The surface-sensitive optical technique of surface plasmon resonance (SPR) imaging is used to characterize ultrathin organic and biopolymer films at metal interfaces in a spatially resolved manner. Because of its high surface sensitivity and its ability to measure in real time the interaction of unlabeled biological molecules with arrays of surface-bound species, SPR imaging has the potential to become a powerful tool in biomolecular investigations. Recently, SPR imaging has been successfully implemented in the characterization of supported lipid bilayer films, the monitoring of antibody-antigen interactions at surfaces, and the study of DNA hybridization adsorption. The following is included in this review: (a) an introduction to the principles of surface plasmon resonance, (b) the details of SPR imaging instrumental design, (c) a short discussion concerning resolution, sensitivity, and quantitation in SPR imaging, (d) the details of DNA array fabrication on chemically modified gold surfaces, and (e) two examples that demonstrate the application of the SPR imaging technique to the study of protein-DNA interactions.
A multistep surface modification procedure for the creation of DNA arrays on chemically modified gold surfaces that can be used in surface plasmon resonance (SPR) imaging studies of protein−DNA interactions is demonstrated. The multistep procedure is required to create an array of spots that are surrounded first by a hydrophobic background which allows for the pinning of aqueous DNA solutions onto individual array elements and then to replace that hydrophobic background with one that resists the nonspecific adsorption of proteins during in situ SPR imaging measurements. An amine-terminated alkanethiol monolayer is employed as the base layer, and Fmoc and PEG modifiers are used to create the sequentially hydrophobic and protein adsorption-resistant surfaces, respectively. Specifically, the chemical modification steps are the following: (1) the adsorption and self-assembly of an 11-mercaptoundecylamine (MUAM) monolayer on an evaporated gold thin film, (2) the reaction of the MUAM monolayer with an Fmoc protecting group to create a hydrophobic surface, (3) the photopatterned removal of the alkanethiol followed by (4) the readsorption of MUAM to create an array of MUAM squares (750 × 750 μm) surrounded by a hydrophobic MUAM-Fmoc background that can pin drops of aqueous solution, (5) the attachment of oligonucleotide sequences onto the MUAM squares by the reaction of the amine-terminated surface with the heterobifunctional cross linker SSMCC followed by a coupling reaction to a small volume (0.1 μL) of thiol-modified DNA, and (6) the removal of the Fmoc protecting group followed by (7) a pegylation reaction of the MUAM with PEG-NHS to create a protein adsorption-resistant background. A combination of polarization-modulation FTIR spectroscopy, contact angle, and scanning angle SPR measurements is used to characterize the surface modification procedure. An SPR imaging measurement of the adsorption of single-stranded DNA binding protein (SSB) onto an oligonucleotide array created by this procedure is used to demonstrate the utility of these surfaces.
In situ external reflection infrared spectroscopy at the air-water interface was used to study the influence on phospholipid structure of an endogenous mixture of the two hydrophobic surfactant proteins, SP-B and SP-C, which are thought to play pivotal roles in the adsorption and function of pulmonary surfactant. Mixtures studied were 1:1, 2:1, and 7:1 (mol:mol) DPPC-d(62):DPPG, and 7:1 DPPC-d(62):DOPG, alone and in the presence of 0.5-10 wt % mixed SP-B/C purified chromatographically from calf lung surfactant extract. Perdeuteration of DPPC produced a shift in vibrational frequencies so that it could be differentiated spectroscopically from the phosphoglycerol component in the surface monolayer. CH(2) antisymmetric and symmetric stretching bands ( approximately 2920 and 2852 cm(-1)) along with the analogous CD(2) stretching bands ( approximately 2194 and 2089 cm(-1)) were analyzed, and band heights and peak wavenumber positions were assessed as a function of monolayer surface pressure. Small, near-physiological contents of 1-2 wt % SP-B/C typically produced the maximum observed spectroscopic effects, which were abolished at high protein contents of 10 wt %. Analysis of CH(2) and CD(2) stretching bands and C-H/C-D band height ratios indicated that SP-B/C affected PC and PG lipids differently within the surface monolayer. SP-B/C had preferential interactions with DPPG in 1:1, 2:1, and 7:1 DPPC-d(62):DPPG films that increased its acyl chain order. SP-B/C also interacted specifically with DOPG in 7:1 DPPC-d(62):DOPG monolayers, but in this case an increase in CH(2) band heights and peak wavenumber positions indicated a further disordering of the already fluid DOPG acyl chains. CD(2) band height and peak wavenumber analysis indicated that SP-B/C had no significant effect on the structure of DPPC-d(62) chains in 7:1 films with DPPG or DOPG, and had only a slight tendency to increase the acyl chain order in 1:1 films of DPPC-d(62):DPPG. SP-B/C had no significant effect on DPPC-d(62) structure in films with DOPG. Infrared results also indicated that interactions involving SP-B/C and lipids led to exclusion of PC and PG lipids from the compressed interfacial monolayer, in agreement with our previous report on the phase morphology of lipid monolayers containing 1 wt % SP-B/C.
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