Raman Optical Activity of Biological Molecules

This thesis describes work that has helped to establish Raman optical activity (ROA) as a powerful new chiroptical spectroscopic technique for the study of molecular chirality and conformation of biological molecules in aqueous solution. The first chapter describes the background and recent developments of vibrational optical activity including both the infrared vibrational circular dichroism (VCD) and Raman optical activity approaches. In chapter two, the basic theory of vibrational Raman optical activity is briefly reviewed. It comprises the fundamental theory to describe the vibrational Raman optical activity phenomenon, the rationale of the choice of the backscattering geometry for ROA instrument set-up and the basis of the more advanced ab initio ROA theory for calculation of ROA spectra. A new ROA instrument based on the backscattering geometry and back thinned CCD (Charge-Coupled Device) light detector is detailed in chapter 3. The new ROA instrument represents the contemporary development of ROA instrumentation and the up-to-date sophisticated optical and electronic devices used in the ROA spectrometer. A few basic considerations in ROA instrumentation and the performance of the instrument are discussed. The breakthrough of the instrument sensitivity has enabled ROA spectroscopy to be applied to important biological molecules in aqueous solution for the first time. The following three chapters are devoted to the ROA study of a number of biological molecules including small peptides, polypeptides, proteins and carbohydrates. These ROA data constitute the basis of ROA study for more complicated biological molecules in the future. Chapter 4 deals with ROA studies on model peptides and polypeptides. They comprise L-alanine oligomers: di-L-alanine and tri-, tetra-L-alanine, and a tripeptide L-Pro-L-Leu-Gly-amide for model ?-turn structure plus two polyamino acids poly-L-glutamic acid and poly-L-lysine. Di-L-alanine and its enantiomer are investigated in detail in various aqueous solutions and the results suggest that di-L-alanine could be used as a good model peptide for vibrational optical activity spectra analysis of peptides and polypeptides. ROA is a very local effect that is related to the intrinsic chirality. The most important ROA bands of peptides are in the extended amide 111 region. In chapter 5, the first ROA spectra of eight globular proteins in aqueous solution are reported and a preliminary empirical analysis presented. These protein ROA data clearly demonstrate that ROA is now able to investigate protein structure in the solution phase. The dominant ROA features of proteins arise mainly from the polypeptide backbone. Proteins containing different secondary structure compositions show characteristic ROA patterns. The most prominent ROA features are concentrated in the extended amide III region and are particularly sensitive to reverse turn structure. The ROA band intensity and A value offer a sensitive probe of the rigidity or flexibility of the globular proteins. In the last chapter 6, the ROA spectra of a range of carbohydrates including fifteen monosaccharides, a disaccharide and a cyclodextrin are investigated. The overwhelming ROA spectral information convincingly demonstrates that carbohydrates are particularly favourable samples for vibrational ROA study. ROA measurements on carbohydrates can yield ample stereochemical information with respect to the glycosidic linkage, anomeric configuration, sugar ring chair conformation and intramolecular interaction between adjacent chiral centres. Of all the information available from carbohydrate ROA spectra, the characteristic ROA couplet of the glycosidic linkage is probably the most valuable, which may be used to probe the conformation of disaccharides, oligosaccharides and polysaccharides.