BACKGROUND: After excitation with light photoacids can change the pH in a solution by release of a proton. They have been used mostly for excited state proton transfer studies. In this review the general functionality and mechanisms and the subdivision of photoacids is explained. STATE OF THE ART: Different uses of photoacids are described, covering a wide range of various biochemical topics, focusing on biochemical applications. Examples for the introduced subdivisions are covered. CONCLUSIONS AND OUTLOOK: The areas in which photoacids can be employed are diverse. Photoacids have a promising future in biotechnology and biochemistry and should be considered for upcoming applications, especially in non-invasive control of biochemical reactions.

Many photochemical or photobiological applications require the use of high power ultraviolet light sources, such as high-pressure mercury arc lamps. In addition, many photo-induced chemical, biochemical and biological applications require either a combinatorial setting or a parallel assay of multiple samples under the same environmental conditions to ensure reproducibility. To achieve this, alternative, controllable light sources, such as ultraviolet light emitting diodes (UV LEDs) with high power and spatial control are required. Preferably, LEDs are arranged in a suitable standardized 96-well microtiter plate format. We designed such an array and established the methods required for heat management and enabling stable, controllable illumination over time.

Most biochemical reactions depend on the pH value of the aqueous environment and some are strongly favoured to occur in an acidic environment. A non-invasive control of pH to tightly regulate such reactions with defined start and end points is a highly desirable feature in certain applications, but has proven difficult to achieve so far. We report a novel optical approach to reversibly control a typical biochemical reaction by changing the pH and using acid phosphatase as a model enzyme. The reversible photoacid G-acid functions as a proton donor, changing the pH rapidly and reversibly by using high power UV LEDs as an illumination source in our experimental setup. The reaction can be tightly controlled by simply switching the light on and off and should be applicable to a wide range of other enzymatic reactions, thus enabling miniaturization and parallelization through non-invasive optical means.

ZusammenfassungDurch Integration der experimentell aus der DanYvEsTEm-Ana~yse gewonnenen Energieverteilungsfunktionen der Elektronen werden die Elektronenkonzentrationen im Plasma der positiven Saule yon Ne-und He-Niederdruckentladungen bestimmt und mit den aus der Strombilanzgleichung erhaltenen Werten verglichen. Da in die Komentrationsbestimmung aus der bei der DanYvEsmYN-Analyse ermittelten zweiten Ableitung des Sondenstromes die Lage des Plasmapotentials maDgeblich eingeht, kann man indirekt auf daa Raumpotential schliekn.Eine weitere Moglichkeit, das Plasmapotential zu bestimmen, ergibt sich am der Gegeniiberstellung von gemeesenen und berechneten Energieverteilungsfunktionen. Sowohl im Helium als auch im Neon lessen sich in erster Naherung die berechneten Verteilungsfunktionen abschnitteweise durch Standardverteilungen approximieren. Der Abfall der Verteilungsfunktion bei hohen Energien bt eine Folge der Wirk$amkeit der unelaetiachen StiiDe auf die Einatellung der Energieverteilung der Elektronen. In geeigneter Darstellung tritt bei der 1. Anregungsispannung daa betrachteten Gasa ein Knick in der 2. Ableitung der Sondenchamkteriatik auf, der erperimentell beatimmt wird. Aua der Lage diesea KnickpunkM und aus der Kenntnis der hgungespannung kann des Plasmapotential ald Bezugspunkt in der Energieskala angegeben werden. Beide Methoden weiaen darauf hin, daD das Plasmapotential im Bereich des Nulldurchganges der 2. Ableitung der Sondencharakteristik liegt.

SummaryThe paper deals with the propagation of externally excited signals in dc-discharges. The experimentally observed behaviour of signals can be qualitatively correct explained on the basis of the theoretically developed conception of the simultaneous propagation of ion acoustic wave and electron free streaming wave. There exists a satisfactory quantitative agreement between experimental and theoretical values of damping for both types of waves. The assumption of the existence of the electron free streaming wave explains not only the amplitude modulated measuring signal but also leads to an essentially better agreement between the experimental and theoretical values of damping for ion acoustic waves.