Tyrosine residues motions

The present study has shown that enkephalin binds to PS in a pH-dependent fashion. The binding most likely involves the NHa group of the tyrosine residue of enkephalin and the CO2 group of PS. We have measured for the interaction and find it to be of the order of 5 x 10 M ("pH" 6.3) which is much weaker than the interaction of PS with morphine derivatives. Upon binding of enkephalin to PS the correlation time for internal motion in the backbone of the peptide increases by at least one order of magnitude (from 7.0 x 10" sec rad" ). The Ti of the bound peptide... 

Measiuements of the emission anisotropy A as a function of added collisional quencher are made with the steady fluorescence intensity, which integrates the different weighted fluorescence lifetimes. Quenching emission anisotropy plot of 1 /A vs I (Fig. 5.14) yields for A(o) a value of 0.246 and 0.243 for [L-Met2] DREK and DREK, respectively. These values, lower than that (0.278) measured at - 45 °C for tyrosine at 280 nm (Lakowicz and Maliwal, 1983), indicate that tyrosine residue in both peptides display residual motion independent of the global rotation of the peptide. It is possible to measure the relative importance of the mean residual motions of the tyrosine residues ... 

Oxytocin is a small cyclic polypeptide which contains nine amino acida and a single tyrosine residue. The FD anisotropy decay of the tyrosine floutescence is shown in Figure 11.20. The ED data to 200 MHz (vertical dotted line) show only increasing values of and little change in the modulated anisotropy. Hence, the data contain incomplete information on the anisotropy decay. This situation was improved by the use of instrrrmentation which allowed measurements to 2 GHz. In this case there is detectable shape in the values of Am and one can see that there are conqionents due to two correlation times, 29 and 4S4 ps. The 29-ps conelation tune is due to segmental motions of the rosyl residues, and the 454-ps cmielation time is due to overall rotation of the peptide. 

Fast ccMT >onents in the anisotn y decays are characteristic of many proteins and pq>rides and have been reported in many publications. Picosecond-timescale motions were also reported for tyrosine residues in pro-teins in these studies, a streak camera was used to obtain adequate time resolution. The short correlation rime observed in protons is variable and ranges from 50 to 5(X) ps, with the values being detemuned in part by the rime resolution of the instrument. The shorty correlation time is )proximately equal to that observed for NAIA in w er or fcorrelation times are typically insensitive to protein folding and are not greatly affected ly the viscosity of the solution. 

The mobility of tyrosine in Leu3 enkephalin was examined by Lakowicz and Maliwal/17 ) who used oxygen quenching to measure lifetime-resolved steady-state anisotropies of a series of tyrosine-containing peptides. They measured a phase lifetime of 1.4 ns (30-MHz modulation frequency) without quenching, and they obtained apparent rotational correlation times of 0.18 ns and 0.33 ns, for Tyr1 and the peptide. Their data analysis assumed a simple model in which the decays of the anisotropy due to the overall motion of the peptide and the independent motion of the aromatic residue are single exponentials and these motions are independent of each other. 

Lakowicz et al.(]7] VB) examined the intensity and anisotropy decays of the tyrosine fluorescence of oxytocin at pH 7 and 25 °C. They found that the fluorescence decay was best fit by a triple exponential having time constants of 80, 359, and 927 ps with respective amplitudes of 0.29, 0.27, and 0.43. It is difficult to compare these results with those of Ross et al,(68) because of the differences in pH (3 vs. 7) and temperature (5° vs. 25 °C). For example, whereas at pH 3 the amino terminus of oxytocin is fully protonated, at pH 7 it is partially ionized, and since the tyrosine is adjacent to the amino terminal residue, the state of ionization could affect the tyrosine emission. The anisotropy decay at 25 °C was well fit by a double exponential with rotational correlation times of 454 and 29 ps. Following the assumptions described previously for the anisotropy decay of enkephalin, the longer correlation time was ascribed to the overall rotational motion of oxytocin, and the shorter correlation time was ascribed to torsional motion of the tyrosine side chain. 

It can be seen from these data that the larger hydrophobic side chains are the most buried with the exception that cystine also tends to be quite inaccessible with 26, 72, 84, and 110 completely buried. All of the alanines are exposed, three of the four prolines are very exposed, three of the valines are completely buried as are Met 30, Phe 46, and Ser 90. Phenylalanine 8 is only accessible via a tunnel from the surface which is in fact occupied and blocked by one well-defined solvent molecule. The various residues of each polar amino acid have a wide range of exposure, but the larger residues tend to be most accessible with the exception of the tyrosines, which are quite variable. Residues in the active site region, 11, 12, 41, 43, 44, 45, 119, 120, 121, and 123, tend to be the extremes within each residue type but it should be noted that the motion of His 119 to the active position proposed later (Section VI) would increase the exposure of 11, 12, 41, and 44 and decrease the exposure of 121 and 109 in particular. The hydrophilic residues and especially the hydrophilic portions of these residues are generally ex-... 

Fluorescence depolarization measurements of aromatic residues and other probes in proteins can provide information on the amplitudes and time scales of motions in the picosecond-to-nanosecond range. As for NMR relaxation, the parameters of interest are related to time correlation functions whose decay is determined by reorientation of certain vectors associated with the probe (i.e., vectors between nuclei for NMR relaxation and transition moment vectors for fluorescence depolarization). Because the contributions of the various types of motions to the NMR relaxation rates depend on the Fourier transform of the appropriate correlation functions, it is difficult to obtain a unique result from the measurements. As described above, most experimental estimates of the time scales and magnitudes of the motions generally depend on the particular choice of model used for their interpretation. Fluorescence depolarization, although more limited in the sense that only a few protein residues (i.e., tryptophans and tyrosines) can be studied with present techniques, has the distinct advantage that the measured quantity is directly related to the decay of the correlation function. 

Researches on theoretical topics have not been reported very extensively. A few papers are mentioned here and some others at appropriate points later in the article. Weber has re-examined the famous Perrin equation for quantifying the rotational depolarization of fluorescence. The arguments presented in the paper are applied to the temperature dependence of the local motions of tyrosine and tryptophan residues observed in proteins. 

Because of their spectral properties, RETcan occur fiom phenylalanine to tyrosine to tryptophan. Also, blue-shifted tryptophan residues can transfra the excitation to longer-wavelength tryptophan residues. In fact, energy transfer has been repeatedly observed in proteins and is one reason for the minor contribution of phenylalanine and tyrosine to the emission of most (Hoteins. The anisotropy displi ed by tyrosine and tryptophan is sensitive to both ov l rotational diffusion of proteins and the extent of segmental motion during the excited-state lifetimes. Hence, the intrinsic fluorescence of proteins can provide considerable information about protein structure and dynamics and is often used to study protein folding and association reactions. In this chapter, we present examples of protein... 

---