**1. Introduction**

In recent year, many monoclonal antibody drugs have been developed, and some of them are practically used in therapy [1–3]. With respect to catalytic antibodies, they have extensively been

developed [4–14] for the last two decades from the viewpoint of both basic research and the application, where it has been proved that there are many catalytic antibodies being effective against anti-rabies virus [15], anti-influenza virus [16], anti-*Helicobacter pylori* [17], anti-HIV [7, 8, 10], anti-Alzheimer's disease [14, 18], etc. Interestingly, some of them have been advanced to the stage tested *in vivo* in this decade [15–19]. In the case of catalytic antibodies, some of them play the role as a whole structure of IgG [5, 9, 11, 13], IgA [20], or IgM [21–23]. On the other hand, in some cases, their subunits (light chain or heavy chain) exhibit unique functions [1–4, 6–8, 12]. Once the antibody subunits are separated, the structure of the light or heavy chain becomes flexible and has a tendency to possess structural diversity (or molecular heterogeneity). Regarding structural heterogeneity, it was found about 20 years ago that a whole antibody possesses the structural heterogeneity. These studies were extensively studied by Harris et al. [24] and Nebija et al. [25] using the capillary isoelectric focusing and the 2D-gel electrophoresis [26, 27].

We have also reported about the molecular heterogeneity caused by different electrical charges and different molecular size in mouse monoclonal antibody [28]. This phenomenon is not good for the preparation efficacy, high reproducibility, and practical application. In addition, the structural diversity leads us to ask what structure plays the most important role in exhibiting the catalytic antibody functions. It also provides us with another subject of how we can best make a significant structure with high reproducibility and productivity.

We have recently found a crucial method to solve the heterogeneity problem by using copper ion, which can convert the multi-molecular forms into mono-molecular forms for many recombinant human antibody light chains. In addition, the constant region domain of the light chain (CL) plays an important role in generating a mono-molecular form.

In this review, we will describe a novel method for preparing a single and defined mono-form structure in detail.
