A METHOD FOR STUDYING THE STRUCTURE OF UNIAXIALLY ALIGNED BIOPOLYMERSUSING SOLID-STATE N-15-NMR - APPLICATION TO BOMBYX-MORI SILK FIBROIN FIBERS

Recent advances in the application of solid state nmr spectroscopy to uniformly aligned biopolymers have opened a window through which to vi ew the detailed structure of biological macromolecules that are unable to be seen with standard techniques for structure determination such as x-ray diffraction. Atomic resolution structural details are obtaine d from solid state nmr data in the form of bond orientations, which yi eld the relative positions of specific atoms within the molecule. For static aligned systems such as fibers, in which rapid reorientation ab out the axis of alignment does not occur, it has generally been necess ary to perform trial and error line-shape simulations to extract struc tural details from nmr spectra arising from a single type of nuclear s pin interaction. In the present work, a new method is developed in whi ch solid state N-15-nmr spectra obtained from uniaxially aligned molec ules placed with the axis of alignment both parallel and perpendicular to the magnetic field are analyzed to yield the orientations of speci fic molecular bonds. Analytical expressions are derived that utilize s pectral features read from N-15 chemical shift anisotropy line shapes to calculate a discrete number of possible orientations for a specific site. The N-15-H-1 dipolar interaction is employed to further narrow the number of unique orientations possible for a given site. With this method, a neighborhood of possible orientations is quickly determined , and full line-shape simulations within this region of allowed space can be performed to refine the limits of orientation. This technique d emonstrates the use of a single type of isotopic label to determine th e orientation of a specific molecular group such as a peptide plane wi thin a protein. Results from the application of this method to the Bom byx mori silk fibroin protein provide structural detail that is consis tent with currently accepted structural models based on fiber diffract ion studies.