The formation of striped phases of dialkyl disulfides and dialkyl sulfides on Au(111) and graphite has been monitored by scanning tunneling microscopy (STM) to investigate differences in adsorption of the sulfide to the gold surface. Striped phases formed by C18H37S- moieties after S−S bond cleavage of dioctadecyl disulfide on the gold surface were observed. On the other hand, self-assembled monolayers (SAMs) of dioctadecyl disulfide and dioctadecyl sulfide adsorbed on a graphite surface displayed long-range, well-ordered monolayers with one striped pattern that shows uniform periodicity as a function of molecular length via nondissociative adsorption. In the case of dioctadecyl sulfide SAMs on gold, molecular ordering was identical in periodicity and pattern to the striped phase of SAMs formed by physisorption on the graphite surface. In addition, the striped phase of dioctadecyl sulfide SAMs formed upon a 22 × √3 surface reconstruction of Au(111) was clearly observed for the first time in the SAM system as physisorbed monolayers. From our STM results, we suggest that the formation of dialkyl sulfide SAMs on gold proceeds via a nondissociative adsorption process unlike the case of dialkyl disulfide SAMs, as revealed mainly by macroscopic techniques. In this study, we have successfully revealed and compared the adsorption processes of dialkyl disulfides and dialkyl sulfides for the first time from a nanoscopic viewpoint.

The enzymatic degradation behavior of solution-grown single crystals of poly[(R)-3hydroxybutyrate] (P(3HB)), which were adsorbed on the surface of highly ordered pyrolytic graphite, with an extracellular PHB depolymerase from Alcaligenes faecalis T1 was directly observed by atomic force microscopy. At the initial stage of degradation, enzymatic action of both adsorption and hydrolysis produced smaller crystal blocks by predominant degradation of distorted regions of a single crystal. Subsequently, the hydrolysis progressed from a generated end of the crystal block, resulting in the formation of cracks. The degradation also took place at a middle part of the crystal, yielding slits. Several cracks and slits stood in a line along the long axis of crystal, and further enzymatic hydrolysis combined the cracks and the slits into longer cracks. With these processes, the enzymatic degradation converted a lath-shaped lamellar single crystal into lathlike fingers and crystal fragments. It has been proposed from the degradation texture of one lamellar single crystal that inherent straight degradation pathways exist parallel to the average folding direction of the single crystal of P(3HB). The straight degradation pathway may be a mismatched region of molecular packing of adjacent polymer chains in a lamellar single crystal and may be generated with a crystal growth process as a history line.

This paper reports the initial process of the enzymatic degradation of solution-grown lamellar single crystals of bacterial poly[(R)-3-hydroxybutyrate] (P(3HB)) with an extracellular polyhydroxybutyrate (PHB) depolymerase purified from Alcaligenes faecalis T1. We used a hydrolytic-activity-disrupted mutant of the PHB depolymerase in order to avoid the influence of hydrolytic reaction in the system. The effect of addition of the mutant enzyme upon the P(3HB) single crystals was investigated by turbidimetric assay, high-performance liquid chromatography (HPLC), and atomic force microscopy (AFM). Suspension turbidity of the P(3HB) single crystals increased after addition of the mutant enzyme having no hydrolytic activity. No soluble product from the P(3HB) single crystals with the mutant enzyme was detected by HPLC. AFM observation of the P(3HB) single crystals adsorbed on highly ordered pyrolytic graphite revealed that the mutant enzyme yielded a lot of lengthwise crystal fragments from the P(3HB) single crystals. On the basis of these results, we concluded that the mutant enzyme disturbs the molecular packing of the P(3HB) polymer chain around the loose chain packing region in the single crystal, resulting in the fragmentation. Therefore, it is suggested that the enzymatic degradation of P(3HB) single crystals with a wild-type PHB depolymerase progresses via three steps: (1) adsorption of the enzyme onto the surface of the single crystal; (2) disturbance of the molecular packing of P(3HB) polymer chain in the single crystal by the adsorbed enzyme; and (3) hydrolysis of the disturbed polymer chain by the adsorbed enzyme.

Enzymatic degradation behavior of a lamella of single crystals of poly(R)‐3‐hydroxybutyrate (P(3HB)) with an extracellular polyhydroxybutyrate (PHB) depolymerase purified from Alcaligenes faecalis T1 has been investigated by atomic force microscopy (AFM) in order to obtain further information for the chain packing state of P(3HB) in a lamellar single crystal. Two kinds of P(3HB) single crystals with different molecular weights, denoted respectively as H‐ and L‐P(3HB) for high and low molecular weights, respectively, were prepared. The enzymatic treatment was conducted for P(3HB) single crystals adsorbed on a surface of highly ordered pyrolytic graphite. The enzymatic degradation of both P(3HB) single crystals generates several crevices crosswise across the crystal at an early stage. Subsequently, the enzymatic degradation yields numbers of cracks lengthwise along the crystal. In addition to these common features, the interval between cracks crosswise across a lamella in H‐P(3HB) single crystal is longer than that in L‐P(3HB) single crystal, and each crack has V‐shaped and rectangular shaped morphology for H‐ and L‐P(3HB) single crystals, respectively. Based on these results, it is concluded that a lamella of P(3HB) single crystal has straight degradation pathways, that may correspond to a switchboard region, along the long axis of the crystal, independent of molecular weight of P(3HB) samples, and that a H‐P(3HB) single crystal has broader degradation pathways with longer intervals crosswise across the crystal than a L‐P(3HB) single crystal.