C HEMISTRY - Research: Dr. Kyle A. Beran
Investigation of the interaction between the matrix and the analyte in MALDI processes
Matrix-assisted laser-desorption/ionization
(MALDI) is an important analytical technique in the investigation of a wide range of compounds that
span from a few hundred to tens or hundreds of thousand mass units. Applications of MALDI include the study
of proteins, synthetic polymers, and natural polymers. The cartoon to the right illustrates how the matrix,
after absorbing laser radiation, assists in generating a dense, particulate plume that includes the matrix
(blue) and analyte (pink) molecules (proteins, polymers, etc.).
| Overview |
A large amount of research has been dedicated to the desorption/ionization process since the initial reports on MALDI. However, the underlying chemical and physical processes are not yet entirely understood. There are a number of mechanistic models that have been proposed in order to explain the desorption process; such as a mechanical expansion model, pressure pulse model, thermal-spike model, and a model that suggests desorption occurs due to the conduction of heat from the metal sample target. In addition to the desorption models, there is also extensive research devoted to the possible mechanistic pathway associated with the proton-transfer process from the matrix to the analyte.
Based on these developing models, it becomes apparent that the efficiency of MALDI is almost exclusively dependent upon the chemical and physical properties of the matrix. Specifically, an efficient matrix is one that is characterized as possessing a high molar absorptivity, small heat of sublimation, and a large proton affinity, gas-phase acidity, and gas-phase basicity.
| Research |
The initial goal of this phase of the research project is to identify potentially
efficient matrices based on the proton affinity (PA), gas-phase acidity (GA), and
the gas-phase basicity (GB) [thermodynamic properties] of the matrix. The PA/GB
values are based on the below reaction (the GA values are based on the reverse of
the below reaction):
For this general reaction, the proton affinity is defined as the negative of the enthalpy of reaction.
ΔE = [ET(MH+) – ET(M)] + [ZPVE(MH+) – ZPVE(M)] – 3/2RT
PA = -(ΔET + ΔZPVE – 5.2RT)
ΔET → total energy obtained from B3LYP/6-311+G**//B3LYP/6-31G*
ΔZPVE → zero-point vibrational energy obtained from B3LYP/6-31G*
3/2RT → translational energy of H+
The gas-phase basicity is defined as the negative of the free energy associated with a protonation or deprotonation reaction.
ΔE = ΔET + ΔZPVE + ΔEvib
ΔH = ΔE + RT
ΔS = ΔStr + ΔSrot + ΔSvib
ΔG = ΔH – TΔS; T = 298 K
In the above calculation scheme, the theoretical values for the zero-point vibrational energy, vibrational energy, and the vibrational entropy have been scaled accordingly.
Our computational methodology (DFT) produces accurate and precise PA, GA, and GB values for 2,5-dihydroxybenzoic acid (2,5-DHB), which is one of the most commonly used matrix, and the neutral x,y-DHB isomeric species. Based on the reliability of these results, we have also investigated the PA/GA/GB of various derivatives of the x,y-DHB monomers that include the ionic (anionic and cationic) radicals and sodiated or potassiated salts. Of these matrices, only the 2,5-DHB•+ radical cation is predicted to be the most efficient proton donor of the x,y-DHB monomeric matrices (i.e. the 2,5 radical cation possesses the largest PA/GA within the series of x,y radical species). This result can be interpreted to mean that the 2,5-DHB matrix is one of the most efficient matrices because the 2,5 radical cation derivative is the structure to which the matrix manifests prior to the transfer of a proton to the analyte. However, it was also determined that the PA/GB for the salts (x,y-[DHBNa(K)]) are significantly larger (100 – 150 kJ/mole) than the neutral or radical x,y-DHB acids. In fact, the PA of the x,y-[DHBM-H]-1 series predicts that the 2,5 isomer is the second highest, and within the uncertainty of our theoretical calculations of possessing the largest PA/GB. This result seems to indicate that the GA values of the 2,5 neutral salts are nearly the largest, thereby supporting evidence for the efficient protonation of the analyte by the 2,5 salt monomer. Based on these theoretical results, and the subsequent comparison to experimental values, we have obtained a high degree of confidence in our methodology. Therefore, we can apply our computational scheme to the determination of the thermodynamic properties for other matrix systems. In addition to the thermodynamics of MALDI, we are also beginning the evaluation of the proton-transfer process between the matrix and the analyte.
| What's Next |
- Calculate the thermodynamic properties of additional x,y-DHB derivatives. These would include
oxygen- and/or hydrogen-bridged dimers and benzoic acid trimers (see M552c, MH11 and M552, below).
MH11 M552 M552c - Calculate the thermodynamic properties of aliphatic-substituted carboxylate groups
(see M4, for example) and N-bridged ring systems (see MY10).
MY10 M4 - Rather than determining thermodynamic values for lowest energy conformers, perform a Boltzmann averaging technique on these structurally larger, and larger entropy, matrix candidates in order to obtain a more realistic description of the matrix.
- Calculate (with Boltzmann averaging) thermodynamic values of amino acids, dipeptides, tripeptides, etc.
- Investigate the mechanistic pathways for the proton-transfer process from the matrix to the analyte.
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Updated September 26, 2006 |
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