Conformational Transformation of Poly(β-phenetyl-L-aspartate) in Block Copolymer with Polystyrene in 1,1,2,2-Tetrachloroethane

Abstract

ABSTRACT:Temperature dependences of conformations of poly(-phenethyl-L-aspartate) (PA) in block copolymer systems, in which polystyrene (PS) was attached to either N-terminus (PA n -N-PS m ) or C-terminus (PS n -C-PA m ) of the PA, respectively, were investigated in 1,1,2,2-tetrachloroethane (TCE) solutions by using 1 H NMR spectroscopy.The block copolymers were synthesized by combination of atom transfer radical polymerization and ring-opening polymerization of N-carboxy amino acid anhydride. Whereas PA homopolymer (H-PA n , n ¼ 68) with right-handed helical conformation started to transform to left-handed structure at 70 C, the helical-sense inversion of PA 68 -N-PS 38 occurred at a lower temperature, 40 C. Further transformation from left-handed conformation was not observed for both polymers in the temperature range of 70 C to 110 C. On the other hand, 1 H NMR spectra of PS 70 -C-PA 170 showed coexistence of right-and left-handed helices of PA at 8.2 and 8.8 ppm at 30 C, respectively. These peaks, however, disappeared above 90 C, and a new peak at 8.5 ppm was observed. Thus, the introduction of PS chain into PA termini affected conformational stability of PA in a different way, although the transformational behaviors were not influenced by the difference of the chain length of PS segments. [doi:10.1295/polymj.PJ2007020] KEY WORDS Poly(-phenethyl-L-aspartate) / Block Copolymer / Helical-Sense Inversion / Chain Terminus / NMR / Controlling the higher ordered structure of polymers is one of the most important factors for producing functional polymeric materials. In that context, polypeptides have been frequently investigated and utilized because they can assume various higher ordered structures. For example, poly(aspartate ester)s are known to form various conformations including right-handed helix (RH), left-handed helix (LH), -sheet, and random coil (RC) depending on structure of side-chain groups, temperature, and solvents. [1][2][3][4][5][6][7][8][9][10][11][12][13] In particular, RH and LH conformations are interesting because their formation can be reversibly controlled by thermal stimulus.5-8 The helical-sense inversion has been reported to be caused by difference in the conformational entropy of the side-chain between the two opposite helical structures. 9-13In addition to the side-chain groups, chain termini of poly(aspartate ester)s should have a significant effect on their conformations, thus, allowing us to control them. Although a number of reports addressing the effect has been limited so far, Ushiyama et al. have demonstrated that attaching helical poly(-benzyl-L-glutamate) (BG) segment at the chain terminus of poly(-phenethyl-L-aspartate) (PA) by block copolymerization have a significant influence on the helical-sense inversion of the PA segment.14 Thus, it would be intriguing if we can use other polymer segment in place of the BG, because then more detailed analysis will become possible without undesirable overlapping of the signals in instrumental measurements such as CD, IR, X-ray, and NMR.As...

Transition Metal-Catalyzed Living Radical Polymerization: Toward Perfection in Catalysis and Precision Polymer Synthesis

“…Nickel-catalyzed ROP of R-amino acid-Ncarboxyanhydride (NCA) gives a well-controlled polypeptide. Thus, the combination of metal-catalyzed polymerization and ROP of NCA efficiently achieved polypeptide conjugation onto (meth)acrylate and styrene-based block copolymers (CB-55, 578 CB-56, 447 CB-57,58 579 CB-59, 580 CB-60, 581 CB-61,62 582 and CB-63 334 ) with relatively narrow MWDs (M w / M n ) 1.1-1.4) (Figure 34). Here, γ-benzyl-L-glutamate NCA, β-phenetyl-L-aspartate NCA, carbobenzoxy-L-cysteine NCA, and L-alanine NCA are typically employed as NCA monomers.…”

Helix–coil transformation of poly(γ-benzyl-L-glutamate) with polystyrene attached to the N or C terminus in trifluoroacetic acid–chloroform mixtures

“…These polymers were synthesized following the method presented in our previous report on block copolymers composed of PS and PPLA, 15 using N-carboxy-g-benzyl-L-glutamate anhydride (BLG-NCA) instead of b-phenethyl-L-aspartate-NCA. For synthesis of PBLG n -N-PS m , a PBLG n macroinitiator that was prepared by the ring-opening polymerization of BLG-NCA, 24 followed by amidation of the N-terminal amine with 2-bromoisobutyryl bromide was used for the atom transfer radical polymerization of styrene.…”

“…The result is in good agreement with those of our previous study of the conformational transformation of block copolymers composed of PPLA and PS in 1,1,2,2-tetrachloroethane. 15 PPLA homopolymer in solution is known to undergo a reversible helix-sense inversion from a right-handed helix to a left-handed helix in response to temperature elevation. When PS was attached to the N terminus of PPLA, the helical structure was not deformed during the course of the helix-sense inversion.…”

“…We also observed a decrease in the helical content of poly(b-phenethyl-L-aspartate) (PPLA) after the attachment of PS to the C terminus in a study of thermally induced conformational transformations of PPLA in 1,1,2,2-tetrachloroethane. 15 These reports indicate that the effects of block copolymerization on the helical structure of a polypeptide segment are complicated.…”