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The binding interactions between the pyridine and small noble metal clusters in different sizes (n ) 2-4) have been investigated by using quantum chemical methods. The binding energies of Py-M 2 complexes are obtained at the levels of the Hartree-Fock method (HF), the second-order Møller-Plesset perturbation theory (MP2), the local density functional method (SVWN), the nonlocal density functional method (BLYP, BPW91, G96LYP, G96PW91), and the hybrid density functional method (B3LYP and B3PW91). All calculated results show that the bonding is stronger in pyridine/copper and pyridine/gold than that in pyridine/silver. The bonding mechanism is explored in terms of the bonding molecular orbital properties. The donation interaction of the lone-pair electrons on nitrogen of the pyridine molecule to the unoccupied orbital of each metal cluster plays an important role. The force constants of the internal coordinates of interests are presented. The vibrational frequency shift has been analyzed on the basis of the coupling between the internal vibrational modes of pyridine and the nitrogen-metal stretching modes as well as the metal-metal stretching modes. For lowfrequency Raman spectra of pyridine-small silver cluster complexes, we propose a new assignment to the N-Ag and Ag-Ag stretching vibrations. The calculated infrared intensities of vibrational modes are compared with the experimental spectra.

Using simultaneously scanning small-angle X-ray scattering (SAXS) and UV-vis absorption with integrated online size exclusion chromatography, supplemental with molecular dynamics simulations, we unveil the long-postulated global structure evolution of a model multidomain protein bovine serum albumin (BSA) during acid-induced unfolding. Our results differentiate three global packing structures of the three molten globule domains of BSA, forming three intermediates I, I, and E along the unfolding pathway. The I-I transition, overlooked in all previous studies, involves mainly coordinated reorientations across interconnected molten globule subdomains, and the transition activates a critical pivot domain opening of the protein for entering into the E form, with an unexpectedly large unfolding free energy change of -9.5 kcal mol, extracted based on the observed packing structural changes. The revealed local packing flexibility and rigidity of the molten globule domains in the E form elucidate how collective motions of the molten globule domains profoundly influence the folding-unfolding pathway of a multidomain protein.