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Noncovalent complexes between chicken muscle adenylate kinase and two inhibitors, P 1 ,P 4 -di(adenosine-5')tetraphosphate (Ap4A) and P 1 ,P 5 -di(adenosine-5') pentaphosphate (Ap5A), were investigated with electrospray ionization mass spectrometry under non-denaturing conditions. The nonconvalent nature and the specificity of the complexes are demonstrated with a number of control experiments. Titration experiments allowed the association constants for inhibitor binding to be determined. Problems with concentration dependent ion yields are circumvented by a data evaluation method that is insensitive to the overall ionization efficiency. The K a values found were 9.0 ϫ 10 4 M Ϫ1 (Ap4A) and 4.0 ϫ S oft ionization mass spectrometry (MS), in particular matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI), is well known for its ability to bring high molecular weight biomolecules into the gase phase and to even preserve noncovalent complexes [1][2][3][4][5][6][7][8][9][10]. Besides its established applications, a recurring question concerns the applicability of mass spectrometry to measuring noncovalent interaction strengths [11]. There are different approaches to measure the interaction strengths in the gas phase as well as in the liquid phase. In the former case, the mass spectra should ideally directly represent the solution phase chemistry. Gas-phase methods include cone voltage driven dissociation (called the "VC 50 " method) [12,13], collision-induced dissociation [14,15], guided ion beam tandem mass spectrometry [16,17], blackbody infrared radiative dissociation [18,19] and heated capillary dissocation [20,21].Some mass spectrometric studies have been reported in the literature for the determination of noncovalent interaction strengths. There are three basic methods to determine association constants that are reflective of those in solution: first, it is possible to record "melting curves" by raising the temperature of the analyte solution and to use MS to determine the percentage of intact complexes. Cheng et al. analyzed complementary DNA strands [22] and Fändrich et al. studied the chaperone GimC/prefolding homologue complex [13] with this method. Second, competition experiments can be used to evaluate the stability of noncovalent complexes. In Jørgensen's method [23,24], the relative intensities of the free host and the different complexes were used, whereas Kempen et al. employed the known concentration of a reference complex to determine the concentration of an unknown complex [25]. Third, a titration of a host with a guest (or vice versa) can be used. In this case, the mass spectrometric signals should accurately represent the concentrations of the species in solution. The titration method is documented in a number of publications, all of them convincingly showing the successful use of mass spectrometry for the quantitative determination of noncovalent binding constants of proteins and small molecules [26 -33]. This aspect was the main focus in most previous research; th...

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Two different strategies for coupling liquid chromatography with atmospheric pressure matrix assisted laser desorption/ionization (AP MALDI) are presented. The first method is flow-injection liquid AP UV-MALDI. Compared with previous similar research, the detection limit was improved 10 times to 8.3 fmol using a solution of 50 nM peptide with 25 mM alpha-cyano-4-hydroxycinnamic acid. The applicability of this method to measure oligosaccharides, actinomycin antibiotics, antibiotics, phosphopeptides, and proteins is demonstrated. The upper mass limit achieved with the current instrumentation is 6,500 Da (doubly charged cytochrome c). The feasibility of a second strategy based on single-droplet IR AP MALDI is demonstrated here. Aqueous peptide solutions were successfully measured by this method.