NMR determination of crystallinity for poly(ε-L-lysine)

Crystallinity of poly(epsilon-L-lysine) (epsilon-PL) was discussed by analyzing the differences in the H-1 spin-spin relaxation times (T-2(H)), the C-13 spin-lattice relaxation times (T-1(C)), and the C-13 NMR signal shapes between the crystalline and the non-crystalline phases. The observed H-1 relaxation curve (free induction decay followed by solid-echo method) showed the sum of Gaussian and exponential decays. Similarly, the observed C-13 relaxation curves obtained from the Torchia method were double-exponential. The C-13 NMR spectrum of epsilon-PL was divided into the narrow and the broad lines by utilizing the intrinsic differences in the H-1 spin-lattice relaxation times in the rotating-frame between them, which are attributed to the crystalline and the non-crystalline phases, respectively. Even though the crystallinity is obtained from the identical NMR measurements, the estimated values are different with each other. The crystallinity estimated from the T-2(H) differences was 75.8 +/- 0.1% at 333 K and 60.7 +/- 0.4% at 353 K. From the T-1(C) differences, the value was estimated to be 62 +/- 11%. Furthermore, the value estimated from the NMR signal separation was 54 +/- 5%. In this study we have explained these discrepancies by the difference in susceptibility among the experiments for the inter-phase, which exists in-between the crystalline and the amorphous phases. Furthermore, the estimated crystallinity was ascertained by the X-ray diffraction experiment. (C) 2007 Elsevier Ltd. All rights reserved.