Improved proton affinity measurements for proline and modified prolines using triple quadrupole and ion trap mass spectrometers

Abstract

Proton affinity (PA) of compounds such as proline, cis-3-methylproline, cis-3-ethylproline, cis-3-isopropylproline and cis-3-isopentanylproline was determined by kinetic method with amines as the reference bases. The effective temperatures determined using ion trap and triple quadrupole mass spectrometers were found to be significantly different. In the case of the triple quadrupole instruments, the effective temperature depends significantly on the collision energy. The influence of the apparent basicity (GBapp) on the effective temperature may be used to estimate the difference in protonation entropy (DeltaDeltaS degrees) between the sample and reference compounds. In case of the ion trap mass spectrometer, the variation of the effective temperature as a function of the excitation amplitude is small, so it is difficult to account for the contribution of the entropy effects to the proton affinity value. A better estimation of the PA and DeltaDeltaS degrees values for the investigated molecules is obtained by combining the GBapp and Teff data pairs that are obtained from both the mass spectrometers.

Gas-phase proton affinities of guanidines with heteroalkyl side chains

“…Briefly, in this approach protonbound dimers [A·H + ·B] of the compound of interest A and a reference base B of known PA are generated, mass-selected, and then subjected to collision-induced dissociation (CID); also the results of metastable ion spectra can be analyzed using the kinetic method [31]. According to the concept of the kinetic method, the ratio of the resulting ion abundances of the fragment ions I(A·H + ) and I(B·H + ), respectively, can be considered as a monitor for the difference of the proton affinities, i.e., PA = −RT eff ln[I(A·H + )/I(B·H + )], where the effective temperature T eff represents a phenomenological measure of the mean internal energy of the ions which dissociate within the timewindow characteristic of a particular analyzer and operating conditions [32][33][34].…”

Calculation of proton affinity using the CR‐CCSD[T]/ECP method

“…Absolute values of proton affinities are not always easy to obtain and are often derived from relative measurements with respect to reference molecules. Relative proton affinities are usually measured by means of high-pressure mass spectrometry, with triple quadrupole and ion-trap mass spectrometers [4] or using ion-mobility spectrometry [5].…”

Stereochemical effects during [M-H]− dissociations of epimeric 11-OH-17β-estradiols and distant electronic effects of substituents at C(11) position on gas phase acidity

“…It is well established that the effective temperature depends on the amplitude degree of excitation, the time-scale window of the dissociation processes, and a number of other parameters [43,44]. The measurements of proline basicity, performed by our group [47], underline the advantage of the combination of experimental results obtained with an ion trap and triple quadrupole instruments. Therefore, experiments are performed with both the ion trap and triple quadrupole instruments [47].…”

“…The measurements of proline basicity, performed by our group [47], underline the advantage of the combination of experimental results obtained with an ion trap and triple quadrupole instruments. Therefore, experiments are performed with both the ion trap and triple quadrupole instruments [47].…”