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The subterahertz (THz) region of absorption spectra (2-30 cm-1) of biomolecules reveals low-frequency molecular motions as resonances that can serve as fingerprints specific to molecules. Nanoscale edge effect in subwavelength periodic slit arrays can be used to increase the coupling of incident THz radiation to the biological material under the test due to the local electric-field enhancement, thus improving detection sensitivity and special resolution of THz resonance vibrational spectroscopy. In this paper, propagation of polarized light through 1-D grating made of perfect conducting metallic slits is considered. The analytical integral equation is derived and numerical solution is found in a long wavelength limit. The simulation program is developed that permits to find transmission characteristics of periodic structures and distribution of electromagnetic (EM) field across the slit with the goal to optimize the structure geometry. The perfect conductor approximation gives us an upper estimate of achievable enhancement of electric field in this case.