Moments of Proteins

A sensitive approach for the analysis of molecular structures is the measurement of rotation time constants and optical anisotropy coefficients by relaxation electrooptical methods.The experimental procedure used for electrooptical investigations is relatively simple samples are subjected to electric field pulses, and the response due to field-induced alignment or field-induced reactions is recorded by spectrophotometric techniques. In the dichroism experiments, the absorbance of polarized light is measured under electric field pulses. The measured quantity, linear dichroism (LD), means that anisotropic absorption of plane or linearly polarized light has taken place. 

In the case of proteins, a special variant of the ion polarization, namely, the polarization of protons, may provide an important contribution to this complexity. However, proteins that have so far been investigated at high time resolution do not reveal any contribution from ion or proton polarization. Both 

The results of extinction anisotropy measurements are usually represented in the form of the reduced linear dichroism expressed as follows  

In the presence of a uniform external electric field, the light absorption by the solution is characterized by cylindrical symmetry for rotations around the direction indicated by the field. Thus A may be expressed as 

Subsequently, one gets the following two conditions for excluding the chemical contribution to optical anisotropy of the solution 

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The dipole moments of protein and other gas-phase macroions differ from those of solution or solid state precursors because of (i) the change of z and consequent rearrangement of partial charges over the molecule and (ii) unavoidable geometry distortion upon ionization, even with the softest sources.At still higher z... 

Felder, C.E., Prilusky, J., Silman, I., Sussman, J.L., A server and database for dipole moments of proteins. Nucleic Acids Res. 2007, 35, W512. The server is available at http //bioportal.weizmann.ac.il/dipol/index.html... 

Experimental dipole moments may be used to check the validity of electrostatic calculations. In the past, dipole moments of proteins were often characterized by measurements of dielectric relaxation. More information may be obtained by measurements of the electric dichroism because these measurements provide not only the magnitude of the dipole moment but also the optical anisotropy with respect to the dipole vector. Thus, measurements of the electric dichroism provide a more rigorous test for calculations of electrostatic parameters of proteins. Using the calculations described earlier for pK s of titratable groups, one can predict dipole moments of proteins and their axes given by the principal axes of the rotational diffusion tensor and compare them with electrooptical data. ° One important aspect of comparison of computed and experimental dipole moment is that computations of dipole moments, optical anisotropy, and rotational diffusion coefficients can be used in combination with experimental electrooptical procedures to determine the long-range structure of biomacromolecular assemblies, such as the complexes of DNA and proteins described by Pbrschke et al. so... 

Computed dipole moments of proteins presented here refer to a particular point within the structures of the proteins named center of diffusion (CD). As is known, a calculated dipole moment depends on the choice of coordinate system origin when the total charge on the molecule is not zero. Experimental values of dipole moments are derived from the orientational behavior of molecules under electric field pulses, which obviously is independent of any coordinate system but can be biased by other factors. For example, Wegener " showed that rotational velocity > driven by the external electric field, E at low Reynold s number can be expressed as... 

On the other hand, a chain of the /3-form of the coiled form of the proteins it contains. A few glucose gives the polymer called cellulose moments of thought concerning the profound... 

It should be borne in mind, however, that enzyme proteins, like other proteins, are dipolar ionic structures surrounded by an intense electrostatic field. Horse carboxyhemoglobin, for example, has a dipole moment of 480 Debye units (molecular weight, 67,000). Compared with amino acids, sugars have a rather low dipole moment, due mainly to a considerable measure of mutual cancellation of the constituent link moments resulting from their disposition in space (glycine = 15, a- and... 

Observation of reorientational dynamics of dipolar groups surrounding the fluorophore in response to changes in the dipole moment of the fluorophore occurring upon electronic excitation. Such dynamics result in the appearance of spectral shifts with time,(1 ) in changes of fluorescence lifetime across the fluorescence spectrum,(7,32) and in a decrease in the observable effects of selective red-edge excitation.(1,24 33 34) The studies of these processes yield a very important parameter which characterizes dynamics in proteins— the reorientational dipolar relaxation time, xR. 

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