Multilayer thin films composed of a positively charged protein and cationic polymer layers can be prepared on a solid surface against their electrostatic force of repulsion, by depositing avidin and biotin-labeled polymers [poly(ethyleneimine) (PEI), poly(allylamine) (PAA), or poly(amidoamine) (PAMAM) dendrimer] alternately and repeatedly. In contrast, the thin film does not form when unmodified polymers are used, confirming that the strong binding between avidin and biotin is responsible for the formation of the multilayer films. The loading of avidin in each layer of the multilayer films depends significantly on the molecular geometry of the polymers. The avidin/PAMAM dendrimer films are composed of a monomolecular layer of avidin whereas the avidin multilayers are formed in each layer of the PAA- and PEI-based thin films. The results are rationalized by the different molecular structures of the polymers; PAMAM dendrimer assumes a globular shape, as compared with the linear and highly branched polymer chains of PAA and PEI, respectively.

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The effect of light on polypeptide conformation was investigated by circular dichroism measurements on a series of copolymers composed of /3-benzyl L-aspartate and /3-(m-phenylazo)benzyl L-aspartate, dissolved in the mixed solvents of 1,2-dichloroethane (DCE) and trimethyl phosphate (TMP). The dark-adapted (trans) copolymers are all left-handed helices in DCE and undergo a conformational change from a left-handed helix to a right-handed helix with an increase in the amount of TMP in the solutions. Photoisomerization of the side-chain azobenzene moieties from trans to cis isomers also causes helix reversal in these copolymers at adequate solvent compositions. These copolymers exhibit CD bands in the regions of azobenzene n~r* and ir-ir* transitions which change their CD signs corresponding to the helix reversal.

A layer-by-layer thin film composed of avidin and 2-iminobiotin-labeled poly(ethyleneimine) (ib-PEI) was prepared and their sensitivity to the environmental pH and biotin was studied. The avidin/ib-PEI multilayer assemblies were stable at pH 8-12, whereas the assemblies were decomposed at pH 5-6 due to the low affinity of the protonated iminobiotin residue to avidin. The avidin/ib-PEI assemblies can be disintegrated upon addition of biotin and analogues in the solution as a result of the preferential binding of biotin or analogues to the binding site of avidin. The decomposition rate was arbitrarily controlled by changing the type of stimulant (biotin or analogues) and its concentration. The avidin/ib-PEI assemblies were disintegrated rapidly by the addition of biotin or desthiobiotin, whereas the rate of decomposition was rather slow upon addition of lipoic acid or 2-(4'-hydroxyphenylazo)benzoic acid. The present system may be useful for constructing the stimuli-sensitive devices that can release drug or other functional molecules.

A layer-by-layer deposition of concanavalin A (Con A) and glycoproteins such as glucose oxidase (GOx) and horseradish peroxidase (HRP) afforded multilayer thin films on the surfaces of a quartz slide and a platinum electrode, through biospecific complexation of Con A and sugar residues in the glycoenzymes. Lactate oxidase (LOx), which contains intrinsically no sugar chain, was also built into a multilayer assembly by being modified extrinsically with mannose residues. The enzymes formed monomolecular or sub-monomolecular layers in each layer of the multilayer films. Electrochemical measurements using Con A−enzyme multilayer-modified electrodes revealed that the enzymes are catalytically active in the multilayer films. The Con A−enzyme multilayer films are relatively stable against low concentrations of mannose and urea, although the films destructed gradually in high concentrations of urea solutions (>3 M).

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We fabricated a layer-by-layer (LbL) film composed of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) and poly(allylamine) (PAA) and investigated its pH response by UV-visible spectrometry. When the (PAA/TPPS)5PAA film was immersed in a pH 1.5 solution, J-aggregate bands were observed at 484 and 691 nm. Above pH 3.0, the J-aggregates were completely dissociated and an H-aggregate band was observed at 405 nm. The interconversion between the J-aggregates and H-aggregates in the LbL film was repeatable and controllable by changing the pH of the solutions.