In this Letter, we report on the direct mapping of helicity in the helix-to-coil structural transitions by theoretically simulating the ultrafast electron diffraction (UED) technique for isolated proteins with thousands of possible conformations. A novel approach for producing random coil structures of a given helical protein is presented with proper account of the steric avoidance threshold. A 'resonance' is manifested in the UED patterns of the helical structures. This resonance erodes as the transition to random coils becomes dominant, indicating that the residual helicity in the sample and the ensemble-averaged radius of gyration can be obtained and related to structural dynamics of isolated proteins. (c) 2005 Elsevier B.V. All rights reserved.