Fig 7. False positive spermidine adduct (+128) on Q60 of rUsp/His6.

On first inspection the y3 to y13 ion series, the score of 50.2 and the matched intensity of 89% gave strong support for a spermidine adduct on Q60. However, the absence of y1 and y2 ions leaves open an alternative interpretation. Lysine and spermidine have the same 128 Da mass. The parent ion for the spermidine adduct above is m/z MH⁺³ 600.3044 and that of the Usp peptide APSEIVDFQHSIEQEAK, including K at the C-terminus, is almost identical at m/z 600.3039. It was concluded that the evidence for a spermidine adduct on Q60 is inadequate. The candidate adduct was therefore rejected.