Enantioselective Collision-Activated Dissociation of Gas-Phase Tryptophan Induced by Chiral Recognition of Protonated L-Alanine Peptides.

Enantioselective dissociation in the gas phase is important for enantiomeric enrichment and chiral transmission processes in molecular clouds regarding the origin of homochirality in biomolecules. Enantioselective collision-activated dissociation (CAD) of tryptophan (Trp) and the chiral recognition ability of L-alanine peptides (L-Ala n ; n = 2-4) were examined using a linear ion trap mass spectrometer. CAD spectra of gas-phase heterochiral H+ (D-Trp)(L-Ala n ) and homochiral H+ (L-Trp)(L-Ala n ) noncovalent complexes were obtained as a function of the peptide size n. The H2 O-elimination product was observed in CAD spectra of both heterochiral and homochiral complexes for n = 2 and 4, and in homochiral H+ (L-Trp)(L-Ala3 ), indicating that the proton is attached to the L-alanine peptide, and H2 O loss occurs from H+ (L-Ala n ) in the noncovalent complexes. H2 O loss did not occur in heterochiral H+ (D-Trp)(L-Ala3 ), where NH3 loss and (H2 O + CO) loss were the primary dissociation pathways. In heterochiral H+ (D-Trp)(L-Ala3 ), the protonation site is the amino group of D-Trp, and NH3 loss and (H2 O + CO) loss occur from H+ (D-Trp). L-Ala peptides recognize D-Trp through protonation of the amino group for peptide size n = 3. NH3 loss and (H2 O + CO) loss from H+ (D-Trp) proceeds via enantioselective CAD in gas-phase heterochiral H+ (D-Trp)(L-Ala3 ) at room temperature, whereas L-Trp dissociation was not observed in homochiral H+ (L-Trp)(L-Ala3 ). These results suggest that enantioselective dissociation induced by chiral recognition of L-Ala peptides through protonation could play an important role in enantiomeric enrichment and chiral transmission processes of amino acids.