Investigations of the Mechanism of the "Proline Effect" in Tandem Mass Spectrometry Experiments: The "Pipecolic Acid Effect" AA(Xxx)AA

Abstract. The fragmentation behavior of a set of model peptides containing proline, its four-membered ring analog azetidine-2-carboxylic acid (Aze), its six-membered ring analog pipecolic acid (Pip), an acyclic secondary amine residue N-methylalanine (NMeA), and the D stereoisomers of Pro and Pip has been determined using collision-induced dissociation in ESI-tandem mass spectrometers. Experimental results for AAXAA, AVXLG, AAAXA, AGXGA, and AXPAA peptides are presented, where X represents Pro, Aze, Pip, or NMeA. Aze-and Pro-containing peptides fragment according to the well-established "proline effect" through selective cleavage of the amide bond N-terminal to the Aze/Pro residue to give y n + ions. In contrast, Pipand NMA-fragment through a different mechanism, the "pipecolic acid effect," selectively at the amide bond C-terminal to the Pip/NMA residue to give b n + ions. Calculations of the relative basicities of various sites in model peptide molecules containing Aze, Pro, Pip, or NMeA indicate that whereas the "proline effect' can in part be rationalized by the increased basicity of the prolyl-amide site, the "pipecolic acid effect" cannot be justified through the basicity of the residue. Rather, the increased flexibility of the Pip and NMeA residues allow for conformations of the peptide for which transfer of the mobile proton to the amide site C-terminal to the Pip/NMeA becomes energetically favorable. This argument is supported by the differing results obtained for AAPAA versus AA(D-Pro)AA, a result that can best be explained by steric effects. Fragmentation of pentapeptides containing both Pro and Pip indicate that the "pipecolic acid effect" is stronger than the "proline effect.&quot

Objective Climatological Analysis of Extreme Weather Events in Arizona during the North American Monsoon

Almost one-half of the annual precipitation in the southwestern United States occurs during the North American monsoon (NAM). Given favorable synoptic-scale conditions, organized monsoon thunderstorms may affect relatively large geographic areas. Through an objective analysis of atmospheric reanalysis and observational data, the dominant synoptic patterns associated with NAM extreme events are determined for the period from 1993 to 2010. Thermodynamically favorable extreme-weather-event days are selected on the basis of atmospheric instability and precipitable water vapor from Tucson, Arizona, rawinsonde data. The atmospheric circulation patterns at 500 hPa associated with the extreme events are objectively characterized using principal component analysis. The first two dominant modes of 500-hPa geopotential-height anomalies of the severe-weather-event days correspond to type-I and type-II severe-weather-event patterns previously subjectively identified by Maddox et al. These patterns reflect a positioning of the monsoon ridge to the north and east or north and west, respectively, from its position in the "Four Corners" region during the period of the climatological maximum of monsoon precipitation from mid-July to mid-August. An hourly radar gauge precipitation product shows evidence of organized, westward-propagating convection in Arizona during the type-I and type-II severe weather events. This new methodological approach for objectively identifying severe weather events may be easily adapted to inform operational forecasting or analysis of gridded climate data.Strategic Environmental Research and Development Program (SERDP) through the U.S. Departments of Defense and Energy [RC-2205]; U.S. Environmental Protection Agency; Universidad Nacional Autonoma de Mexico Programa de Apoyo a Proyectos de Investigation e Innovation Tecnologica (UNAM PAPIIT) [IA100916]Published Online: 4 November 2016; 6 month embargoThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]