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Electrospray ionization mass spectrometric investigation of ammonium ion complexes with anomeric 2,3‐O‐isopropylidene‐1α‐ and ‐1β‐D‐ribofuranosyl azides: anomeric and kinetic isotope effects in ammonium affinities
Author(s) -
Kralj Bogdan,
Kocjan Darko,
Kobe Joz̆e
Publication year - 2001
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.533
Subject(s) - chemistry , anomer , electrospray ionization , kinetic isotope effect , ammonium , ion , ammonium acetate , mass spectrometry , analytical chemistry (journal) , crystallography , medicinal chemistry , stereochemistry , deuterium , organic chemistry , chromatography , high performance liquid chromatography , physics , quantum mechanics
[Methanol + ammonium acetate] solutions of anomeric 2,3‐ O ‐isopropylidene‐1α‐ and 1β‐ribofuranosyl azides were investigated by electrospray ionization mass spectrometry (ESI‐MS). The compounds included d 6 ‐labeled and/or unlabeled isopropylidene groups that enable the identification of peaks characteristic of the ammonium‐attached monomeric (MNH 4 + ), ammonium‐bound homodimeric ([M] 2 NH 4 + ) and heterodimeric ([MNH 4 M 1 ] + ) complex ions in ESI mass spectra of solutions of a pair of compounds. The intensities of the product ion peaks obtained by the collisionally activated ammonium‐bound dimeric ions are related to the secondary isotope effect k α /k αd6 = 0.88 and k β /k βd6 = 1.25 or to isotope plus anomeric effects k α /k βd6 = 1.43 and k β /k αd6 = 0.59 in the ammonium affinities of these compounds. The calculations of solely anomeric effects in the ammonium affinities of α and β anomeric compounds obtained from the data presented previously give two series of values: k α /k β = (k α /k αd6 )(k αd6 /k β ) = 1.49 and k αd6 /k βd6 = (k αd6 /k β )(k β /k βd6 ) = 2.12 or k α /k β = (k α /k βd6 )(k βd6 /k β ) = 1.14 and k αd6 /k βd6 = (k αd6 /k α )(k α /k βd6 ) = 1.63. The disparities of these results indicate the different structures of hydrogen bonding in ammonium‐bound dimeric complexes which decompose to monomeric ions with different rate constants. Comparison of experimental results obtained by the qualitative approach of the kinetic method and ammonium affinities of these compounds calculated by the semi‐empirical molecular orbital method (AM1) show that the [MNH 4 M 1 ] + dimeric complex ions dissociate to the most stable MNH 4 + and M 1 NH 4 + monomeric ions. The obtained relative order of ammonium affinities of these compounds is: αd 6 > α > β > βd6. Copyright © 2001 John Wiley & Sons, Ltd.