Photoresponsive Polypeptides. Photochromic and Conformational Behavior of Spiropyran-Containing Poly(l-glutamate)s under Acid Conditions

Abstract

High molecular weight poly(l-glutamic acid) was chemically modified with a spiropyran reagent to give polypeptides containing 35 and 85 mol % spiropyran units in the side chains. To investigate the photochromic and conformational behavior in acid conditions, the polypeptides were dissolved in hexafluoro-2-propanol (HFP) and a small amount of trifluoroacetic acid was added (TFA, c = 5 × 10-4 g/mL). In the absence of acid, the same polypeptides were found to respond to light, giving reversible coil/α-helix transitions. In the presence of TFA, by contrast, photoisomerization of the chromophores in the side chains did not result in any conformational variation of the macromolecular main chains, and CD spectra showed that the macromolecules adopted a random coil structure, both in the dark and after light exposure. However, when appropriate amounts of cosolvents such as methanol (MeOH) or trifluoroethanol (TFE) were added to the HFP/TFA solutions, the system again responded to light, giving reversible coil/α-helix transitions of the macromolecular structure. The extent of the photoinduced conformational changes depended on solvent composition and the photoresponse could be modulated by the combined action of light and chemical environment. For HFP/MeOH solvent mixture, it is possible to trigger the photostimulated coil/α-helix transition in a narrow window of solvent composition, so that the system is characterized by a gated photoresponse. The molecular processes responsible for the observed photoinduced conformational changes are discussed on the basis of absorption and fluorescence results. These results also allow us to set up a detailed picture of the molecular mechanisms responsible for the described chemo-modulated photosensory system.

Spiropyran-based dynamic materials

“…1 Photochromism and acidochromism of spiropyran. (A) Reversible transformations between the four states: spiropyran (SP) 1, merocyanine (MC) 2, protonated merocyanine (MCH + ) 3, and protonated spiropyran (SPH + ) 4 (note that although 4 is represented with the extra proton on the spiro N atom, it is also possible that the proton resides on the spiro O atom, or on the nitro group 306 ). (B) UV-Vis spectra of the parent spiropyran (1 0 ,3 0 ,3 0 -trimethyl-6-nitrospiro[chromene-2,2 0 -indoline]) before (gray) and after (purple) UV irradiation (5 min; I = 0.7 mW cm À2 ), and after the addition of 20 eq.…”

Development of a photoresponsive cell culture surface: Regional enhancement of living‐cell adhesion induced by local light irradiation

“…1). 9–11 Figure 2 shows the absorption spectra of a PMN solid thin film formed on a glass substrate after irradiation with visible light (500–600 nm) and UV light (350–400 nm). By irradiating colorless thin film of PMN with UV light, an absorbance peak at 566 nm increased indicating that isomerization from the spiro form to the merocyanine form proceeded.…”

Photochromic Responses in Polymer Matrices

“…Knowledge of photochemical and thermal processes in the ring opening and closure pathways lead to systems with better photochemical and thermal stability. The techniques applied include protonation of the MC-phenoxide moiety [58], synthetic modification of the SP with crown ethers [59], or a 7-trifluoromethylquinoline group [60], and by intramolecular bidentate metal ion chelation [61].…”