Circular dichroism spectra of oriented and unoriented deoxyribonucleic acid films—A preliminary study

Abstract

The circular dichroism and absorbance spectra of both oriented and unoriented DNA films ( Escherichia coli and calf thymus DNA's) were studied as a function of salt of the nucleic acid (sodium and lithium), salt ion concentration and the relative humidity (100% hydrated down to 34% r.h. † † Abbreviations used: r.h., relative humidity; CD, circular dichroism. ). At 92% or higher humidities sodium and lithium salts of sheared or sonicated E. coli and calf thymus DNA showed a CD spectra that was essentially the same as that of the nucleic acids in solution. Presumably these films are of the DNA in the B form. At humidities in the range of 75 to 66% r.h. films with 3% sodium content by weight showed a CD spectra very similar to that of double-stranded RNA in solution or the A form geometry. However, LiDNA at low humidity showed a different CD spectra in which the positive 275 nm peak disappears and which shows similarities with the CD spectra of DNA solutions in which the salt concentrations are in the range of 4 to 6 m for both NaCl and LiCl. This type of spectra is presumably a C form geometry type. The calf thymus DNA at low humidities and low sodium ion concentrations shows very high magnitude ellipticities and a non-conservative type CD spectrum which is probably due to aggregation artifacts or strong optical interaction between close packed molecules in the film. In the case of the oriented DNA films it is shown that the CD spectra obtained, which shows a very high angular dependence as the film is rotated about the axis of the beam, can be interpreted as the sum of two components. By the use of Jones matrix calculus a general formula is developed which gives the apparent CD spectra of films having both linear and circular dichroism and linear and circular birefringence. For films that are reasonably thin and are only partially oriented these formulas reduce to a particularly simple form, the sum of the actual circular dichroism term plus a linear dichroism component which is a function of the orientation of the fast axis of the oriented DNA film with respect to the CD apparatus parameters. Using the formula, both the CD spectra and a linear dichroism spectra are obtained giving an insight into the orientation of the optical transitions of the DNA as it is found in the film. These equations give evidence for the possible existence of an n−π ∗ transition below 240 nm and probably centered at 225 nm.