| The study of biomolecules promotes the development of life science, medicine and chemistry etc.The non-biological application of biomolecules is a challenging research,and is strongly potential. Recently,biomolecules have been applied to catalyzer,molecular identification,optical memory and molecular lead etc.The study of bimolecules promotes the progress of the experimental and theoretical research for molecular interaction,resonance Raman effect,surface enhancement Raman scattering.β-carotene molecule has the function of collecting optical energy,energy transfer,effective singlet oxygen quencher and free radical trap.We know that molecule structure has closely related with the property and function of molecule.Especially the molecular chain structural order of this kind of long chain conjugated polyene effects on the property and function of molecule. Moreover effect of the optimized order of polyene on the optical property is focused by the researcher.But recent years the progress is slow for scare research method,for example the polyene molecular chain structural order study of the extremely low concentration is not found on the papers.The study to the molecular chain structural order is urgent by the needs of biological science and material science.Raman cross section(RCS) is an important parameter which can shows the molecular microscopic property as well as the Raman shift and the line width.It relates not only to the incident frequency and the structure of a scattered molecule but also to the environment where the scattered molecules are located.Furthermore,it indicates the light-scattering capacity of a particular molecule and the distribution of electric cloud. The paper calculated the Raman cress section ofβ-carotene on different molecular environment and at the same time the chain structural order is studied by the Raman spectra and absorbing spectra.1 The RCSs ofβ-carotene in different solvents and the molecular chain structural order.Under 514.5nm excitation laser the twe basic characteristic Raman line are1520cm-1,1150cm-1,which originate,respectively,from carbon-carbon single and double-bond stretching vibration of the conjugated backbone. The cyclohexane line 1444cm-1 is regard as internal standard,shown in fig.1.Fig.1 Resonance Raman spectra ofβ-carotene in aqueous solution at 10-5M concentration1 CS2 2 C6H6 3 CH3OH 4 C2H5OH 5 CCL4 Table 1 The relative RCS for C=C and C-C stretching modes of aqueousβ-Carotene at 10-5M at different solventsThe relative RCS for C=C and C-C stretching modes of aqueousβ-carotene at different solvents were computed.Table 1 shows the relative RCS of aqueousβ-carotene at solvents,in the paper the relative RCS ofβ-carotene is 5.1*107 in benzene.From the relative RCS we can find that the absolute RCSs for C=C and C-C stretching mode ofβ-carotene is larger than general RCS(10-30 cm2 molecule-1Sr-1)for about 106～107 times. Figure 2 and Figure 3 show that the intensity of Raman is increasing linearly with the increase of the solvents.The slope of Figure 2 is a little steeper than Figure 3.It can be seen the Raman bands of C=C stretching is more sensitive to around environment than C-C stretching. Figure 4 and Figure 5 show the relative RCSs ofβ-Carotene increase With the increase of the solvents refractive index,especially the relative RCS ofβ-Carotene in CS2 is the largest.Figure 6 show the absorbing spectra ofβ-Carotene in different solvent.With the increase of solvents refractive index the wavelength peak shift red,indicating that the molecules chain become extension.ExtendedЛ-electron giving a strong electron-phonon coupling bring CC coherent vibration,meaning a large RCS.The increasing factor to the RCS is about 10～102.Fig.2 Relative Raman intensity of C=C(1520cm-1) inβ-Carotene and Refractive index of solventsFig.3 Relative Raman intensity of C-C(1150cm-1)inβ-Carotene and Refractive index of solvents Fig.4 Relative RCSs for C=C(1520 cm-1) stretching mode of aqueousβ-Carotene in solventsFig.5 Relative RCSs for C-C(1150 cm-1) stretching mode of aqueousβ-Carotene in solventsFig.6β-Carotene absorption spectra in solvents2 The study of RCSs and molecular chain structural order in polar solvent (water)at low concentrationUsing Teflon-AF2400 liquid optical fiber,RCSs for C=C and C-C stretching modes ofβ-carotene in aqueous solution at low concentrations(10-6-10-10 M) were determined by measuring Raman intensity.It was unexpected that the RCS increased rapidly with the decrease of concentration at extremely low concentration range(10-8-10-10 M),larger than general RCS (10-30cm2molecule-1Sr-1) for 109 times.Fig.7 shows the dependence of the changing tendency of RCS for C=C and C-C stretching mode of aqueousβ -Carotene on the concentration.Each point on the curve is the value of the RCSs at various concentrations.The molecular chain tended to become extension with the concentration decreasing,Despite the fact that the tendency is not very clearly.Fig.7 The relative changing tendency of RCS for C=C stretching mode ofβ-Carotene in aqueous solution with the decreasing concentration. The solid dots are the RCS values of 1520cm-1 band versus various concentrations.The inset is corresponding to C-C stretching mode (1155cm-1).3 The study on RCS and molecular chain structural order in nonpolar solvent(CS2) at low concentration.RCS for C=C and C-C stretching modes ofβ-carotene in aqueous solution at low concentrations(10-6-10-11 M) were determined by measuring Raman intensity.It was unexpected that the RCS increased rapidly with the decrease of concentration at extremely low concentration range (10-8-10-11M),which is larger than general RCS(10-30cm2molecule-1Sr-1) for 1010 times.We fit the RCS ofβ-carotene and the concentration according to the data on Table 2:σR(C)=A exp(-C/B)+D Where A B D is fitted parameter;C isthe concentration ofβ-carotene.To the C=C stretching mode,A=38838,B=1.9E-10,D=287,the correlation factor is R2=0.995;To the C-C stretching mode,A=1869,B=2.0E-9,D=36 the correlation factor is R2=0.986.The dot is the experimental data,and the solid line is fitted line.From the figure8 and figure9,you can see that the tendency of RCS of C-C stretching mode forβ-carotene is the same as one of the C=C stretching mode.With the concentration decreasing the Raman line shift red,shown in figure10.The band line width become narrow,which is attributed to the CC coherent from the electon-phonon coupling.In the nonpolar solventsβ-carotene Raman band shift red is clearer than that one in polar solvents,which means that the chain structural order ofβ-carotene in nonpolar solvents is better than that one in polar solvents.We give experimental evidence of the changes in the structure of the molecular backbone induced by the solvents.Table2 RCS ofβ-carotene at different concentration Fig.8 The relative changing tendency of RCS for C=C stretching mode ofβ-Carotene in aqueous solution with the decreasing concentration.Fig.9 The relative changing tendency of RCS for C-C stretching mode ofβ-Carotene in aqueous solution with the decreasing concentration.Fig.10 The Raman shift in different concentrationThe study of RCS forβ-carotene in different solvents can supply a emulating environment where the subject,especially biological samples exist,which is a potentially powerful analytical tool for life sciences, medicine and biological environment.Resonance about RCS of biomolecules at extremely low concentrations supplies experimental data and theoretical basis for the microscope mechanism of molecular interaction, which is helpful to know the natures of electron-phonon coupling. These researches are not only basic research,but also applicable one. All the studies will be beneficial to the bioanalytical and biomedical application,surface chemistry etc.
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