42481-B4
Investigation of Solvation Effects in Small Peptides Using 13C NMR
During the course of this ACS-PRF grant, my undergraduate research collaborators and I have investigated the effect of solvation on peptide secondary structure using high-field solution-state and solid-state NMR. While it is an accepted fact that the solvent plays a critical role in determining the secondary structure, there is a dearth of detailed, quantitative spectroscopic data for small molecules, which serve as a testing ground for computational structure prediction models. Small peptides in solvated environments often interconvert between multiple conformations on a time scale rapid with the NMR time scale. The detection of those conformations and the quantification of the corresponding populations present an additional experimental challenge. The problem of multiple, interconverting conformations goes well beyond the biophysical realm and is readily generalized to conformationally flexible molecules in viscous condensed phases.
At the start of this project, we focused our efforts on the investigation of the partially solvated conformational states of the alanine dipeptide (Ac-Ala-NHMe) using a flash freeze technique. After struggling with many technical difficulties in producing kinetically frozen states, we have shifted our focus to the family of uncapped alanine- and glycine-containing dipeptides (four) and tripeptides (eight). These short peptides have a very high surface-area-to-volume ratio, and as such, experience high solvent exposure. They also, most likely, toggle between multiple low-energy conformations due to the reduced steric hindrance. Our goal has been to extract information about the conformational distribution in water from 13C chemical shifts, and to a lesser extent from 15N shifts. Chemical shifts are exquisitely sensitive to many experimental factors, such as concentration, temperature, sample geometry, solvent, and pH. By controlling these factors, we can coax detailed information from chemical shifts. The interpretation of the experimental data is aided by high-level ab initio calculations of the compounds in a polarizable continuum dielectric model (to simulate an aqueous environment) to survey the low-energy conformations. Hyperfine quantities, such as the chemical shift, are notoriously difficult to calculate to the high level of accuracy demanded by modern high-field spectrometers, and this project is providing an excellent testing ground for those approaches.
We have measured and assigned the 13C chemical shifts for all twelve di and tripeptides. Since the space of possible conformations is fairly large in these compounds, we have used a combined molecular dynamics and quantum chemical approach to search for the low-energy conformations. These typically number between 2 and 6 within 3kT of the lowest energy state. Upon completion, we will be in position to compare Boltzmann-weighted computed chemical shifts with the experimental values. A manuscript detailing the results from the dipeptide portion is in the end stages of preparation. We expect to submit it to a top-tier ACS journal by the end of 2008. All the calculations are being done locally by Oberlin undergraduate research collaborators on an NSF-funded parallel computational cluster. The NMR data were collected on a local 600 MHz NMR spectrometer, also funded by the NSF, by the same undergraduates.
