Amplified vibrational circular dichroism as a probe of local biomolecular structure

We show that the VCD signal intensities of amino acids and oligopeptides can be enhanced by up to 2 orders of magnitude by coupling them to a paramagnetic metal ion. If the redox state of the metal ion is changed from paramagnetic to diamagnetic the VCD amplification vanishes completely. From this observation and from complementary quantum-chemical calculations we conclude that the observed VCD amplification finds its origin in vibronic coupling with low-lying electronic states. We find that the enhancement factor is strongly mode dependent and that it is determined by the distance between the oscillator and the paramagnetic metal ion. This localized character of the VCD amplification provides a unique tool to specifically probe the local structure surrounding a paramagnetic ion and to zoom in on such local structure within larger biomolecular systems.