The structure modified by L-valine under pH regulation

L-valine modified polyacetylene chains are a class of conjugated polymers with chiral side chains. The stability and transition behavior of their helical structures are highly dependent on the regulation of environmental pH. This transition mechanism involves the synergistic changes between side chain interactions and main chain conformations, which can be analyzed in the following aspects:

From a molecular structure perspective, in L-valine modified polyacetylene chains, hydrophobic valine side chains (containing isopropyl groups) are covalently linked to the polyacetylene main chain. The amino groups (-NH₂) and carboxyl groups (-COOH) at the ends of the side chains endow the molecules with pH responsiveness. Under different acid-base conditions, these groups undergo protonation or deprotonation, changing the charge state and polarity of the side chains, thereby affecting intermolecular interactions.

In an acidic environment (low pH), the amino groups of the side chains are protonated (-NH₃⁺), while the carboxyl groups remain neutral (-COOH), making the molecules overall positively charged. The repulsive force between positive charges weakens the tight packing of the side chains; at the same time, the hydrogen bonding between polar groups and water molecules is enhanced, leading to a loose spatial arrangement of the side chains. The helical structure of the polyacetylene main chain may unwind or form a looser helical conformation with an increased helical pitch due to the loss of stable support.

When the environmental pH is close to neutral, the amino and carboxyl groups of the side chains may form intramolecular or intermolecular hydrogen bonds (interaction between -NH₂ and -COOH). The chiral structure of valine induces the main chain to form a stable single helical configuration (such as left-handed or right-handed helix) through steric hindrance. At this time, the hydrophobic interactions of the side chains (van der Waals forces between isopropyl groups) and hydrogen bonds jointly maintain the rigidity of the helical structure, and the planarity of the conjugated system in the main chain is also enhanced.

In an alkaline environment (high pH), the carboxyl groups of the side chains are deprotonated (-COO⁻), while the amino groups remain neutral (-NH₂), making the molecules overall negatively charged. The repulsive force between negative charges further stretches the side chains, destroying the original hydrogen bond network and hydrophobic stacking. The stability of the main chain helical structure decreases, and it may unwind or transform into a random coil conformation. If the pH is too high, strong alkaline conditions may also cause the cleavage of covalent bonds between the side chains and the main chain, directly leading to the disintegration of the molecular structure.

In addition, the pH-induced helical structure transition is reversible: when the pH cycles within the range of acidic-neutral-alkaline, the charge state and intermolecular forces of the side chains adjust dynamically, and the polyacetylene chains can switch back and forth between the unwound-helical-unwound states. This reversibility endows it with potential application value in intelligent materials (such as pH-responsive sensors and drug carriers). Its conformational changes can be characterized by ultraviolet-visible spectroscopy, circular dichroism, etc. For example, the formation of a helical structure is accompanied by the shift of characteristic absorption peaks or the enhancement of chiral signals.

In summary, the helical structure transition of L-valine modified polyacetylene chains is essentially the synergistic change of side chain charges, hydrogen bonds, and hydrophobic interactions under pH regulation. By affecting the spatial arrangement of the side chains, it indirectly regulates the conformational stability of the main chain, ultimately achieving the reversible conversion between helical and non-helical states.