Biomolecules and glasses display profound analogies in their structure and dynamics. At low temperatures, amorphous solids exhibit characteristic anomalies in their thermodynamic and spectroscopic properties. These anomalies are attributed to low-energy excitations which are generally described as two level tunneling systems. On the other hand, biomolecules have been snown to exist in very many different, thermally accessible, coufigurational substates, each characterized by slightly different energy. In that context, the characteristics of glasses and spin glasses find a correspondence in biomolecules: below a critical temperature there exists a large number of energy valleys separated by effectively infinitely high barriers, the system is non-ergodic and properties vary from valley to valley (substate to substate). The substates possess a hierarchal structure and relaxation is non-exponential in time, as suggested for glasses. In particular, it has been suggested that the glass-like states in proteins could be due to local arrangements of certain atoms or groups of atoms and that hydration water could play a crucial role in the dynamics of conformational transitions.

Different aspects concerning the similarity between proteins and glasses have been investigated by Electron Paramagnetic Resonance spectroscopy. Some interesting insights into the role of solvent in determining the glass-like behaviour have been gained. These studies have been conducted in some metalloproteins, containing either copper as well as ferric ions, and in some paramagnetic ion-doped glasses formed by water and a second component.

The spectral features of both classes of systems display a significant strain in some spin Hamiltonian parameters, which has been attributed to the random distribution of the electric ligand fields around the metal ions. By suitable theoretical models, it has been shown that the statistical modulation of the spin hamiltonian parameters is due to the distribution of the conformational energies both in the amorphous systems and in biomolecules. The results are discussed also in connection with similar data in literature.