Helix control in polymers

Abstract

The helix is a critical conformation exhibited by biological macromolecules and plays a key role in fundamental biological processes. Biological helical polymers exist in a single helical sense arising from the chiral effect of their primary units—for example, DNA and proteins adopt predominantly a right-handed helix conformation in response to the asymmetric conformational propensity of D-sugars and L-amino acids, respectively. In using these homochiral systems, nature blocks our observations of some fascinating aspects of the cooperativity in helical systems, although when useful for a specific purpose, “wrong” enantiomers may be incorporated in specific places. In synthetic helical systems, on the contrary, incorporation of non-racemic chirality is an additional burden, and the findings discussed in this review show that this burden may be considerably alleviated by taking advantage of the amplification of chirality, in which small chiral influences lead to large consequences. Peptide nucleic acid (PNA), which is a non-chiral synthetic DNA mimic, shows a cooperative response to a small chiral effect induced by a chiral amino acid, which is limited, however, due to the highly flexible nature of this oligomeric chimera. The lack of internal stereochemical bias is an important factor which makes PNA an ideal system to understand some cooperative features that are not directly accessible from DNA.

Chirality: Amplification of chirality in PNA:PNA duplexes is limited

“…Although oligomer length is a difficult variable in the PNAs, owing to synthetic and solubility limitations on molecular mass, Nielsen and co-workers 10 , realizing the value of the PNA system as a probe of cooperativity, carried out an early experiment in which they took advantage of their findings on the chiral effects of terminal amino acids to explore how the terminal chiral influence extended through a series of oligomers from 8 to 12 units long. In this experiment, the initial stereochemical bias is propagated through the PNA:PNA duplex by cooperative interactions throughout the helical conformation, with a propagation of chirality via the 'sergeant and soldiers' type 7,8 . The deciding observation was that although the circular dichroism (CD) intensity of the chromophore associated with the base stacking in equal molarities of the duplex increased from the 8-to the 10-mer, the 10-and 12-mers were of the same intensity ( Figure 3).…”

“…synthetic helical polymers to allow far less chiral input than is found in DNA to be capable of controlling helical senses [6][7][8] . For example a helical polymer constructed of very few nonracemic chiral units disbursed among many achiral units is adequate in a highly cooperative system to control the helical sense of large portions of the chain ('sergeant and soldiers' experiment) 7,8 (Figure 2a).…”

“…In another approach, a helical polymer constructed of a mixture of nearly racemic chiral units randomly dispersed along the chain will take a helical sense of the units in the majority ('majority rule experiment') 7,9 (Figure 2b).…”

Exploration of mandelicacid‐basedpolymethacrylates: Synthesis, properties, and stereochemical effects

“…[11][12][13][14][15][16][17] Furthermore, the design of hierarchical tiers of chirality in synthetic polymers has produced multiple studies towards application in next-generation functional materials. [17][18][19][20] Amygdalin (Scheme 1) is a natural product found in the kernels of various stone fruits and seeds with concentrations depending largely on plant species and cultivar. 21 Some common fruit seeds noted for high amygdalin content include apricots (6-55.6 mg/g), cherries (2.7-3.9 mg/g), peaches (0.9-43.3 mg/g), and plums (2.6-12.7 mg/g), as well as in bitter almonds (0.3-68.5 mg/g).…”

“…While many petroleum‐based precursors lack stereochemistry, nature purposefully utilizes specific chirality in its design to produce advanced function and respond to chiral molecules in different ways 11–17 . Furthermore, the design of hierarchical tiers of chirality in synthetic polymers has produced multiple studies towards application in next‐generation functional materials 17–20 …”