Drug Dbq P Protein CMVP A144L

Fig. 2.25 Positive ion ESI mass spectra of the molecular ion region of DBQ (MW 489 Da) in a titration study of the GPC spin column eluates of DBQ (Drug, D) incubated with CMVP A144L (Protein, P) with molar ratios of 1, 10, 20 and 40 of [D]/[P]. A reference ESI mass spectrum of DBQ and CMVP A144L of a molar ratio of 1:1 is also displayed. The amount of protein in all the experiments were kept constant at 50 pmol.

adducts are more abundant. Nevertheless, these simple GPC spin column/ESI-MS titration experiments demonstrate the usefulness of the technique to determine whether non-specific binding is likely with ligands to specific protein targets when excess ligand is present. Furthermore, as in this case, if excess drug non-covalently binds to the protein target, the ligand is probably unsuitable as a drug candidate. However, if as the reaction D/P ratio increases and the curve for the product D/P ratio asymptotically approaches 1, the ligand is reacting at a specific site in the protein target, most likely the active site. Recently, condensed

Table 2.5 Mole ratios for titration of DBQ drug (D) and cytomegalovirus protease A144L (P) during incubation (Reaction) and after passing through a GPC spin column (Coeluted).

Spectrum* D/P Mole Ratio*

Reaction Coeluted

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