SynopsisDNA-oriented samples of various origins were studied under different conditions of humidity and sodium chloride content by means of infrared spectroscopy.( 1 ) Oriented DNA ( M . Lysodeikticus, E. wli, calf thymus and salmon sperm) films a t 3-475 sodium chloride yield polarized infrared spectra which show drastic changes at relative humidities (r.h.) between 94% and O%, indicative of conformational changes:The measurements of the infrared dichroism a t frequencies of about 1230 cm-1 and a t about 1090 cm-1 allow one t o determine the orientation of the phosphate group, whereas the measurements at 1710 cm-1 characterize the base orientation. At humidities higher than 90% r.h. (B form) the bisector of OPO forms an angle of 70" relative to the helix axis, whereas at lower humidities, between 75% and 50% r.h. (A form) a rotation t o about 45" is observed. Simultaneously, the 0-0 line of phosphate group changes its orientation from 55" to 65" t o the helix when B + A transition takes place. The results are in general agreement with that of X-ray diffraction and allow one t o determine the orientation of the phosphate group with greater precision.( 2 ) The B-A conformational change is not observed for satellite DNA, isolated from Cancer pagums, of which the guanine + cytosine content is below 5%. As a function of decreasing humidities, one observes the transition:B form + disordered form A diagram of conformational changes of DNA's as a function of base composition and of r.h., suggests that B-A transition will occur for DNA of relatively higher G + C content, whereas for high (A + T ) content, base sequence may be of importance.The B-A transition is prevented in DNA at relatively high or very low sodium chloride content.

Polydeoxynucleotides of different base sequence, the alternating poly[d(A-T)]-poly[d(A-T)], crab satellite DNA, on the one hand, and double-stranded homopolymer complexes poly[d(A)]-poly[d(T)], poly[d(I)]-poly[d(C)], on the other, display significant differences in their conformation and conformational transitions. Infrared linear dichroism investigations indicate that the alternating poly[d(A-T)]-poly[d(A-T)], enzymatically synthesized, adopts a lower humidity a well-expressed A* form in which stability is relatively small,i.e., restricted to limited relative humidity. This A form is characterized by the orientation of the bisector of the phosphate OPO group at 34 degrees with respect to the helical axis, which is slightly lower than that of DNA. In contrast, for the homopolynucleotide double-stranded complex poly(dA)-poly(dT) and also for poly(dI)-poly(dC), the B yields A conformational change is not observed. Instead poly(dA)-poly(dT) exists at lower humidity in a stable modified B form. Thus the present results indicate that homo(dA)-homo(dT) double-stranded sequences prevent the B yields A structural transition. All AT-containing polydeoxynucleotides and crab satellite DNA adopt a high humidity a modified B form characterized by the orientation of the bisector of the phosphate group OPO at 64 degrees with respect to the helical axis which is significantly lower than 68-74 degrees observed in DNAs. The base pairing geometry in poly(dA)-poly(dT), poly[d(A-T)]-poly[d(A-T)], and also in poly(dI)-poly(dC) is apparently a Watson and Crick type. Thus the observed differences in conformation are not due to different base pairing scheme. It is suggested that in DNAs of high AT content the presence of homo(dT)-homo(dA) sequences and the relatively low stability of the A form of d(A-T) alternating sequences may inhibit the change to the A form. A possible role of these sequences in DNA recognition by protein is suggested.

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The presence and assignment of low frequency infrared bands characteristic of the A, B and C forms of DNA (Salmon Sperm and Micrococcus Lysodeikticus) and of synthetic poly (dA‐T): (dA‐T) and of polyribonucleotides‐A form are considered. The vibrational modes that may arise from the phosphate diester stretch of DNA backbone are investigated in the region of 750 cm−1–900 cm−1. The proposed assignment of the IR bands occurring at about 837 cm−1 to the vibrations of the phosphate diester C–O–P–O–C chain in a specific conformation allow characterization of the backbone geometry in the B and C forms. The bands situated at 812 cm−1 and 860 cm−1 are undoubtedly characteristic of the backbone phosphodiester chain of the A form. The possibility of other contributions to the 812 cm−1 band is also discussed.

From infrared linear dichroism studies, direct evidence is given for a new Na DNA form, which is similar to the C form of Li DNA. This C-like form in Na DNA occurs under conditions of very low NaCl content (at high relative humidity), high NaCl content (at low relative humidity), and low G + C content of the DNA (at low relative humitiy). The C form of Li DNA was originally discovered by x-ray diffraction, but has not previously been characterized by infrared spectroscopy. The C form is primarily characterized by the orientation of the O2- -O3 line of the phosphate group, which is about 48 degrees (+/-2 degrees ) with respect to the helical DNA axis and which is different from that of the B and A forms. Na DNA molecules rich in A + T may entirely or partially adopt the C form at low relative humidity. Since from infrared evidence, the C-like form is found not only in Li DNA at low relative humidity but also in Na DNA under different conditions, the C form may be biologically important. A possible role for recognition by proteins is suggested.