Enhancement of ultraviolet photodissociation efficiencies through attachment of aromatic chromophores.

Two N-terminal derivatization reagents containing aromatic chromophores, 4-sulfophenyl isothiocyanate (SPITC) and 4-methylphosphonophenyl isothiocyanate (PPITC), were used to increase the dissociation efficiencies of peptides upon ultraviolet photodissociation (UVPD) at 193 nm. The resulting UVPD spectra are dominated by C-terminal ions, including y, z, x, v, and w ions, and immonium ions. The attachment of the PPITC or SPITC groups leads to a reduction in the number and abundances of N-terminal ions because the added phosphonate or sulfonate functionalities result in neutralization of some of the N-terminal species, ones that might normally be singly protonated in the absence of the negatively charged sulfonate or phosphonate groups. In addition, the greater photoabsorptivities of the PPITC- and SPITC-derivatized N-terminal product ions enhanced their secondary photodissociation, leading to formation of immonium ions.