UV difference spectra

A liner containing glass wool was installed in the injection port of the GC to trap polymer residues. A solution containing a known amount of the carbamate in THF along with sulfolane as an internal standard was used to establish the concentration of carbamates in the PMMA and PPMA matrix. Quantum yields were then determined. Product ratios were calculated from UV difference spectra (taken on a Beckman DK-2A spectrometer) of films before and after photolysis. 

The effect of heat denaturation on the native mixed soy 7S and 11S globulins and on the enzyme-modified soy 7S and 11S proteins is shown by the UV difference spectra (Figure 4). The strong negative peak at 232-233 nm indicates rupture of the secondary structure which has occurred in both the native and enzyme-modified protein. The enzyme-modified protein may retain more order and be somewhat more stable to heat denaturation than the native protein. Loss of secondary structure is also apparent in the circular dichroism spectra (Figure 5) with the change in character of the curve below 220 nm. The spectrum resembles that reported by Koshiyama and Fukushima (11) for 7S protein in 0.25% SDS/0.01M tris buffer. 

Fig. 18. Typical UV difference spectra, showing the increase in absorbance, A A, during the binding of (a) V3+ and (b) Yb3+ to human lactoferrin. The spectra are shown as a superimposed series of spectra, corresponding to successive increments of the added metal ion. Below each set of spectra is the corresponding titration curve, showing that in each case two metal ions are bound per molecule (r is the ratio of moles of the metal iron added per mole of protein).

The chemistry of the group 13 metals makes binding studies difficult, as careful control of pH and bicarbonate concentration is necessary to prevent formation of species such as Al(OH)4 and Ga(OH)4". Nevertheless, UV difference spectra have shown that Al3+, Ga3 +, In3+, and Tl3 all form transferrin complexes with two metal ions per molecule (126, 144-146). Ae values imply the ionization of two tyrosines per bound metal ion, as for other specifically bound metals. NMR studies using 13C-enriched bicarbonate show virtually identical spectra for Al3+ and Ga3+, implying equivalent metal-anion environments (99). 

The actinides plutonium, neptunium, protoactinium, and thorium (151,173) bind to transferrin. The larger Th4+ ion (radius, 0.94 A) still binds to both sites, although binding to the second site (probably the N-terminal site) is significantly weaker than that to the first and apparently involves only one Tyr ligand compared with two Tyr in the other (151). Although UV difference spectra for Pu4+ are equivocal (174), it seems likely that two Pu4+ are bound. The likely carrier properties of transferrin for Pu4+ makes the design of competitive chelators of some importance (151). 

The concept that transferrin complexes of larger metal ions may not exhibit the same closed structure as that of Fe3+ is not incompatible with spectroscopic studies. UV difference spectra reflect tyrosine coordination, and it is known that the open , metal-free structure has both tyrosines close together, adjacent to the (bi)carbonate site on domain 2 (80), but far (8 to 9 A) away from the remaining Asp and His ligands... 

III IV Fig. 11 8 UV difference spectra 0.5---ImM EDTA, pH7,+NaC104 tRNA pretreated with EDTA ... 

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