Spiropyran Derivative of an Elastin-like Bioelastic Polymer: Photoresponsive Molecular Machine to Convert Sunlight into Mechanical Work M. Alonso, † V. Reboto, † L. Guiscardo, † A. San Martı ´n, † and J. C. Rodrı ´guez-Cabello* ,‡ Dpto. Quı ´mica Analı ´tica, E.U.P., Universidad de Valladolid, Francisco Mendizabal 1, 47014 Valladolid, Spain, and Dpto. Fı ´sica de la Materia Condensada, E.T.S.I.I., Universidad de Valladolid, Paseo del Cauce s/n, 47011 Valladolid, Spain Received August 1, 2000 Revised Manuscript Received October 17, 2000 Photoresponsive polymers are able to respond to light, giving reversible variations of their structure and conformation that are accompanied by variations of their physical properties 1 that could be exploited in many future technological developments. One of the most important group of photoresponsive polymers is poly(R-amino acids) conjugated with photochromic side chains. 1 These polymers respond to light, giving revers- ible coil-R-helix (or -sheet) transitions. 1 However, these materials display these properties in suitable solvents that invariably have a bad environ- mental consideration. Furthermore, their efficiency is intrinsically low, since the number of attached photo- chromic moieties needed to yield a significant coil-R- helix transition is always high. 1 Fortunately, these two main drawbacks could be overwhelmed by the use of a new family of synthetic polypeptides, the bioelastic elastin-like polymers as substitutes of the conventional poly(R-amino acids). This has been demonstrated by a pioneer work from Strzegowski et al. on an azo deriva- tive of this kind of polypeptide. 2 The inverse temperature transition showed by bioelas- tic polymers is a molecular transition from a relatively extended and disordered chain to a regularly folded -spiral that takes places in water solutions and hydro- gels of these polymers 3 on raising the temperature. The term “molecular machines” can be applied to these polymeric molecules because of their exclusive ability to convert many types of energy into useful mechanical work on folding; 3,4 an ability that, in the limit, can be displayed by a single molecule. 4 Following the path open by Strzegowski et al., 2 we report herein the photomodulation of the inverse tem- perature transition of a modified bioelastic polypeptide that is able to respond to sunlight-darkness or, alter- natively, sunlight-UV cycles. This polymer provides a route to photoresponsive materials capable of sunlight photomechanical transduction without the need for either solvents other than water or UV radiation and with a high efficiency. The copolypeptide used in this work was prepared in the following manner. The random copolypeptide poly- [0.74(VPGVG), 0.26(VPGEG)] was synthesized as de- scribed by Gowda. 5 Stoichiometry and purity were routinely checked by 13 C and 1 H NMR, elemental and amino acid analysis, and chromatographic methods. The mole fraction of both pentamers was determined by amino acid analysis. Some of the physical properties of the final polymer were also checked. The inverse tem- perature transition (T t ) for this polymer takes place at 32 °C (result not shown). The spiropyran photochromic compound 1-(-hydroxy- ethyl)-3,3-dimethyl-6′-nitrospiro-(indoline-2,2′[2H-1]ben- zopyran) (Sp-OH) was synthesized following the method of Zaitseva. 6 Finally, for conjugation of the photochromic molecule to the polymer, 55 mg of the copolymer (0.034 mmol of glutamic acid residues) was dissolved in 1 mL of N,N′-dimethylformamide. To this solution were added 5.5 mg (0.037 mmol) of pyrrolidinopyridine, 44 mg (0.214 mmol) of dicyclohexylcarbodiimide, and 76 mg (0.214 mmol) of Sp-OH. Sp-OH and the carbodiimide were added in excess to increase the yield of the conjugation reaction. 7 The solution was stirred at room temperature for 3 days. One drop of 1 M acetic acid was added to facilitate precipitation of the dicyclohexyl urea byprod- uct. The precipitate was removed by precipitation into excess diethyl ether, washed repeatedly with ether, and dried in air at room temperature. The yield was 42 mg (ca. 76%). Thin-layer chromatography revealed no con- tamination by unconjugated Sp-OH. Conjugation reached the 45% of the glutamic acid side chains, as revealed by UV-vis spectroscopy. 7 Figure 1 shows the changes in the UV-vis absorption spectrum of a 5 mg/mL solution of the conjugated polymer in 0.01 N phosphate buffer (pH ) 3.5) for the dark-adapted and the UV-exposed sample (250-400 nm from a 500 W Hg lamp equipped with a CVI Laser Corp. SUG-11-1.00 band-pass filter) and for the sunlight- exposed sample. The sunlight-exposed sample showed the pattern of the relatively apolar spiro form, while the dark-adapted and the UV-radiated polymer showed the expected absorption features of the merocyanine form, which is of higher polarity but is not zwitterionic in acid media (see Figure 2). According to the literature, ir- radiation with UV light restores the open merocyanine structure of the spiro compounds and has the same effect as the dark adaptation but with a higher rate. 7 Figure 3 shows that the inverse temperature transi- tion is sensitive to the state of the spiropyran chro- mophore. When buffered at pH 3.5, phase separation † Dpto. Quı ´mica Analı ´tica. ‡ Dpto. Fı ´sica de la Materia Condensada. * To whom correspondence should be addressed. Telephone +34 983 423680; Fax +34 983 423544; e-mail cabello@wfisic.eis.uva.es. Figure 1. UV-vis absorption spectra of the conjugated polymer after sunlight and UV exposition and dark adaptation. 9480 Macromolecules 2000, 33, 9480-9482 10.1021/ma001348h CCC: $19.00 © 2000 American Chemical Society Published on Web 11/30/2000