Side Chain Dynamics in Poly(γ-benzyl L-glutamate) as Studied by High-Resolution Solid State 13C Nuclear Magnetic Relaxation in Rotating Frame

Abstract

ABSTRACT:The rotating frame 13 C spin-lattice relaxation time for poly(y-benzyl Lglutamate) (PBLG) in the solid state was measured by the cross polarization/magic angle spinning (CP/MAS) NMR. From the 13 C spin-lock frequency dependence of 'observed' relaxation time Tf p, it was revealed that the spin-lattice process with a relaxation time T 1 P is dominant rather than the spin-spin process. Thus, T 1 P measurement gives the information about the dynamical behaviors in PBLG. The temperature dependence of T 1 P was investigated above room temperature. The T 1 P temperature curves for side chain carbons showed a minimum, whose value is much larger than the expected one for the isotropic motion with a single distribution of the correlation time. It was suggested that side chain motion is highly anisotropic and/or the distribution of the correlation time is excessively broad. However, the temperature causing such a T 1 P minimum is very close to that for previously reported /J-process corresponding to motion of whole side chain. Further, T 1P results showed that the motion of benzyl group at the end of side chain contributes dominantly to the /J-process. The effect of MAS rate on T 1P is briefly discussed.KEY WORDS CP/MAS 13 C NMR / Rotating Frame Relaxation/ SpinLocking Frequency / Poly(y-benzyl L-glutamate) / Side Chain Dynamics / The physical properties of a-helical poly-(peptide )sin solid state have been investigated extensively from various points of view. 1 -3 It has been reported that the side chain of poly(glutamate) undergoes considerable motion above a room temperature. The side chains of some polypeptides show glass-like transition which is similar to the glass transition in amorphous polymers. 4 The temperature dependence of correlation frequency of molecular motion measured by dielectric and mechanical relaxations obeyes the WilliamsLandel-Ferry (WLF) equation, 5 which is usually applicable to the main chain motion. This implies the cooperative motion of side chains. Thus, it is of interest to reveals the relation between the local mobility of the side chain and cooperativity of side chain motion.t To whom correspondence should be addressed.It is well known that solid state 13 C highresolution NMR is one of the most useful methods for studying local structures and dynamics of polymers. The measurement of 13 C relaxation times gives us the information about the local mobility of side chains, which is very different character from 1 H relaxation measurement, such as 1 H spin-lattice relaxation time in rotating frame, 1 H T 1 P. 6 It is known that 1 H T 1 P is susceptible to the homonuclear spin-diffusion process, which thermally mixes throughout the sample and 131

Side Chain Dynamics of Poly(γ-[κ-2H1]benzyl L-glutamate) and Poly(γ-[ζ-2H2]benzyl L-glutamate) by Solid State 2H NMR

“…9 The present results from the 2 H NMR experiments are located on the curve obtained from other relaxation experiments. This fact shows that the motional mechanism at high temperature follows the WLF type.…”

“…It is considered that rapid and large amplitude motions of the side chain are caused by multibond internal rotations at high temperatures. 9 In order to see such a situation, allowed conformations which occur during jump motion were calculated for a single side chain. Projections of a side chain on the plane perpendicular to the helix axis is shown in Figure 9(a).…”

“…Local internal motions along the side chain were recently investigated using nuclear magnetic relaxation measurements by CP/MAS 13 C NMR. 9 The temperature dependence of T1P for each carbon in the side chain showed a minimum at around 350 K indicating that there are motions whose rates are of the order of 50 kHz in the side chains and that the motional process mainly corresponds to cooperative motion of whole side chains. It was suggested that side chain motions are highly anisotropic and/or the distribution of the correlation times is excessively broad.…”

“…It was suggested that side chain motions are highly anisotropic and/or the distribution of the correlation times is excessively broad. 9 In this work, we investigated the side-chain dynamics of PBLG by means of the solid state 2 H NMR, in order to elucidate motional mode and rate of the side chain motions. 2 H NMR is a powerful technique of studying the local molecular motions.…”

Side Chain Motion and Structure of Racemic Poly(γ-benzyl glutamate) as Studied by Solid State High-Resolution 13C Nuclear Magnetic Resonance

“…For PBLG, it was reported that Tf P mainly reflects T1P process. 15 To confirm this for PBDLG, dependence of T 1 P on the 13 C spin-lock frequency, / 1 , was measured, because T 1 P and TcH have very different / 1 dependences. Since T 1 P is proportional to 1 + (2nf 1 )2r 2 , where r is the correlation time, T 1 P depends on the second power of / 1 at most (slow motion limit).…”