High-throughput approaches towards the definitive identification of pharmaceutical drug metabolites. 2. An example of how unexpected dissociation behaviour could preclude correct assignment of sites of metabolism

Understanding collision-induced dissociation of dofetilide: a case study in the application of density functional theory as an aid to mass spectral interpretation

Fragmentation of molecules under collision-induced dissociation (CID) conditions is not well-understood. This may make interpretation of MSMS spectra difficult and limit the effectiveness of software tools intended to aid mass spectral interpretation. Density Functional Theory (DFT) has been successfully applied to explain the thermodynamics of fragmentation in the gas phase by the modelling the effect that protonation has on the bond lengths (and hence bond strengths). In this study, dofetilide and four methylated analogues were used to investigate further the potential for using DFT to understand and predict the CID fragmentation routes. The products ions present in the CID spectra of all five compounds were consistent with charge-directed fragmentation, with protonation adjacent to the cleavage site being required to initiate fragmentation. Protonation at the dissociative site may have occurred either directly or via proton migration. A correlation was observed between protonation-induced bond lengthening and the bonds which were observed to break in the CID spectra. This correlation was quantitative in that the bonds calculated to elongate to the greatest extent gave rise to the most abundant of the major product ions. Thus such quantum calculations may offer the potential for contributing to a predictive tool for aiding the accuracy and speed mass spectral interpretation by generating numerical data in the form of bond length increases to act as descriptors flagging potential bond cleavages.

Predicting collision-induced dissociation spectra: semi-empirical calculations as a rapid and effective tool in software-aided mass spectral interpretation

RATIONALE

Fifteen molecules were modelled using quantum chemistry, prior to interpreting their collision-induced dissociation (CID) product ion spectra, in a 'blind trial' to establish if calculated protonation-induced bond elongation could be used to predict which bonds cleaved during CID. Bond elongation has the potential to be used as a descriptor predicting bond cleavage.

METHODS

The 15 molecules were modelled with respect to protonation-induced bond length changes using Density Functional Theory (DFT). Significant bond elongations were highlighted to flag potential bond cleavages. CID product ion spectra, obtained using positive ion electrospray ionisation (Waters Synapt G1), were interpreted to establish if observed bond cleavages correlated with calculated bond elongations. Calculations were also undertaken using AM1 (Austin Model 1) to see if this rapid approach gave similar results to the computationally demanding DFT.

RESULTS

The AM1-calculated bond elongations were found to be similar to those generated by DFT. All the polarised bonds observed to cleave (n = 82) had been calculated to elongate significantly. Protonation, possibly via proton migration, on the most electronegative atom in the bond appeared to initiate cleavage, leading to a 100% success rate in predicting the bonds that broke as a result of protonation on a heteroatom. Cleavage of carbon-carbon bonds was not predicted.

CONCLUSIONS

Cleavage of the polarised bonds appears to result from protonation on the more electronegative atom of the bond, inducing conformational changes leading to bond weakening. AM1-calculated bond length changes act as a descriptor for predicting bond cleavage. However, the impetus for cleavage of the unpolarised bonds may be product ion stability rather than bond weakening.

Using density functional theory to rationalise the mass spectral fragmentation of maraviroc and its metabolites

Tandem mass spectrometry (MS/MS) is widely used for the identification of metabolites at all stages of the pharmaceutical discovery and development process. The assignment of ions in the product ion spectra can be time-consuming and hence delay feedback of results that may influence the direction of a project. A deeper understanding of the processes involved in generation of the product ions formed via collision-induced dissociation may allow development of chemically intelligent software to aid spectral interpretation. Current commercially available spectral interpretation software takes a mainly arithmetical approach resulting in extensive lists of numerically plausible ions, many of which may not be chemically feasible. In this study, high-resolution MS/MS spectra were obtained for maraviroc and two of its synthetic metabolites, and structures for the product ions proposed. Density functional theory (DFT) based on in silico modelling was undertaken to investigate whether the fragmentation observed was potentially a result of bond lengthening (and hence weakening) as a consequence of protonation of the molecule at the most thermodynamically stable site(s). It was determined that for all three compounds, where the product ions resulted from simple bond cleavages (not rearrangements), the bonds that cleaved had been calculated to elongate after protonation. It was also noted that the protonated molecule may represent a mixture of singly charged protonated species and that the most basic sites in the molecule may not necessarily be the most thermodynamically stable for protonation.