Photoresponsive polypeptides: Photochromism and conformation of poly (L‐glutamic acid) containing spiropyran units

Abstract

SYNOPSISSpirobenzopyran units were bound to the side chains of poly (L-glutamic acid) and partially methylated poly (L-glutamate ) s. The modified polymers were found to exhibit "reverse photochromism" in hexafluoro-2-propanol ( HFP ) , so the samples kept in the dark were characterized by an intense absorption band in the visible range of the spectrum, which was completely erased upon exposure to sunlight or irradiation at 500-550 nm.The CD spectra showed that the macromolecules adopted a random coil conformation in the dark, whereas the bleached solutions after exposure to light displayed the typical CD pattern of the a-helix. The back reaction in the dark was accompanied by the progressive decrease of the helix content and recovery of the original disordered conformation. The photoinduced conformational changes resulted in large and reversible viscosity variations.When spiropyran side chains were converted to "spiropyran salts" of trifluoroacetic acid, the system was still photochromic, but the macromolecules were disordered both in the dark and light conditions. However, when appropriate amounts of methanol were added as a cosolvent to the HFP solutions, the system responded to light, giving reversible variations of the a-helix content. Irradiation at appropriate solvent compositions allowed modulation of the extent of the photoresponse. 0 1993 John Wiley & Sons, Inc. INTRODUCTIONPhotochromic compounds are able to change their color, or more generally their absorption spectra, depending on whether they are exposed t o light or dark conditions. This reversible process is due to the capability of the photochromic molecules to exist in two different states whose relative concentration depends on the wavelength of the incident light. ' Polymers containing photochromic moieties in the side chains have recently attracted considerable interest, mainly in connection with phenomena other than merely color change. In fact, the photoisomerization of the side chains can produce reversible variations of the macromolecular conformation which, in turn, can be accompanied by reBiopolymers, Vol. 33, 1505-1517 (1993) Most work has been carried out with polypeptides containing azobenzene units in the side chains, [5][6][7][8][9][10][11][12][13][14] in which the generation of cis-and trans-azo photoisomers requires artificial sources of uv light. In recent preliminary reports we have described spiropyran-modified poly ( L-glutamic acid) l5 and poly (L-lysine) ) l6 in which the photomodulation of conformation was obtained by exposure to sunlight.Studies on poly (spiropyran-L-glutamate ) , including kinetics of the conformational decay in the dark1' and molecular dynamic simulations, have been also recently performed.In this paper we describe the photochromic and conformational behavior observed in poly ( L-gluta- 1506FISSI ET AL. mate ) s having various contents of spiropyran units in the side chains. Investigation has also been carried out in acidic conditions where photochromic units are present as "spiropyran salts." The ...

Synthesis and photochromic response of a new precisely functionalized chitosan with “clicked” spiropyran

“…The latter is generally performed by the condensation between spiro-compounds containing hydroxyl (Ciardelli, Fabbri, Pieroni, & Fissi, 1989) or carboxyl groups (Edahiro, Sumaru, Takagi, Shinbo, & Kanamori, 2006Fissi, Pieroni, Ruggeri, & Ciardelli, 1995Hirakura et al, 2004) and complementary functional groups into the macromolecule. Photo-responsive spiro-compounds have been attached to biopolymers from renewable resources such as polypeptide (Ciardelli et al, 1989;Fissi et al, 1993Fissi et al, , 1995Pieroni, Fissi, Viegi, Fabbri, & Ciardelli, 1992), pullulan (Hirakura et al, 2004), methylcellulose (Arai, Shitara, & Ohyama, 1996) and dextran (Edahiro et al, 2006), through condensation, or chitin (Fu et al, 2008) through graft polymerization.…”

Molecular switching in nano-structured photochromic films of biopolymers

“…Eight-five percent spiropyran-containing poly (L-glutamate), PSG, was synthesised as already described [4]. The chemical structure is reported in Fig.…”

“…On the same LB films of PSG, we recorded Circular Dichroism (CD) spectra. Previously acquired CD spectra of dark-adapted and irradiated PSG in HFP solution [4] showed that the spectrum of dark-adapted PSG corresponds to the spectrum of random coil polypeptides with a maximum at 212 nm.…”

Spectral and photochromic study of spiropyran

“…It has a tremendous demand in sensory [1] as well as biomedical field [2]. This 'smart' compound is able to elicit responsive behaviour on changes in the environment [3]. Light is one of the stimuli that capable of performing those conformational and chemical changes in particular system [4].…”