Use of Microcoil Probehead for Determination of the Structure of Oriented Silk Fibers by Solid-State NMR

Lamellar Structure in Alanine-Glycine Copolypeptides Studied by Solid-State NMR Spectroscopy: A Model for the Crystalline Domain of Bombyx mori Silk Fibroin in Silk II Form

Bombyx mori silk fibroin (SF) fibers with excellent mechanical properties have attracted widespread attention as new biomaterials. However, the structural details are still not conclusive. Here, we propose a lamellar structure for the crystalline domain of the SF fiber based on structural analyses of the Ala Cβ peaks in the ¹³C cross-polarization/magic angle spinning NMR spectra of (Ala-Gly)_m (m = 9, 12, 15, and 25) and ¹³C selectively labeled (Ala-Gly)₁₅ model peptides. Namely, three Ala Cβ peaks with relative intensities of 1:2:1 obtained by deconvolution were assigned to two kinds of β-sheet and a β-turn, which are interpreted as a lamellar structure formed by repetitive folding using β-turns every eighth amino acid, for which the basic structure is (Ala-Gly)₄ in an antipolar arrangement. The dynamics and intermolecular arrangement were further studied using ¹³C solid-state spin-lattice relaxation time observations and the rotational echo double resonance experiments, respectively.

Submillimeter coils for stimulated-echo spectroscopy of a solid sodium ion conductor by nonselective excitation of MHz broad 23Na quadrupolar satellite spectra

In solids the detection of ionic motion covering the time range of milliseconds and longer is often accomplished using stimulated-echo spectroscopy. For spectral line widths much below or much above 1MHz nonselective or fully selective radio-frequency pulse excitation, respectively, is typically applied in such experiments. To enable the study of samples with quadrupolarly broadened satellite spectra featuring intermediate widths (in the lower MHz range) the present work exploits microcoils. Using such coils, stimulated-echo spectroscopy can be performed under conditions of nonselective excitation for instance with ²³Na as a nuclear probe. Nutation experiments carried out used to assess the coil performance. The impact of second-order quadrupolar interactions is studied using explicit density-matrix calculations. The applicability of the present approach is successfully tested for a sodium orthophosphate based solid ion conductor.

Microscopic structural analysis of fractured silk fibers from Bombyx mori and Samia cynthia ricini using 13C CP/MAS NMR with a 1mm microcoil MAS NMR probehead

Conformational changes have been studied in silk fibers from the domestic silkworm Bombyx mori and a wild silkworm Samia cynthia ricini as a result of fractured by stretching. About 300 samples consisting of only the fractured regions of [1-13C]Ala or [1-13C]Gly labeled silk fibers were collected and observed by 13C CP/MAS NMR spectra. The total amount of these fractured fibers is only about 1mg and therefore we used a home-built 1mm microcoil MAS NMR probehead. A very small increase in the fraction of random coil was noted for the alanine regions of both silk fibroins and for the glycine region of B. mori silk fibroin. However, there is no difference in the spectra before and after fractured for the glycine region of S. c. ricini silk fibroin. Thus, the influence of fracture occurs exclusively at the Ala region for S. c. ricini. The relationship between sequence, fracture and structure is discussed.

The generation of intense radiofrequency fields in microcoils

Large amplitude radiofrequency (rf) fields hold great promise in wide line NMR where it becomes possible to excite the full breadth of the line in a quantitative fashion. Applications in quadrupole NMR and in NMR of paramagnetic systems benefit greatly from intense fields. Spin manipulations in multiple quantum NMR experiments, specifically, the generation of multiple quantum coherence in MQ NMR, are more efficiently produced using intense rf fields. In this work we describe a microcoil probe that produces an rf field of 25 MHz, more than five times larger than the greatest rf field reported in the literature. We accomplish this in a robust, 127 microm diameter solenoid coil using 1 kW of rf power.