## 3 Supramolecular Organization of Peptide-Synthetic Polymer Hybrids
On the other hand the ABA triblock copolymer (_i.e._, A, PLP and B, PEO) mainly showed the form II of PLP and a significant crystallinity of PEO. In any case, PLP displayed the form II indicative of a left-handed helix when both architectures were dissolved in water. This form was also the most favorable for PLP hybrids with poly(L-lysine) or poly(L-glutamic acid) as blocks when they were dissolved in water or trifluoroacetic acid (TFA). The synthesis of amphiphiles containing PLP II, a hydrophilic helix independent of pH and temperature within the physiological range, was postulated to have significant applications on the formation of nanoconstructs for drug and gene delivery. Stimuli-responsive polypeptide block copolymers, as a type of block copolymers, are very appealing systems due to their polymorphism of morphologies in selective solvents (e.g., micelles, star micelles, vesicles, tubules, fibrils or other kind of supramolecular aggregates). The most frequently used stimuli in these kinds of systems are pH, light, temperature, redox potential, magnetic field or ultrasounds [79]. The last efforts in this area have been made so as to develop new photo- or thermoresponsive systems which will be discussed below. [PERSON] _et al._[80], have recently presented a photosensitive diblock copolymer constituted by poly(glutamic acid) (PLGA) and poly(ethylene oxide) (PEO) containing spiropyran (SP) as side-chain motifs. These side-chains could undergo conformational changes (from \(\alpha\)-helix to random coil and vice versa) with UV and visible light, respectively (Scheme XII). The reversible aggregation-dissolution-aggregation process in water solutions in response to a suitable light exposure history can be visualized in Figure 1. Before UV irradiation, when the peptide block was mostly hydrophobic due to the close form of the spiropyran (SP), the sample showed flowerlike micellar structures composed of dark regions (_i.e._, PLGASP domains) and bright domains corresponding to PEO. After UV exposure (\(\lambda=350\) nm), the aggregates disrupted entirely resulting in single soluble copolymer molecules due to the switch associated with the SP form to MC form (merocyanine open form). After a period in visible light irradiation, flower-like micellar aggregates were recovered again in sizes comparable as UV prior exposure. Because the stimulus used was noninvasive UV source and the block copolymer had a bioinspired architecture, this photoresponsive rod-coil block copolymer can be used as model system to study photoinduced drug release processes or light-controlled biomedical applications. Other recent works involved temperature as stimuli to achieve responsive systems basically based on gel-sol transformations. One of these examples was the study conducted on reverse thermal gelation of poly(alanine) end-capped poly(propylene glycol)-poly(ethylene glycol)-poly(propylene glycol) copolymers (PA-PLX-PA) [81]. The structure-property relationship of the sol-gel transition was investigated by varying PA and PLX lengths and L-Ala/DL-Ala ratio. It was found that changes in the secondary structure of PA (_i.e._, from random coil to \(\beta\)-sheet) and a decrease in molecular motion of PLX were observed by increasing the temperature. Moreover, the molecular weight of each block and the composition of PA played a critical role in determining the sol-gel transition temperature. Therefore, it was observed from the phase diagrams that when the PA block length increased, at a fix PLX length, the sol to gel transition temperature decreased. On the other side, as the hydrophilic PLX length increased at a similar PA length, the sol to gel transition temperature increased due to the increase in hydrophilicity.