Towards protein photoresponse engineering: terahertz determination of conformational flexibility of photonic proteins

Summary form only given. Biological systems are intriguing as candidate photonic materials in that they offer a number of advantages over semiconductor engineered materials, namely, large quantities of uniform size and spectral response can be readily produced, the systems lend themselves to self assembly and with the advent of protein engineering it is now possible to genetically engineer the response. One major impediment to this engineering is the lack of understanding and predictability of the photoresponse with mutation. For example the protein bacteriorhodopsin has been developed as a optical storage material and is also being investigated for modulation applications using the high index change that occurs in the M state of its photocycle. The photoresponse of this system has been altered through mutagenesis. Both the spectral response and the time response can be changed with exchange of certain amino acids in the polypeptide chain. The reason for the time response change is not well understood and at present methods to find optimum materials are largely combinatorial. A number of investigators have suggested that conformation and conformational flexibility are closely linked to protein function and that the alteration of conformational flexibility with residue exchange may be the main reason for such highly varying photocycling times. We show that both the conformation and conformational flexibility can be measured using THz TDS. While the ultraviolet and visible absorbance spectra are unable to differentiate between the mutant and the wild type, the far infrared absorbance is markedly different.