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43861-G6
Helix Macro-Dipolar Effects on the Acidities of Cysteine-Containing Helical Peptides

Jianhua Ren, University of the Pacific

We focus on the development of a quantitative model of the helix macro-dipolar effects on the acidities of the cysteine-containing peptides. We studied two groups of model peptides, the cysteine-polyalanine peptides (Group I) and the cysteine-polyglycine peptides (Group II). Within each group, two series of peptides were considered, one with the cysteine residue at the N-termini, and the other with the cysteine residue at the C-termini. Group I contains Cys-(Ala)_n-NH₂ and (Ala)_n-Cys-NH₂, where n = 3-9. Group II contains Cys-(Gly)_n-NH₂ and (Gly)_n-Cys-NH₂. All peptides were synthesized by employing the standard protocols of solid-phase peptide synthesis using an apparatus assembled in my laboratory.  The gas-phase acidities of the peptides were determined by using a triple-quadrupole mass spectrometer coupled with an electrospray ionization (ESI) source. The extended Cooks' kinetic method was applied by carrying out the collision-induced-dissociation experiments at several collision energies. The resulting data were analyzed to yield the gas-phase acidities of the peptides and the entropy effects on the systems studied. The conformations of the peptides were modeled through quantum chemical calculations as well as molecular dynamics simulations. Computations also yield the theoretical gas-phase acidities.

Both experimental and computational studies suggest that the gas-phase acidities of the same type of peptides increase systemically as the peptide chain lengths are increased. The trend of the acidity enhancement is likely due to the stabilization of the negatively charged thiolate group (S¯) through internal salvation. The thiolate anion is better solvated with a longer peptide chain. The results also show that the N-cysteine peptides, Cys-(Ala)_n-NH₂, is significantly more acidic than the corresponding C-cysteine peptides, (Ala)_n-Cys-NH₂. For the peptides longer than seven residues, the difference in acidity may be explained by the effects of the helix macro-dipole moment. The N-terminal thiolate anion is stabilized by the favorable interaction with the positive end of the macro-dipole moment, while the C-terminal thiolate is destabilized by the unfavorable interaction with the negative end of the macro-dipole moment. Molecular dynamics simulations clearly show that the deprotonated N-cysteine polyalanine peptides exist in stable helices, while the deprotonated C-cysteine peptides adopt globular conformations. The difference in acidity for the short peptides is sort of surprising, considering the common notion that short peptides do not form stable helices. Simulated annealing revels that the low energy conformations of the deprotonated Cys-(Ala)_3,4,5-NH₂ peptides are populated with partial helical loops, while those of the deprotonated (Ala)_3,4,5-Cys-NH₂ are mainly random coils. The results from this study strongly suggest that the helical conformation has a significant influence on the acidities of gas-phase peptides. Similar to that of the cysteine-polyalanine peptides, the N-cysteine-polyglycine peptides are more acidic than the corresponding C-cysteine-polyglycine peptides. But the differences in acidities are smaller. This is likely due to the fact that polyglycine peptides are more flexible and have a small propensity to adopt helical conformations.

As an extension of this project, we investigate the conformational effects on the proton affinities of a group of lysine-containing peptides. The initial results show that the C-terminal lysine-polyalanine peptides are more basic than the corresponding N-terminal lysine-polyalanine peptides. The factors that influence the proton affinities will be investigated. In addition, we begun to investigate a new type of polymers, the N-methyl substituted polyglycine based peptoids. We examined the fragmentation characteristics of several alkali metal adducts of the peptoids by using a MALDI-LTQ (linear ion trap) instrument. Rich fragmentations were observed. This study may lead to a practical method of obtaining sequence information of peptoids.

The PRF-type G grant has led two papers published in the leading chemistry journals and four more manuscripts to be submitted for publication. The results have been presented in several national and international conferences. The grant has a significant impact on the PI's professional carrier in both research and teaching. The PI has developed a research program and has received a grant from the National Science Foundation. The PI has also developed a teaching program that focuses on training undergraduate research students. The students involved in this research have gained significant research experiences. One graduate student received a Master's degree and two undergraduate students went on to graduate/professional schools.

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