  The first realization of a biomolecular OR gate function with double-sigmoid
response (sigmoid in both inputs) is reported. Two chemical inputs activate the
enzymatic gate processes resulting in the output signal: chromogen oxidation,
which occurs when either one of the inputs or both are present (corresponding
to the OR binary function), and can be optically detected. High-quality gate
functioning in handling of sources of noise is enabled by "filtering" involving
pH control with an added buffer. The resulting gate response is sigmoid in both
inputs when proper system parameters are chosen, and the gate properties are
theoretically analyzed within a model devised to evaluate its noise-handling
properties.
