Poly(amino acids) by XPS : Analysis of Poly(L- Leucine)

Poly(L-leucine) was analyzed as a model compound representative of poly(amino acids) with alkyl pendant groups. Two specimens of different origins and molecular masses were examined. One (A) was pressed on an indium foil to allow recording time of flight secondary ion mass spectrometry (TOF SIMS) spectra in parallel and, therefore, insure good electrical charge dissipation. The other one (B) was placed in a stainless steel trough and then pressed. The C 1s peak was recorded first and recorded again at the end; no difference was found between the two records, indicating that poly(L-leucine) did not suffer beam damage. The C 1s peak of specimen A was split into three components: one fixed at 284.8 eV and due to C —(C,H), the other two found at 286.1 and 287.8 eV and due to C N and O C N, respectively. The O 1s peak centered at 531.2 eV and due to C CN showed a tail which indicated the presence of a small component at 533.0 eV, possibly due to water or C C H of contaminating compounds. The N 1s peak was symmetric and centered at 399.7 eV. The position of the peak components of specimen B were coincident with specimen A within 0.1 eV. The ratio of the molar concentration, with respect to N, of [C CN], [O C N], and [C N], on the one hand, and of [C(C,H)], on the other hand, fitted the expected values of 1 and 4 within 5%, except for one component of one specimen which showed a deviation of 15%. The choice of the background shape (S shape or linear) has no significant influence on the results. These spectra may be used as references for the characterization of polypeptides and proteins.