Influence of Geometry of Metal Complexes on Conformational Changes in BSA at Air/Water and Solid Surfaces

Abstract

Two new metal complexes with N 2 O 2 tetradentate ligand, viz. [Cr(naphprn)(H 2 O)2](C10 4 ).2H 2 O (1) and [Ni(naphprn)] (2), have been synthesized, and their interactions with bovine serum albumin (BSA) in Langmuir films atthe air/water interface and as Langmuir-Blodgett films (LB films) at the solid/liquid interface have been evaluated. The π-A isotherms of BSA and BSA + 1 and BSA + 2 show the average limiting areas 40.4, 17.3, and 24. 8 nm 2 , respectively. Circular dichroism (CD) studies show 2 brings minor changes in the a helix coil equilibria of native BSA whereas 1 increased the helicity of the protein considerably, suggesting strong metal ion chelation. This was further substantiated by the rate of change of surface potential values of BSA with the two metal complexes. BSA binds with 1 within 138 s, while 2 binds to the protein at a slower rate and the reaction completes after 188 s. The compact structure arising in BSA + 1 films compared to BSA + 2 and the correlation to surface energy profiles of BSA were investigated by using the static contact angle of liquids on solid surfaces. The calculated solid surface tensions indicate the lowest polar component γ s P for BSA + 1 (γ s P = 12.1 mN/m) on a high surface energy (hsfe) substrate such as glass and the highest value on a low surface energy (lsfe) substrate such as Teflon (γ s P = 22. 8 mN/m) compared to those for pure BSA or BSA + 2. A comparison of the results obtained from the acid -base parameter of the surface tension revealed a similar tendency. From the advancing contact angle measurements, the spreading rates of the protein with the two different metal complexes suggested overall protein unfolding and or reorientation driven by hydrophobic interactions.