Minor tautomers of nucleic acid bases can result by intramolecular proton transfer. These rare tautomers could be stabilized through the addition of methyl groups to DNA bases. A comprehensive theoretical study of tautomers of methylated derivatives of guanine, adenine, cytosine, thymine, and uracil was performed. Molecular geometries of all tautomers were obtained at the density functional theory and MP2 levels with the 6-31G(d,p) basis set, and single-point calculations were performed at the CCSD(T)/6-311G(d,p) level. Tautomers obtained by protonation at the preferred protonation site for methylated isolated bases were compared to their nonmethylated counterparts. The effects of methylation on the relative stabilities of nucleic acid base tautomers are also studied and discussed in this work. The results suggest that some sites on the bases may not be mutagenic and may even stabilize the canonical Watson-Crick form. The results also indicate that a number of methylation sites can stabilize the tautomers, suggesting possible mechanisms for mutagenic changes.

The degradation in image quality, produced by compression, can be regarded as multidimensional in nature, involving tone, colour, sharpness and noise. By measuring each of these categories separately, and then combining to form a single figure of merit or quality metric, a better understanding of the effects of compression on quality can be achieved. This approach is superior to the use of single overall measures of 'quality', such as the Mean Squared Error (MSE) measurement of distortion. Tone reproduction is assessed using cascaded transfer functions. Colour reproduction is assessed using both the CIE (1976) L*a*b* Colour Difference measure and a colour metric, the Colour Reproduction Index, based on a proven model of colour vision. Image sharpness and noise are evaluated using the Modulation Transfer Function (MTF) and the Noise Power Spectrum (NPS) respectively. These results are combined, together with characteristics of the display system and a 'typical' observer, in an image quality metric based on the Square Root Integral (SQRI) metric. Comparisons are made between the various methods of assessment, and objectivelymeasured quality is correlated with results from subjective scaling experiments. Problems with the application of methods based on linear systems analysis are described and possible solutions suggested.

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A method for imaging the conjunctiva to yield greater contrast between the vasculature and the scleral background, by optimisation of the imaging system, is described. A simple modification to the image capture system, based on first principles calculations, is shown to give images with much improved information content. This will enhance the assessment of hyperaemic changes induced by foreign materials, indicating the degree of biocompatibility, e.g. with contact lenses and ophthalmic medications. Observation of vascular abnormalities associated with pathological conditions may assist in their diagnosis and monitor progressive changes.

Summary A method for imaging the conjunctiva to yield greater contrast between the vasculature and the scleral background, by optimisation of the imaging system, is described. A simple modification to the image capture system, based on first principles calculations, is shown to give images with much improved information content. This will enhance the assessment of hyperaemic changes induced by foreign materials, indicating the degree of biocompatibility, e.g. with contact lenses and ophthalmic medications. Observation of vascular abnormalities associated with pathological conditions may assist in their diagnosis and monitor progressive changes.

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