/z that are not detected upon traditional CID. The resulting IRMPD mass spectra afford more complete sequence coverage for both model peptides and tryptic peptides from cytochrome c. We compare the effectiveness of this derivatization/IRMPD approach to that of a common N-terminal sulfonation reaction that I n recent years, there has been tremendous effort devoted to the application of mass spectrometry to the field of proteomics [1, 2] in large part because of the success of tandem mass spectrometry for elucidation of primary sequences and modifications of peptides and proteins [3][4][5][6][7]. Several ion activation methods have been developed in this context, including collision induced dissociation (CID) [8,9], electron capture dissociation (ECD) [10]. electron-transfer dissociation (ETD) [11], pulsed-Q dissociation (PQD) [12], surface induced dissociation [13], and photodissociation (PD) [14 -19]. Each method has its own particular strengths and shortcomings, with CID being the most widely used due to its relatively well-understood underpinnings. However, the resulting CID mass spectra can be cluttered with redundant b and y ions, as well as ions created by uninformative losses of small organic molecules like water and ammonia. Hence, the interpretation of such data manually or via de novo algorithms remains challenging [20,21]. ECD and ETD provide complementary c and z ions and have proven more useful for tracking post-translational modifications (PTMs); however, these activation methods generally offer lower dissociation efficiencies than CID and are highly dependent on the charge state of the selected precursor ion [10,11]. PD methods, including both infrared multiphoton dissociation (IRMPD) and ultraviolet photodissociation (UVPD), have also shown promise because of their potential for high-energy deposition and tunability [14 -19, 22-35]. Since photoabsorption is not a collision-based process, it does not alter the kinetic energies of ions and thus minimizes any ion losses due to scattering.Photodissociation also offers particular advantages for ion trap mass spectrometers. For example, photoactivation is independent of the trapping voltage, unlike CID. In CID, the rf voltage during ion activation influences the energy deposition of collisional activation as well as defines the lower m/z range. The potential for greater energy deposition in CID occurs at the expense of storage of lower m/z ions, ones which might be key diagnostic ions for...