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Ultraviolet resonance Raman spectroscopy of the integral membrane protein OmpA : elucidating structure and tryptophan microenvironment of folded and unfolded states


A 325 residue integral membrane protein, outer membrane protein A (OmpA), has been investigated using ultraviolet resonance Raman spectroscopy at 207.5 nm and 228 nm to elucidate differences in secondary structure and tryptophan microenvironment of various OmpA mutants. OmpA was investigated in its native form containing five tryptophans, as a trp-less mutant, as five full-length single-trp mutants, and as five truncated single-trp mutants in which the soluble tail (̃148 residues) was cleaved from the transmembrane domain (̃177 residues). A wavelength-dependence study of a single-trp OmpA mutant was performed at nine different wavelengths between 206.5- 236.5 nm to determine the optimal wavelength that provides the greatest resonance enhancement of the tryptophan modes in OmpA. This wavelength was found to be 228 nm, which is slightly red-shifted from the absorption maximum of tryptophan in solution. Excitation of the protein backbone using 207.5 nm light revealed secondary structure changes between the folded and unfolded states of full-length and truncated single-trp mutants of OmpA, W57 and W57t, respectively, with few noticeable spectral differences between the two mutants. At 228 nm, OmpA wild-type, trp- less and 10 single-trp mutants unfolded in KPi, folded in SUVs, and folded in OG were examined to identify changes in tryptophan structure and local environment. This study has revealed changes in hydrophobic environments, hydrogen -bonding, and the torsion angle, [Chi]²'¹

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