An electrospray ionization-flow tube study of H/D exchange in the protonated serine dimer and protonated serine dipeptide

Serine is known to undergo a chiroselective self-directed oligomerization to form a singly-protonated octamer whose structure is made up of four dimer units. This has led to interest in the protonated dimer. An electrospray ionization-fast flow technique has been employed to study the hydrogen/deuterium (H/D) exchange reactions of the protonated dimer of serine and of the protonated dipeptide of serine with ND3 and CH3OD, respectively. Ammonia-d(3) reacts with the dimer with an overall rate constant of (6.7 +/- 0.4) x 10(-10) cm(3) molecule(-1) s(-1). On a time scale of similar to12ms, the protonated dimer was observed to undergo very efficient consecutive exchange with ND3 of all of its nine labile hydrogens at ND3 concentrations of less than or equal to1.2 x 10(12) molecules mL(-1). Reaction with CH3OD is less efficient in view of the lower proton affinity of the latter. In addition, the dipeptide was observed to exchange its seven labile hydrogens with ND3 somewhat less efficiently than the dimer. The results are interpreted as indicating a rather open structure for the protonated dimer allowing easy complexation by ND3 and involving very labile protons. This conclusion is discussed in the light of the known computed structures of the protonated dimer [Anal. Chem. 73 (2001) 3646]. H/D exchange of the dimer demonstrates a single structure-the ion-molecule form. Serine is not basic enough to stabilize a zwitterionic salt-bridge structure for the dimer. (C) 2003 Elsevier Science B.V. All rights reserved.