IRMPD Conformations of Metalated Polypeptides

1. IRMPD Conformations of Metalated Polypeptides Robert C. Dunbar1, Jeffrey D. Steill2, Nick C. Polfer3, Jos Oomens2 1Case Western Reserve University 2FOM Institute for Plasma Physics, (Netherlands) 3University of Florida • Introduction • Conformation of metal ion complexes as a function of: • Length of peptide chain (dipeptides, tripeptides) • Size, electronic properties and charge of the metal ion. • Nature and sequence of side-chains Side chain sequence effects Charge-solvated (CS) complex Zwitterion (Salt Bridge, SB) complex Chain length effects Metal ion size and charge Methods Ions were electrosprayed from 1 mM peptide plus 1 mM metal salt. They were cooled in a hexapole storage trap for ~1 sec and then introduced into the FTICR cell (4.7 T homebuilt instrument at FOM Institute). IRMPD dissociation was accomplished with ~10-20 macropulses (at 5 Hz, ~50 mJ/pulse) from the tunable free electron laser FELIX. The spectrum was swept over a few hundred wavelength points with averaging of 4 ion trapping/ dissociation cycles per wavelength point. Spectra were calculated as photo-appearance spectra of all major ionic fragments. Computations were DFT/B3LYP and DFT/MPW1PW91. Basis 6-31+g(d,p). Frequencies scaled by 0.975. SB Experiment Experiment CS SB • PheAla gives CS.AlaPhe gives SB. • PheAla allows chelation by metal ion and both carbonyls, giving enhanced ion solvation. Thus CS is favored. Ca+2shows a mixture of CS and SB ions Sr+2 (not plotted) shows a larger component of SB than Ca+2 Ba+2PheAla (CS) • Dipeptide is SB • Tripeptide is CS • Larger ion promotes SB. Ca+2 Ba+2 • Higher charge promotes SB. K+ Ca+2, Sr+2 Metal-ion/carbonyl interactions Conclusions • Going from dipeptide to tripeptide shifts towardCS (more chelating C=O’s) • Smaller metal ions favor CS (stronger C=O chelation) • Higher charge shifts towardSB (stronger electrostatic salt bridge effect) • Chelation by side chain favors CS when geometry is favorable (sequence dependent) Basis of Interpretation CS conformation is favored by large number of chelating C=O’s and by strong ion chelation Strength of chelation by carbonyl groups is probed by red shift of C=O stretching frequency. Unperturbed frequencies: • Amide C=O: ~1700 cm-1 • Terminal C=O: ~1800 cm-1 Observations The tripeptides (MAla3) all have CS conformation. Terminal C=O and amide C=O show similar trends. Interaction is stronger for: • Small metal ions (Ca+2, K+) • High metal ion charge (Ba+2, Ca+2) Acknowledgments This work is financially supported by the “Nederlandse Organisatie voor Wetenschappelijk Onderzoek” (NWO). RCD acknowledges the National Science Foundation for travel support. The FELIX staff, and particularly Dr. Lex van der Meer and Dr. Britta Redlich, are gratefully acknowledged.