Không giống như các axit ribonucleic, polyme sợi duy nhất, DNA hình thức doublestrand xoắn, sắp xếp một thang xoắn, bao gồm hai chuỗi bổ sung. Bổ sung Điều này xảy ra thông qua liên kết hydro giữa các phân tử giữa hai cặp cơ sở, giữa adenosine của một sợi và thymidine của người khác, và tương tự giữa cytidine và guanosine. | MACROMOLECULAR MASSES AND SIZES BY SCATTERING TECHNIQUES 181 to be scattered in a solid angle per unit volume of the sample. The total scattering section can be split into two contributions one coherent and the other noncoherent dơ dQ dơ dữ c da dtt In an experiment of light or X-ray scattering both scattered and incident radiations are always out of phase which is also true for the neutron scattering by most of the isotopes but not for all certain isotopes like protons scatter neutrons in phase. In contrast deuterium scatters neutrons out of phase and in a coherent manner. The concept of coherent scattering length was introduced to account for the phase of the scattered neutrons 2 ddd i bij b takes positive values for isotopes scattering out of phase and negative values for in-phase scattering centers. Thus for hydrogen we have bH X 10-12 cm and for deuterium bD X 10-12 cm Under these conditions the scattered intensity can be written as I Q K MocP Q where Kn is the contrast factor which is expressed as Kn tebi- Eb M2 i u Mu is the molar mass of a repetitive unit. The first sum refers to repetitive units and the second sum refers to solvent molecules. For neutron scattering as in the case of light scattering the total scattered intensity corresponds to the difference between scattering by the repetitive units and by the solvent molecules. Recalling that in light scattering the contrast factor K is written as 2n2n2 dn 2 T40 Na dC In X-ray scattering the scattering cross section corresponds to d Ơ d r2 Z 2 182 DETERMINATION OF MOLAR MASSES AND STUDY OF CONFORMATIONS MORPHOLOGIES where re is the radius of an electron X 10-15m and AZ is the difference between the number of electrons of constitutive polymer atoms and those of solvent. AZ can be deduced from the electron density of the various components pe u repetitive unit and pe s solvent AZ 2 pe u - pe s 2V2 where Vu is the volume of a repetitive unit. As indicated earlier the intensity .
