On the basis of the known sequences and structures of myoglobin, and alpha and beta hemoglobin, a possible correlation between certain amino acids in the sequence and the location of the helical and non-helical parts of the structure is suggested. The presence in the sequence of four critical groups; proline, aspartic acid, glutamic acid, or histidine appears to be necessary (although the last three are not sufficient) for a helical disruption to form. Additional support for this correlation is obtained from analyses of proline replacement in mutant and variant proteins. A mechanism based on hydrophobic bonding is proposed as a rationale for the apparent behavior of these groups. On the basis of these rules and correlations, secondary structures can be proposed for lysozyme and tobacco mosaic virus protein which are consistent with several pieces of evidence.

The fluorescence of cattle rod outer segments (dried) and of rhodopsin in solution lies in the range of 575 to 600 millimicrons with a quantum efficiency of 0.005 if excitation is in the visible band near 500 millimicrons. The emission is abolished by bleaching at -196 degrees C but can be reversibly regenerated by irradiation with light of longer wavelength (600 millimicrons). This behavior reflects the known interconversion of rhodopsin to prelumirhodopsin at this temperature.

Publication costs assisted by The U. S. Atomic Energy CommissionThe photoisomerization of all-trans, 13-cis-, and 11-cis-retinal was carried out using energy transfer techniques. In this way the isomerization was initiated in the retinal triplet state. From the triplet the quantum efficiencies found were 0.17 and 0.35 for the all-trans and 13-cis isomer, values comparable to the efficiencies measured by direct singlet irradiations; however, 11-cis-retinal had a measured triplet isomerization efficiency of 0.75, a value much higher than that measured by direct photoisomerization. These findings are in qualitative agreement with recent theoretical predictions providing that the higher steric factors in 11-cis-retinal are considered.

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