Based on non-equilibrium Green's function and density functional theory, a first-principles study of the transport properties of two benzene-pyridines sandwiching the sigma barrier of ethyl is performed. Using symmetric leads, strong negative differential resistance (NDR) effects with high peak-to-valley ratios (PVRs) are present under low bias. When using asymmetric leads, the PVR can be modulated to higher value with the unchanging of voltage (V-peak) corresponding current peak (I-peak). Our investigations indicate that pyridine "alligator-clip" can be used as very good low bias molecular NDR devices. (C) 2017 Elsevier B.V. All rights reserved.