The reactivity of gaseous ions of bradykinin and its analogues with hydro-and deuteroiodic acid

The kinetics of attachment of hydroiodic acid to gaseous protonated bradyki nin, des-Arg(1)-bradykinin, des-Arg(9)-bradykinin, and their respective met hyl esters are reported. Rate constants range from 9.3 x 10(-11) cm(3)-s(-1 ) for attachment of hydroiodic acid to the (M + H)(+) ion of bradykinin to much less than 3 x 10(-12) cm(3)-s(-1) for a slow-reacting component of a p opulation of (M + H)(+) ions derived from des-Arg(9)-bradykinin. In the cas es of the (M + 2H)(2+) ions from bradyErinin and the (M + H)(+) ions from d es-Arg(9)-bradykinin, the rate data could not be fit with a single rate con stant, indicating the presence of at least two non-interconverting ion popu lations. For the (M + 2H)(2+) ions, it was demonstrated that the two reacti ng structures could be induced to interconvert upon gentle activation. The attachment sites are the most basic neutral sites of the molecule, viz., ar ginine and the N-terminus in the case of bradykinin and its analogues. A si mple picture is proposed to estimate the rate constant for hydroiodic acid attachment to a fully exposed neutral basic site. The picture is based upon the assumption that a significant degree of proton transfer from hydroiodi c acid to the attachment site occurs and therefore estimates that the captu re radius is the distance at which the endoergicity of the proton-transfer reaction is just compensated for by the Coulomb attraction of the ion pair. Storage of the ions in the presence of deuterioiodic acid (DI) showed evid ence for hydrogen/deuterium exchange. Two competing mechanisms can lead to H/D exchange, one of which involves initial DI attachment to a neutral basi c site. However, experimental evidence suggests that the gas phase H/D exch ange reactions result primarily from a "relay-type" mechanism proposed for similar systems reacting with D2O. These results provide important new info rmation to facilitate the use of hydroiodic acid attachment kinetics and H/ D exchange kinetics using DI as chemical probes of three-dimensional gaseou s polypeptide ion structure.