Protein Nanotubes Comprised of an Alternate Layer‐by‐Layer Assembly Using a Polycation as an Electrostatic Glue

We present the synthesis and structure of various protein nanotubes comprised of an alternate layer‐by‐layer (LbL) assembly using a polycation as an electrostatic glue. The nanotubes were fabricated by sequential LbL depositions of positively charged polycations and negatively charged proteins into a porous polycarbonate (PC) membrane, followed by release of the cylindrical core by quick dissolution of the template with CH2Cl2. This procedure provides a variety of protein nanotubes without interlayer cross‐linking. The three‐cycle depositions of poly‐L‐arginine (PLA) and human serum albumin (HSA, Mw=66.5 kDa) into the porous PC template (pore diameter, Dp=400 nm) yielded well‐defined (PLA/HSA)3 nanotubes with an outer diameter of 419±29 nm and a wall thickness of 46±8 nm, revealed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. The outer diameter of the tubules can be controlled by the pore size of the template (200–800 nm), whereas the wall thickness is always constant, independent of the Dp value. The (PEI/HSA)3 (PEI: polyethylenimine) nanotubes showed a slightly thin wall of 39±5 nm. CD spectra of the multilayered (PEI/HSA)n film on a flat quartz plate suggested that the secondary structure of HSA between the polycations was almost the same as that in aqueous solution. The three‐cycle LbL depositions of PLA and ferritin (Mw=460 kDa) or myoglobin (Mb, Mw=1.7 kDa) into the porous PC membrane also gave cylindrical hollow structures. The wall thickness of the (PLA/ferritin)3 and (PLA/Mb)3 nanotubes were 55±5 nm and 31±4 nm; it depends on the globular size of the protein (ferritin>HSA>Mb). The individual ferritin molecule was clearly seen in the tubular walls by SEM and TEM measurements. Rolled sandwich: Protein nanotubes (see figure) comprising an LbL assembly of human serum albumin, ferritin or myoglobin with a polycation as an electrostatic glue were prepared by templating synthesis using a porous polycarbonate membrane. The tubule wall thickness is dependent on the globular size of the protein at the same number of deposition cycles.