We present new quantitative regulations of the binding-affinity using dynamical single-molecule detection system with X-rays. In the study of antigen-antibody interactions, we found that structural fluctuations of single-molecules were negatively regulated by antigen-binding. Although strategies to produce ligand-induced stability have been well studied from the macro aspect both theoretically and experimentally, our dynamical single-molecular experimental results are first observations with angstrom accuracy in the real-time and space. It is considered that those negative regulations of protein structural fluctuations with binding event are related to biological functions. In addition, we clarified that ratio between antigen-binding condition and no-binding one in observed structural fluctuations are extremely relative to the binding-affinity. These results indicate that the phenomena of protein-ligand interactions considered as stable states can be defined as results of dynamical processes at the single-molecule level. Such new quantifications from angstrom-level structural fluctuations can be applied to various biological science and biotechnologies. (c) 2007 Elsevier Inc. All rights reserved.