Martin F. Jarrold

Background:

• NATO Postdoctoral Fellow, University of California, Santa Barbara
• Member of Technical Staff, Physics Research Division, AT&T Bell Laboratories
• Professor, Department of Chemistry, Northwestern University
• Dow Chemical Company Research Professor, Northwestern University
• 3eme Cycle en Chimie Lecturer

We're interested in issues involving the nanometer length scale, ranging from the melting and freezing of size-selected clusters, to weighing viruses, and the self-assembly of peptide nanostructures. We're involved in instrument design and in the development of new methods.

Small is Different: Melting and Freezing of Small Particles. The properties of small particles are different from the bulk material, and depend on the number of atoms. For clusters containing less than 1000 atoms, adding or subtracting even a single atom can make a huge difference. We're investigating the melting and freezing behavior of metal clusters in this size regime. The figure below shows an example of these studies, it shows the heat capacity recorded as a function of temperature for size-selected Al₅₃ clusters. The peak in the heat capacity at around 600K is due to the latent heat and we take the center on this peak to be the melting temperature. Bulk aluminum melts at 934K, so Al₅₃ has a depressed melting temperature. The melting temperatures fluctuate with cluster size, in some cases increasing or decreasing by hundreds of degrees. We are currently designing an experiment to investigate super-cooling in size-selected clusters.

Weighing Molecular Heavyweights: Megadalton Mass Spectrometry. We're developing methods to determine accurate masses for objects in the mega- to gigadalton size range (10⁶-10⁹ amu) such as viruses, complexes, and nanoparticles. Large ions can have hundreds or even thousands of charges and can have a distribution of masses, making conventional mass spectrometry methods untenable. One solution to this problem is to simultaneously measure m/z (the mass to charge ratio) and z (the charge). A measurement of the average m/z and average z yields an average value for m. However, if m/z and z are measured for individual ions the mass distribution can be obtained. The m/z and z for an individual ion can be measured by a technique called charge detection mass spectrometry. The challenge is to measure z with sufficient accuracy.

Assembly by Design: Peptide Nanostructures. Molecular self-assembly is an attractive bottom-up approach to generating nanoscale architectures. We're exploring the nanostructures that can be generated by the adsorption of peptides on inorganic surfaces. An example from our recent work is shown on the left. This AFM image shows the features generated by adsorption of Ac-K₆(LALLAAL)₃K₆-NH₂ (K6) on mica. K6 is boloamphiphile with a helical hydrophobic core and hydrophilic polylysine ends. The dots in the image are separated by about 7nm and arise from the adsorption of helical bundles of 4-6 peptides. The morphology of the surface can be controlled by changing the peptide sequence. For example, reducing the length of the hydrophilic polylysine ends leads to worm-like structures instead of dots. The morphology also responds to changes in the environmental parameters like the ionic strength, demonstrating potential applications as stimulus-responsive materials.Please visit our group web site (nano.chem.indiana.edu) to get more information on our research projects.

Selected Publications:

Size-Sensitive Melting Characteristics of Gallium Clusters: Comparison of Experiment and Theory for Ga17 and Ga20 , with S. Krishnamurty, S. Chacko, D. G. Kanhere, G. A. Breaux, and C. M. Neal, Phys. Rev. B 2006, 73, 045406.

Second Order Phase Transitions in Amorphous Gallium Clusters, with G. A. Breaux and B. Cao, J. Phys. Chem. B 2005, 109, 16575-16578.

Melting, Pre-Melting, and Structural Transitions in Size-Selected Aluminum Clusters with Around 55 Atoms, with G. A. Breaux, C. M. Neal, and B. Cao, Phys. Rev. Lett. 2005, 94, 173401.

Entropic Stabilization of Isolated β-Sheets, with Ph. Dugourd, R. Antoine, G. A. Breaux, and M. Broyer, J. Am. Chem. Soc. 2005, 127, 4675-4679.

The Mobile Proton in Polyalanine Peptides, with M. Kohtani, J. E. Schneider, and T. C. Jones, J. Am. Chem. Soc. 2004, 126, 16981-16987.

Gallium Cluster "Magic Melters", with G. A. Breaux, D. A. Hillman, C. M. Neal, and R. C. Benirschke, J. Am. Chem. Soc. 2004, 126, 8628-8629.

An Application of Evolutionary Methods to Polypeptide Folding: Comparison with Experiment for Unsolvated Ac-(Ala-Gly-Gly)5-LysH , with M. Damsbo, B. S. Kinnear, M. R. Hartings, P. T. Ruhoff, and M. A. Ratner, Proc. Natl. Acad. Sci. USA. 2004, 101, 7215-7222.