Some problems concerning mixers and detectors for stopped flow kinetic studies.

This conference is taking place on the 55th anniversary of the introduction of rapid mixing methods to the field of biology by Hartridge and Roughton (1923a). Their specific aim was to study the reactions of hemoglobin with its ligands (Hartridge and Roughton, 1923b). The tenacity with which nature holds onto its secrets is nowhere better exemplified than with this remarkable molecule. The very nature of the reaction of hemoglobin with its ligands (02, CO, and NO) and its effectors (H+, 2,3diphosphoglycerate[phosphates], CO2, and salt), together with its tetrameric cooperative intramolecular interaction, taxes our ingenuity and technology to explore, and we still do not understand these reactions in detail. However, methods and techniques discussed at this meeting offer the greatest promise so far for forcing this important model of molecular dynamics to yield its secrets. During the past ten years a number of advances have been made that are only now bearing fruit, not just in hemoglobin research but also in many other areas of biology. The wide range of papers and posters presented in this Discussion bears witness to the breadth of application that has occurred as reliable commercial instruments have become available. These instruments have in general a time resolution of 1 ms, limited primarily by flow velocity and stopping time; they are useful for solutions whose viscosity is essentially that of water. The state of the art in mixing and flow systems up to 1972 has been reviewed recently by Chance (1974). In this talk I would like to point out some of the technical problems that still exist, with the hope that this discussion will stimulate research efforts to solve them. Since the introduction of the rapid reaction apparatus and early experimental work of Hartridge and Roughton (1923a, 1924, 1926), Roughton and Millikan (1936), Millikan (1936), and Chance (1940a,b), very little quantitative work has been done on the theory of mixing and flow for the particular needs of stopped flow. Schlichting (1968), Pai (1954), Kay (1963), Hinze (1959), Davies (1972), and