Anisotropy Decays of Melittin

Melittin also binds to lipid vesicles because of its nonpolar surface in the a-helical state. Hiis binding results in a dramatic change in the anisotropy decay (Bg-ure 17.18), which now shows a correlation time near 12 ns, but there is considerable uncertainty in this value. This result illustrates one of the difficulties with using tryptophan as an anisotropy probe. It is difficult to obtain much information after 10 ns, or three times the mean lifetime, owing to the low remaining intensity. In neral, me 

Protein Anisotropy Decays as Observed in the Frequent Domain 

Figuie 17.19. FD anisotrofiy decays of Unee singleuyiiai laa pro-tciiu a RNase Ti e, monellin a. adrenococtieottopin (ACTH (I-24)X Data from Ref. 73. 

The extent of segmental mobility also affects die modulated miisolropy (r ) data. The values of at low modulation frequency reflect the steady-state anisotropy and are thus lower for the proteins with higdio segmental flexibility. The transition of toward the hifdi-frequency value of ro occurs at higher frequency for proteins with greater flexibility. It is apparent from these data that the FD anisotropy data are highly sensitive to the form of the anisotropy decay and/or the dynamic properties of the proteiiis. 

Why is spectral relaxation rarely observed in proteins One partial answo is because diis ect is uaudly not considered and tbe heterogeneity of protein decays is interpr ed in terms cmifornuitioial stales. Also, die time-dependent shifts are small and somewhat difficult to observe. Finally, spectral relnXStion probably occurs mostly on a subnanosecond timescale, so that diis process is mostly complete prior to emission. 

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To illustrate the nature of the anisotropy decays the equivalent time-dependent anisotropies are shown as an insert. These were calculated from the frequency-do-main data. For Sj Nuclease the plot of log r(t) versus time is mostly linear with a slope of (12 nsec) This is the portion of the anisotropy decay due to overall rotational diffusion of the protein. The rapid component in the nuclease anisotropy decay is seen only near the t = 0 origin. The anisotropy decay of melittin is much more rapid, which reflects the greater motional freedom of the tryptophan residue in this disordered polypeptide. Because of the segmental motions which depolarize the... 

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