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neutral molecules should be available to ionize. A second
complication is that we could not be certain that exactly the
same quantity of each tagged molecule was introduced into the
spectrometer. This prevents us from drawing quantitative
conclusions about relative photoionization yields. Nevertheless,
the data displayed in Figures 5 and 6 indicate that sufficient
quantities of tagged biomolecules were present to produce ample
ion yields through the MALDI process.
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We conclude from these studies that the binding of a
chromophore to bradykinin and a few smaller biological
molecules does little to facilitate their two-photon ionization
by ultraviolet laser light. Some of the photon energy imparted
to the derivatized peptides is channeled into fragmentation of
the chromophore label at the ether linkage. However, the
fragment ion yield is very poor. Therefore, although we observe
evidence that some derivatized bradykinin ionizes before it
fragments, the poor yield of this process suggests that it is not
efficient as suggested by Becker and Wu.23 In fact, these
experiments reveal that these molecules are quite resistant to
laser-induced photoionization, despite the long alkyl chain added
between chromophore and peptide in hopes of confining
excitation to the chromophore. The jet cooling utilized by Frey
and Holle,22 Li and Lubman,25 Weyssenhoff et al.,26 and Anex
et al.28 appears to be essential for photoionizing biological
molecules of even modest size.
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Acknowledgment. This work has been supported by the
National Science Foundation. The authors thank Stephen
Antonelli (Indiana University) for his assistance with the
synthesis of NHA and NDA and Ernest Davidson for helpful
discussions.
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