EFFECTS OF POLYPEPTIDE CONFORMATION AND SURFACE BONDING ON STATIC SECONDARY-ION MASS-SPECTRA

For the first time, the alpha-helical homopolypeptide poly(gamma-benzy l-L-glutamate) (PBLG) was synthesized by solid-phase methods. A 10 res idue PBLG sample (10-mer) was prepared which exhibited a beta-sheet co nfiguration, and a 15 repeat unit PBLG sample (15-mer) was made which displayed partial alpha-helical character. Solvent-cast thin films of these peptides were deposited on both gold and silver and analyzed by time-of-flight static secondary ion mass spectrometry (ToF-SSIMS). The PBLG 10-mer spectra consisted of a strong molecular ion (cationized w ith Na or Ag) and less-pronounced fragments spaced by the benzyl gluta mate mass (n-mers) attributed to SSIMS fragmentation. Contrary to this , the PBLG 15-mer did not provide an intact molecular ion, and only lo w-mass n-mer species were observed. These n-mers are believed to be a product of SSIMS-induced molecular fragmentation that is enhanced by t he peptide alpha-helical conformation. A disulfide moiety (SS) was als o coupled to the 10-mer and 15-mer N-termini (10-merSS and 15-merSS) t o provide a functional group for self-assembly on gold. Molecular ions were observed from solvent-cast films of these samples, verifying the presence of the SS cap. However, self-assembled monolayers of these p eptides on gold generated weak SSIMS signals which primarily reflect t he strong binding of the SS group to the substrate.