Advanced material: a-glycine crystals

Figure 1. Two antiparallel sets of hydrogen bonds (arrows) bind glycine layers.

Glycine is the smallest and most highly represented of the 20 amino acids that comprise the primary structures of all proteins. It is especially prominent in loops and irregular regions connecting sequences parts of the protein that have folded into more ordered secondary structures.

We have recently discovered that the electrical conductance of a-glycine crystals exhibit extraordinarily large increases (4 orders-of-magnitude) as the temperature is decreased. The dependence of the conductance on temperature also displays strong hysteresis; an effect that is mirrored in equally extraordinary variations of the electrical capacitive properties with temperature. The crystal exhibits a very unusual inductive behaviour (negative capacitance) not expected for a passive “device”.

Figure 2. Anomalous dependance of the conduc-tance of glycine on temperature.

Our recent neutron diffraction studies of a-glycine have revealed a small temperature dependence of spatial separations of the glycine layers and minor changes in the interlayer hydrogen bonds. However, the neutron diffraction data cannot explain directly the magnitude of the changes that we have observed in the electrical properties of these crystals.

Figure 3. Anomalous dependance of the capacitance of glycine on temperature.

The investigations on glycine will be extended to highly ordered, essentially “2-dimensional”, glycine structures on 111-atomically-flat silicon using a new method that we have developed for attaching organic molecules to Si-H surfaces (see ‘Organics-on-silicon’ article). The aim is to determine the origin of the anomalous electrical properties in the crystal and to shed light on any biological significance of these anomolous electrical properties.

This work is a collaboration between the School of Physics (Biophysics), School of Biotechnology and Biomolecular Sciences and Biosciences Division (Los Alamos National Laboratory).

Benno Schoenborn, Terry Chilcott, Paul Langan,
Hans Coster and Kevin Barrof



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