ELECTRON-SPIN RESONANCE STUDY OF ANISOTROPIC ROTATIONAL REORIENTATION OF SPIN LABELS ATTACHED TO THE SIDE-CHAINS OF SOLUBLE POLY(METHACRYLAMIDE)-TYPE COPOLYMERS

Soluble copolymers of N-(2-hydroxypropyl)methacrylamide (HPMA) with N-methacryloylated ω-amino acids, which form side chains with 4-aminoperdeuterio-2,2,6,6-tetramethylpiperidinyl-1-oxy (PD-Temp-NH2) attached to their end, were investigated. The EPR spectra of methanolic solution of these copolymers recorded at 313-173 K were analyzed by using a model of anisotropic but axially symmetric rotational reorientation of the spin label. An analysis of these spectra in the motional narrowing region showed that the symmetry axis z′ of the rotational-diffusion tensor describing this rotational reorientation was an axis close to that of the N-O· bond, and that, depending on the type of side chain, the rotational reorientation about this axis is four to six times faster than about the remaining two axes. The correlation time, τR, characterizing the rate of rotational reorientation of the spin label decreases monotonically with increasing side chain length. Linear log τR vs. 1/T dependences were obtained and were characterized by a low activation energy, E = 15 ± 2 kJ/mol, that was the same within the limits of experimental error for all the copolymers. Slow motional spectra were given a preliminary interpretation with a model of very anisotropic rotational reorientation. © 1979 American Chemical Society.